Studio HVAC: All about mini-split systems, HRV's and ERV's.

[Soundman2020]

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What do you need in your recording studio, to keep it cool and comfortable?
A mini-split system? An HRV? An ERV? Or just ventilation ducts?
(Updated October 2, 2020)

This is often a confusing issue for first-time studio builders: deciding on the heating, cooling, and ventilation: What is a "mini-split"? What is an HRV? Do you need them?

Let's take a look at what they do, and what you will need, or not need...

A "mini-split" system is what most people would just call a "wall mounted air conditioner". It looks like this:

mini-split-system.jpg (15.02 KiB) Viewed 58402 times

mini-split-system.jpg (15.02 KiB) Viewed 58402 times

There are two parts to it, as you can see: The top one goes inside the room (usually high up on a wall somewhere), and the bottom one goes outside. There's also a bundle of pipes and cables that joins the two units together.

The outdoor unit is called the "compressor", and the one you put up on the wall inside your room is called the "condenser", or the "Air Handler Unit (AHU)", or more commonly just the "indoor unit".

The reason why it is called a "split system" is simply because the "whole" is split into two separate "parts", unlike older style window air-conditioners that have everything inside one single unit. With a typical "window" unit, both the compressor and condenser are inside the same physical box, but with a split system, the noisy part (the compressor) is in a separate box from the indoor unit (AHU).

A mini-split system basically does four things for your studio:
1) It circulates air around, inside the room.
2) It heats that air as it passes through, if you need that, when you have it in "heat" mode (not too common that you need that in studios).
3) It cools air as it passes through, if that's what you need, when you have it in "cool" mode.
4) It de-humidifies the air as it passes through, and that happens in either of two modes: the specific "de-humidify" mode, or more generally in normal "cooling" mode.

The reason why it dehumidifies while cooling is simple: as the warm air passes over the cold vanes on the cooling coils inside the unit, moisture in that air will condense on the vanes, as liquid water, which is then drained away through a "condensate drain" pipe. This happens if you want it or not, just because you have warm moist air passing over a cold surface. (Hold a spoon above your kettle when it is boiling, and watch how the water condenses on the cold spoon, and drips off.) In fact, that condensation process actually robs you of cooling power, because the simple fact of condensing the water on the vanes inside the indoor unit, also heats up those cooling vanes: and if the vanes are warm, then they cannot make the air cold! This is why you can have an air conditioner running full bore on a hot humid day, and it seems to be blowing warm air on you, not cold: Because all of its cooling power is used up, 100%, just by condensing the water out of the humid air! There's no cooling power left to actually cool the air.

So that's what the Mini-split does. But what it does NOT do, is provide fresh air, or remove stale air! it just sucks in room air through the intake on its top face, passes it through a simple dust filter inside, then over the cooling vanes, then back into the room through the bottom front slot.

It needs to move a certain volume of air every hour, to ensure that the room stays cool / warm / de-humdified all over, and the amount of "air movement" is determined by the size of the room. Obviously, smaller rooms don't need to have so much air circulating, but larger rooms do. It's easy to calculate: You need to circulate at least 6 "room changes per hour" (preferably 8 ) through the mini split, to ensure that your room air is "conditioned" properly. If you don't move enough air, then it will take a long time to heat or cool the room, or to control the humidity, and there will also be permanent "hot spots" or "cold spots" in the room, that the conditioned air never really reaches. So, for a room that has a volume of, for example, 2,000 cubic feet, you need the mini split indoor unit to move 2,000 x 6 = 12,000 cubic feet per hour. But HVAC is usually rated in "CFM" for "Cubic Feet Per Minute", rather than cubic feet per hour, so you divide that number by 60 (because 60 minutes in one hour), and you get the answer: in this hypothetical case you would need a unit that is able to move 200 CFM of air (200 cubic feet per minute). Note that this has nothing at all to do with the SPEED that the air moves! This is just about the VOLUME of air that you need to move, to condition the room properly. I'll get back to the speed issue later. For now, let's just look at volume and flow rate: cubic feet per minute (in the metric system, this is usually specified as "liters per second" or "liters per minute).

So, in summary so far, that's the mini-split, and what it does: It heats, cools, and de-humidifies, while circulating enough air through the room to keep it comfy. But it does nothing at all for your fresh air / stale air needs.

(And yes, you do need a mini-split in your studio. If you think you don't need one, then you should probably read this: why your studio needs proper HVAC. )

Thus, in addition to your mini-spilt, you need to bring fresh air into the room from the outside world, and you also need to exhaust the same amount of stale air from the room back into the outside air. So you need one "fresh air supply duct" coming into the room, and one "stale air exhaust duct" going out of the room.

That fresh air might come in directly, or it might first be "pre-" processed in some way. In moderate climates, where the indoor temperature and humidity are not too different from outdoors, you can just bring that fresh air right into the room directly, passing it through a filter to get the dust, pollen, and other pollutants out, then into the room (through the silencer boxes, of course!). You do the same with the stale air: just dump it straight out into the world, through a duct (and another silencer), and you are done.

However, in a more extreme climate, the incoming air might be a lot hotter, or a lot colder, than the indoor air, and the exhaust air would be in the same condition: a lot hotter, or colder, than the outdoor air. Since it costs you money to heat that air or cool that air as it goes through the mini-split, it makes sense to do as little heating and/or cooling as possible. And dumping out stale air that you just spent money heating/cooling, is wasteful: It's almost like throwing money out the window!

For the rest of this example, let's assume that you live in a very hot climate ... somewhere in Texas, maybe. (The same principles that I'm going to talk about here, also apply to very cold climates, except in reverse.)

So the air coming in from outside your Texas studio is hot, and you want to have cool air inside the room. Your mini-split will cool it for you, of course, so the smart thing to do is to put that fresh air inlet vent directly above the mini-split air intake on the indoor unit, so it sucks in the hot air, cools it, then sends it into the room. The mini-split would also be sucking in some room air along with that hot air, so they mix together inside the air intake, then get cooled and sent back to the room. That would certainly work: putting the hot outside air directly into the mini-split, but the poor mini-split would be working full time, just cooling down all that hot outside air! It costs money to run that thing, and having it going full bore 24/7 is expensive. At the same time, you would be dumping out the same amount of stale air to the outside world.... which would be cold air, that you just spent a lot of money cooling down, only to dump it overboard!

This is not efficient: you are wasting money like that: it costs money to run the mini-split compressor in order to cool the air, then you just throw away that cold air later. ("Throwing money out the window.")

This is where an HRV comes in (we'll get to ERV's in a bit: first the HRV).

An HRV is a "Heat Recovery Ventilator". The basic concept is that the outgoing cold stale air cools down the incoming hot fresh air. Or more correctly: the incoming hot air heats up the outgoing cold air, and thus gets cooler itself. The HRV exchanges heat between the two air streams. The two air streams don't actually mix, of course: they go through separate "pipes" inside the unit, and the "pipes" are in contact with each. The heat is transferred through the thin metal walls of the "pipes". In most HRV's they aren't really "pipes" at all, but rather long narrow rectangular cavities with thin metal walls on both sides, and a gap of just a couple of mm (fraction of an inch) between them for the air to flow through. These rectangle metal cavity things are interleaved, and stacked in many, many layers, with the two different air paths going through alternating layers. So the outgoing exhaust air goes through layers 1, 3, 5, 7, 9 etc, and the incoming fresh air goes through layers 2, 4, 6, 8, 10, etc. Thus, each air path has a thin metal plate on each side, and just beyond that plate is the "other" air path, going the other way.

It works something like the radiator in your car, where the water flows around inside the cooling vanes, and the air flows around the outside. So the HRV has BOTH air streams passing through it: Both the incoming fresh air stream and also the outgoing stale air stream pas through the unit. Here's a diagram to show how that works:

HVAC-HRV-Heat-Recovery-Unit-fantech.net.jpg (115.52 KiB) Viewed 58401 times

HVAC-HRV-Heat-Recovery-Unit-fantech.net.jpg (115.52 KiB) Viewed 58401 times

One common way of making the rectangular "layers" in the core of an HRV, is using some type of "honeycomb", or "corrugated" metal foil, inside (or even folded wires), to keep the two side foil "plates" in place, so they don't collapse (they are very thin). And the two air paths usually cross over each other, at 90°. So this is what two adjacent layers looks like, in theory:

HVAC--HRV-CORE-diagram-two-layers--simplified-1.jpg (23.52 KiB) Viewed 58377 times

HVAC--HRV-CORE-diagram-two-layers--simplified-1.jpg (23.52 KiB) Viewed 58377 times

And here's what it looks like when multiple layers are stacked:

You can see the two air flow streams crossed over, going through alternate layers of thin corrugated metal.

Here's another way of doing it, in cheaper (but less efficient) HRV units, where only one flow path has the corrugated cores in them, while the other is an open rectangle:

HVAC--HRV-core--staked-layers-diagram--HRV01-FXD.jpg (56.65 KiB) Viewed 58377 times

HVAC--HRV-core--staked-layers-diagram--HRV01-FXD.jpg (56.65 KiB) Viewed 58377 times

It's less expensive to make, but less efficient since the corrugations also pick up more heat from each air flow, and help transfer it to the other. There's a few places on the internet that show you how to build your own HRV this way, DIY, using various types of corrugated non-metal materials. I have never tried that myself, so I am not sure how well it would work, but if you are on a really tight budget for your studio, then it's worth a try.

And here's what a typical complete HRV "core" looks like (commercial, not DIY!):

You can see the layers of corrugated metal in there, crossing over each other. In this picture, one air flow stream would enter the left front face and exit through the right rear face (not visible in the picture), while the other air flow stream would enter the left rear face (also not visible here), and exit through the right front face.

A good HRV can recover a LOT of heat, saving you a lot of money, over time. Top-of-the-line units can recover 90% (or more) of the heat, and even so-so units can recover 60% or 70%, so it's well worthwhile. Every watt of heat that you recover is one watt that you don't have to pay for from running the Mini-split compressor.

Now for ERV's. ERV stands for Energy Recovery Ventilator, and the concept is very similar, except that an ERV can also transfer the humidity between the two air streams. The HRV only transfers heat, but the ERV transfers both heat and humidity. As I mentioned above, dealing with humidity also costs you money, because that's part of what happens when the humid air condenses inside your Mini-split. The de-humidifying process robs you of cooling power, so you need a larger, more powerful, and more expensive unit, that can de-humidify the air completely and still have some cooling capacity left over to actually cool the air. So if you live in a hot, humid climate, you would need a very much over-size unit to do the job... which costs you more money to buy, and more money to run (higher electricity bills).

That's where the ERV comes in: it transfers a lot of the humidity (more correctly: water vapor in the air) from one air stream to the other air stream. In a humid climate, the incoming humid air condenses on a set of cold plates inside the unit, that is part of a wheel, and those plates are then rotated into the out-going air stream, which picks up the moisture from the plates (by evaporation) and carries it back outside. Thus, that humidity never gets into your studio, so your Mini-split does not need to deal with it. The wheel with the plates on it gets the fancy name "enthalpy wheel", but don't worry about that. Here's a diagram showing how an ERV works:

and here's a typical small unit opened up, so you can see the wheel inside

There's another type of ERV where the core looks very similar to an HRV, except that it is not made from metal: the thin walls between the two air flow streams are made from some type of special porous material that allows the humidity to pass through from one side to the other, but prevents the air from getting through. You can hardly tell the difference between the cores of these ERVs and the core of an ordinary HRV just by looking at it... except that the ERV core is a lot more expensive!

A quick note about terminology here: Some HVAC people use the name "ERV" for BOTH types of unit! That's incorrect, of course, and leads to confusion, but they do it anyway. An HRV only recovers heat, and just passes the humidity straight through. An ERV recovers both heat AND humidity. When deciding if you do need one or the other, be very careful as you talk to your supplier: he might be talking about ERVs, when he actually means HRV....

HRV's and ERV's are not perfect, of course: some heat still gets lost and transferred back into the room, and some humidity makes it in as well. So don't think that an ERV will do the same job as a dehumidifier or mini-split: it won't. Not even close. But they still save you a lot of money in the operating costs of the mini-split, because they do manage to move quite a bit of heat, and humidity, and they might also allow you to use a smaller (less expensive) mini-split, because the HRV/ERV takes some of the load off the mini-split.

One final point about HRVs. The HRV (or ERV) usually has a fan in it that sucks in fresh air from outside and also blows the stale air back outside, and therefore also causes air to circulate into the room, and back out again. Sometimes that fan isn't enough to do the complete job, and you will need a booster fan in one of your ducts.

So, to summarize:

1) the Mini-split circulates, heats, cools and de-humidifies, but does not supply fresh air or remove stale air.
2) The ducts and silencers supply the fresh air and remove the stale air.
3) An HRV or ERV transfers heat (and humidity) between the fresh-air stream and the stale-air stream.

Now lets put all of that together in a system, to see where the parts go...

Here's a rough diagram of how that works in general, but not totally accurate for the case of a mini-split:

That's more for a furnace system, or a ducted mini-split, but if you ignore the large box on the right side, then that's pretty much what you will have in a typical studio, if it uses an HRV or ERV. This diagram also does not show the silencers! More about those later... (When I have time, I'll do something in SketchUp to show more what the full system looks like for a studio, with silencers.)

So, there is one duct that brings in fresh air, passes through the ERV then, through a silencer, into the room. And there is another duct that leaves the room carrying stale air, goes on to the ERV, then to the outside.

But why do you need to bring in fresh air at all? After all, if you see one of these mini-split things in a house, shop, or small office, you don't have to have ducts that bring air to it! Why do you need to do that in a recording studio? Simple: because unlike a house, your studio is sealed perfectly air-tight... twice over! In a typical house, there are multiple ways that air can get in and out of a room, through all sorts of gaps, cracks, and holes, but in a studio there are no paths: it is sealed hermetically....

The reason you bring in any fresh air at all, is so you can stay alive! You need to breathe inside the room, and each breath you take brings in oxygen (O2) to your lungs.... and each breath you exhale sends out carbon-dioxide (CO2), methane, carbon-monoxide, and a bunch of other gasses, as well as moisture. So as you breathe, you "use up" some of the O2, reducing the amount left in the room, and you also "add" some CO2, increasing the CO2 concentration in the room. After a while, it gets unpleasant, because the air has too much CO2 in it, and not enough oxygen. After a while more, that gets worse and your health starts suffering: headaches, vision problems, coordination problems, lack of concentration, drowsiness, fatigue, eventually you pass out, and in very extreme cases, you die. That probably won't happen in your studio (hopefully!), but if you don't ventilate, it can get nasty in there quite quickly.

There is a minimum amount of O2 you need to put into the room for each person in there, to keep them healthy (and alive), as well as a minimum amount of CO2 that you need to remove for the same reason.

So it's easy! You just need your air duct or HRV or ERV to supply enough air that everybody gets the right amount of oxygen, and has the right amount of CO2 removed! Simple! Except that it isn't... because the amount of O2 you need, and the amount of CO2 that you produce, depends on your activity level: if you are laying down asleep, you don't need much at all, but if you are running full speed on a treadmill you need a LOT! And if you are jamming hard on the drums, you need a lot, but if you are sitting at the console you don't need so much... So you can't just use one figure for everyone in the room. You need to account for what they are doing.

The recommendation used to be: "15 liters per minute of fresh air per person" (about 0.5 CFM), but that is changing. There's something recent called "sick building syndrome" (SBS), which describes how people suffer a higher level of health issues in poorly ventilated buildings, and more up-to-date recent research shows that you need supply at least 23 liters per minute per person before there's a significant drop in SBS. The new recommendation in some places is 30 l/m per person, which is about 1 CFM per person. But that's for people at rest. For heavy activities, it could be ten times that. To be safe, I assume 10 CFM per person. Even though people in studios are not running full tilt on treadmills, they can still be working hard, so I play it on the safe side and give them plenty of fresh air. So, for a typical studio live room (LR) that might have a band with ten people in it, that would be 100 CFM, or for a control room that might have 5 or 6 people in it, that would be maybe 60 CFM.

Since there's a very aprox. relationship between the size of a room and how many people you can fit inside it, and since we already know how to figure the internal circulation rate that the room needs for cooling/heating/dehumidifying (6 changes per hour), I just use a very simple rule of thumb: whatever your circulation rate turns out to be, you need have about 20% to 40% of that as your supply rate for fresh air to the room. Thus, the hypothetical room I mentioned above that had a volume of 2,000 cubic feet, and needed 200 CFM or air circulating through it, would also need somewhere between 40 and 80 CFM of fresh air coming in, and the same amount of stale air going out. That's a very rough "rule of thumb" that will get you in the ball-park, but it's a bit more complex than that to figure out accurately.

But there's a big problem actually implementing this in practice: Studios have acoustic isolation walls around them! And we need to chop huge holes in those walls to get the ducts in! That's a problem, because even small holes will trash your isolation. That's where "silencer boxes" come in.

An HVAC silencer box (sometimes called a "baffle box") is just a long box made from heavy material (usually one or more layers of thick wood), that is installed exactly at the point where the duct would normal go through the wall or ceiling to the register that delivers the air into the room. The silencer replaces the last few feet of duct before the wall (or ceiling), and has a heavy wooden "sleeve" that passes through the wall or ceiling, usually with the register on the other end. Something like this:

That's the type of "dual split flow" silencer that I developed years ago for most of the studios I design, but might be a bit too sophisticated for smaller home studios. However, it illustrates the principle. This silencer box sits above the inner-leaf ceiling, and below the outer-leaf ceiling or roof. The air supply duct comes into the middle of that, then the air flow splits out two ways, down the two "arms" of the silencer. You can also see the wooden "sleeves" with the registers on the end, that poke down from the bottom of the arms: those are the ones that go through the actual studio ceiling (or wall, in some cases), into the room. You can see this type of "Soundman dual split silencer" in a high-isolation studio, here: Small studio with high isolation: how to build one.

A silencer allows the air to flow through unimpeded, but has a series of solid plates or "baffles" inside, as well as special insulation, and other features, that stop the sound getting through. Careful here! HVAC supply companies also sell "silencers", that are just round metal tubes, similar to a car exhaust muffler, with elements inside to reduce the sound level.... but those are no good for studios. They are fine for offices, houses, shops, churches, etc., and the typical noises you find in those places, but they don't work very well for the low frequencies and high sound levels that you typically find in recording studios. You need proper, custom-designed silencers for your studio.

Now, if you are looking for very high isolation for your studio, then you need TWO silencers for each place where a duct passes through a wall.... because studio walls consist of two "leaves" separated by an air gap, so you need one silencer on each leaf. For high isolation, you can't have one silencer "sleeve" that passes through both leaves, since that would create a path along the sleeve itself for transmitting sound between the leaves, reducing isolation... So your studio silencers should be designed specifically for YOUR studio, for the air flow rate, air flow velocity, and level of isolation (also called "insertion loss" by HVAC people). For medium isolation you can sometimes get away with a single box per wall, and for low isolation you might not even need a box at all: in some cases, a simple run of flex duct might give you enough isolation. But don't count on it doing a lot!

So, summarizing again:
1) the mini-split heats/cools/circulates, and does so at a rate based on the room volume.
2) the HRV/ERV brings in fresh air and removes stale air at a rate of about 20% to 40% that of the mini-split
3) the air going to/coming from the HRV/ERV, moves through ducts to get into the room, and those ducts have to pass through the walls
4) you need silencers at each point where a duct goes through a wall. The silencer allows the air to get through, but stops sound getting through.
5) silencers need to be designed specifically for studios, and for the specific room. Commercial "off-the-shelf" cylindrical silencers are not up to the job.

But why do you need a fancy "silencer"? Why can't you just cut a round hole in the wall, and put the duct through that?

Because it would completely destroy your isolation! A huge hole like that, big enough for a duct, would let through MORE sound than is blocked by the entire wall.

In fact ANY hole in a studio wall is "huge"! Even a pencil-sized hole is "huge", as far as isolation goes. People often don't realize just how much sound can get through a very tiny gap, but it's an eye opener. I wish I could find a better version of this graph, but you can just make out the details, if you look closely:

loss-through-tiny-cracks-and-reduction-effect-of-small-gaps-on-TL.jpg (28.71 KiB) Viewed 58401 times

loss-through-tiny-cracks-and-reduction-effect-of-small-gaps-on-TL.jpg (28.71 KiB) Viewed 58401 times

That shows how much isolation you lose from tiny gaps and cracks in your wall. The horizontal axis is how much isolation you thought you would get from a wall (ceiling, door, window, etc) if it was perfectly sealed with no gaps or cracks, and the vertical axis shows how much you actually get if there is a tiny air gap. The various curves on there are for various "open area" percentages, starting with 0.01% at the top. That's the very top curve, with the mildest reduction in TL... where you already get a whopping 10 dB loss if you were aiming for 50 dB! Yup. that really is 0.01%. One hundredth of one percent. Yup, it really does cost you 10 massive decibels.

In real terms, let's say you have a wall ten feet long, by 8 feet high: that's 80 square feet, which is 11,520 square inches. 0.01% of that is 1.15 square inches. So all you need for that wall, is a hole about the size of your pinky, and that trashed your 50dB isolation down to 40 dB.

Look at it from a different point of view: let's say that instead of a pinky-sized hole in the wall, there's actually just a tiny crack under the wall, only 1/32" high (less than 1 mm), and three feet long (about one meter). That adds up to the same thing! about 1 square inch.

This is why you'll see me repeat over and over again, almost like a mantra, that sealing your room air-tight is absolutely critical for isolation. Tiny gaps cost you big-time. Sealing everything is the best possible thing you can do to get good isolation.

So, getting back to the point of HVAC ducts: even the smallest HVAC duct is a "huge hole", compared to that pinky-sized thing. Thus, you do need silencers.

Then there's the issue of air flow velocity (speed) again. That's actually what determines how big the ducts need to be. Lets' talk about the same hypothetical room above: We needed 200 CFM air volume circulating around inside that room, through the mini-split indoor unit to keep the room comfortable, and we need to supply 40% of that rate as fresh air, so 80 CFM is what we need to bring in. That's the air flow rate or air flow volume. But the air velocity at the registers cannot be higher than 300 fpm (or it will be too noisy), and 200 fpm is better. So, simple math: we need 80 Cubic Feet Per Minute, and 200 Feet Per Minute: simple division: 80/200 = 0.4 square feet. That's how big the air path needs to be, to produce a flow velocity of 200 fpm for a flow rate of 80 CFM. So the HVAC register in the wall/ceiling will need to have a cross sectional area of 0.4 square feet, which is 57 square inches. Thus, you'd need a 6" x 10" register poking through the wall. A rather huge hole!

Sure, you could relax some of the above numbers a bit, and maybe get down to a 4" x 6" register, but that's still a huge hole, when you look at that graph above! 24 square inches on that 10 foot long wall I used as the example, would be 0.2% of the wall. That's the 5th curve down from the top, and it shows that your 50 dB isolation is now down to about 26 dB... which is WORSE than a typical house wall (2x4 studs, 1/2" drywall on each side, is about 30 dB).

Conclusion:
1) You need a mini-split system for your room that is able to circulate all of the air in the room, at least 6 times per hour, and that also has sufficient cooling capacity to deal with the humidity in the air, and also the heat in the air, so the room can be at a pleasant temperature.
2) You need to bring in enough air to keep everyone alive inside the room, and you need to exhaust the same amount of stale air out to the outside world.
3) If you live in a very hot or very cold climate, then you can probably benefit from an HRV or ERV.
4) Your ducts that supply the fresh air an remove the stale air need to be big enough to allow that air to move slowly, so it does not make any noise that would be picked up by the mics in the Live Room, or annoy you in the Control Room, as you try to mix.
5) Your ducts are big "holes in the wall", so you need to put silencer boxes on the. A silencer box allows air to get through the "Hole on the wall", but stops sound from getting through.

One final point: the system I showed right at the top of this page is the typical type of mini-split system you commonly see, and it is usually known as a "ductless mini-split", or an "unducted mini-split", because it does not connect to any type of duct: you just hang it on the wall, and run the pipes to the compressor. But there's another type of mini-split, called a "ducted mini-split", that looks like this:

HVAC-ducted-AHU-plus-compressor-complete-system-BIG-ENH-2.jpg (31.56 KiB) Viewed 58364 times

HVAC-ducted-AHU-plus-compressor-complete-system-BIG-ENH-2.jpg (31.56 KiB) Viewed 58364 times

The compressor unit (outdoors) is the same, but the indoor unit here looks different: In fact, it is sort of like the other indoor unit, but without the pretty plastic covers on it, and with flanges mounted on bot sides, so you can connect it to ducts.

This one is meant to be used along with ducts (obviously!), so it does not go inside your studio: instead, it goes OUTSIDE your studio, and only the ducts go from there into the actual studio. Here's some additional photos of various styles of ducted mini-split air handler unit:

HVAC--mini-split-ahu-typical-indoor-unit-ducted.jpg (11.73 KiB) Viewed 58344 times

HVAC--mini-split-ahu-typical-indoor-unit-ducted.jpg (11.73 KiB) Viewed 58344 times

But why? What's the advantage? Well, there are several advantages. Firstly, is noise: even though the typical ductless mini-split units are quiet, they are still not silent. They do make noises: there's a fan inside that moves air through the room and airflow is noisy, there's a motor that drives the fan, and another motor that drives the moving vanes on the front that waft back and forth to disperse air into the room, there's the noise of the condensate water dripping (or running!) down its drain pipe, there's the noise of the refrigerant moving through the cooling pipes outside the unit, and the coils inside it, and there are sometimes also electron-mechanical "clicking" noises from the relays that kick in and out to control the various functions. All of those noises can really mess with your tracking sessions, as they can get into the mics in the tracking room, or vocal booth, and even just in the control room those noises can be distracting. Studios are VERY quite inside, so you can hear noises that you would not normally notice if they happened in your house. So, it would be good if you could get that unit out of the room, and put it somewhere else: and that's exactly what the "ducted" units are for! You put it outside the isolated, quiet part of your studio (your "isolation shell"), and just run ducts from it, into the room and back, with silencers. This is not a problem, since you have to run ducts into the room in any case, to provide the fresh air and pull out the stale air, so you just use those exact same ducts with the same silencers! There's no change here... except that the unit is now outside your studio, where you can't hear it at all. Then, you just hook up your fresh air supply duct (maybe coming for your HRV or ERV if you have one) to the intake side of the ducted unit, and pick a spot further away on the same duct to take out the stale air.

So, with a ducted mini-split, it sucks air out of the room through the "return" duct, cools/heats/dehumidifies that air, then sends it back into the room through the "supply" duct. At the same time, some of the air coming back through the "return" duct is dumped outside (maybe through an HRV/RV), and your fresh air comes in just before the air intake to the ducted unit. In this case, the ducted unit is usually just called the "Air Handler Unit" (AHU).

And that's it! That's the complete system, and is the usual way of doing HVAC for serious studios (home studios, project studios, and commercial studios).

Here's what such a system looks like, with labels on all of the bits and pieces, so you can see how it all comes together:

I often say this, but it bears repeating again: HVAC is a BIG DEAL for studios, and is very often overlooked in early planning. Even when there's a good, reputable HVAC contractor involved, they very often do not have experience with the very special needs of recording studios, and they want to do it the same way that they would for a house, office, shop, school, church, etc. Then everyone wonders why you can hear the drums in the bathroom, coming trough the ceiling vent.... They might be experts in mini-splits and ERVs and installing all of that, and calculating cooling and humidity, and etc., which is great, but they probably know very little to nothing about acoustic isolation of recording studios, so you'll have to help them along with that part.

Studios are so unlike houses/offices/shops/schools/etc.. They need special design, that takes into account things that are not even on the radar for HVAC contractors that do houses, shops, offices, etc.

So take care with your design: make sure you get enough air into your room at slow enough speeds through large enough ducts, with hefty enough silencers, and also that you circulate enough air through the mini-split that it keeps the room comfy. If you do all of that, then your studio will be a "cool" place to make music in...

- Stuart -

Click here for more "Studio Building Reference Information".


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[Marius]

Stuart –

You are amazing. For the last few days I have been fretting about all this, and you have made it very clear! And I am receiving it on my birthday! Happy Birthday to me !

I think it is admirable that you are so willing to give out this valuable information.

My control rooms will be 23’ x 20’ x 12.5’ = 5750 cubic feet. And my big room will be 35 x 28 x 12.5 = 12,250 cubic feet. So, since my rooms are large, it might be better to use the “occupancy method” to compute the requirements for my ERV – right?

But, this really boils down to what the ASHRAE requirement are for San Antonio, Texas… I can ask the MEP about that.

Marius

[Soundman2020]

Source of the post You are amazing. For the last few days I have been fretting about all this, and you have made it very clear! And I am receiving it on my birthday! Happy Birthday to me !

I think it is admirable that you are so willing to give out this valuable information.

Happy Birthday, Marius! Many happy returns!

Thanks for the kind words about the information here. It's the type of stuff studio-owners and builders need to know, but nobody wants to tell them! I don't get why it has to be such a "big secret". It's very important.

So, since my rooms are large, it might be better to use the “occupancy method” to compute the requirements for my ERV – right?

Right, but I think you'll find that you won't be too far off the "rule of thumb" that I give here, by the time you take it all into account.

For your facility, I'd suggest that you consider a system with a single centralized AHU and ducts leading to/from each room. It would be less expensive like that, and simpler. By the time you install 7 or 8 individual ductless mini-split systems, one for each room, at a couple of grand each (just to buy: plus installation!), you could easily have paid for a larger centralized system that can deal with the entire building. I'd take a close look at both options, to see which one works out cheaper. Don't consider the cost of the ducts and silencers in that comparison, since that would be pretty much the same in both cases. Just look at the costs for the actual ductless systems, vs. the cost for the full building system. Also, the full building system would condition the air for the corridors, offices, bathrooms, storage rooms, etc. Not just for the studio rooms.

Food for thought!


- Stuart -

(PS. I updated the above article with a lot of additional information, after you saw it, so you might want to go back for a second read.... )

[Marius]

Thank you, Stuart. I am anxious to re-read the article now that you updated it.

One of the appealing things about dedicated mini-splits in the sound rooms was that I could control each individually. I have 3 control rooms and 1 large room… There will be lots of times when the rooms are unoccupied.

And, I have been in so many studios where one room is a lot cooler or warmer than others – but there is only one thermostat!

So, according to my thinking I would need four units – three identical for the three control rooms and 1 larger for the big room. But my LARGE room is 12,250 cubic feet! That would have to be a powerful mini-split. And one mini split would be dumping the cool air only in one spot in the room – right? Or, do they somethings route the cool air to several places with “internal” ducts?

Again, thanks for all the help.

[SoWhat]

Greetings Stuart,

Thanks for the additional info! The ducted mini split makes MUCH more sense for a studio application. One question: does the area around the air handler remain open, or does it filled with insulation similar to the rest of the air gap between the wall assemblies?

Thanks very much indeed.

All the best,

Paul

[Soundman2020]

One of the appealing things about dedicated mini-splits in the sound rooms was that I could control each individually. I have 3 control rooms and 1 large room… There will be lots of times when the rooms are unoccupied.

That's why you have a system controller, when there are multiple zones. There are thermostat-like controls in each room, as well as sensors at least for the temperature, but hopefully also for humidity and CO2 level. The controller takes that into account, and opens or closes electrically operated dampers (more correctly "VAV boxes") in the ducts, to control the air flow volume to each room, as well as bypass dampers (VAV's) that allow air to completely bypass some parts of the system, when not needed.

This is what typical VAV boxes looks like:

HVAC-VAV-SIMPLEST---damper-variable-air-volume-hvac2cSML2.jpg (15.92 KiB) Viewed 58325 times

HVAC-VAV-SIMPLEST---damper-variable-air-volume-hvac2cSML2.jpg (15.92 KiB) Viewed 58325 times

HVAC--VAV--barcol-air-vav_1.jpg (10.03 KiB) Viewed 58326 times

HVAC--VAV--barcol-air-vav_1.jpg (10.03 KiB) Viewed 58326 times

HVAC--VAV-box-2PNT-ENH.png (245.24 KiB) Viewed 58326 times

HVAC--VAV-box-2PNT-ENH.png (245.24 KiB) Viewed 58326 times

The top one is the simplest: just a butterfly-vane inside, where the position is controlled by the motor on the outside. The bottom one is more sophisticated, as it has built-in sensors for air flow and temperature, and also has a "pre-heater" built-in, to warm the air a bit if it is too cold.

So with a VAV system and a controller, the adjustment is mostly automatic: if there are no people in the room, then the temperature in that room won't rise and neither will the CO2, so the controller will sense that and reduce the airflow to that room, without affecting other rooms. As people enter the room, their body heat starts raising the temperature, and their breathing increases the CO2 levels and humidity, so the controller senses that and ramps up the airflow to that room. More people in the room, results in more air flow to keep them happy. Fewer people, lower air flow: more efficient, saves you money. You could also program the controller to allow a higher temperature in unoccupied rooms, so it doesn't waste cooling power on that one. A good system controller can keep all of the rooms quite comfortable, and the thermostat on the wall in each room can be used to dial in additional changes, as needed, independent of all the other rooms.

And, I have been in so many studios where one room is a lot cooler or warmer than others – but there is only one thermostat!

A poorly designed system, indeed! Inefficient, too: wasting money. Those were probably not VAV systems, and had no central controller. That makes it cheap, but not pleasant.

There's basically two types of building HVAC system: constant flow, and variable flow. With constant flow, there is no control, other than "on/off". If the one single temperature sensor says the air coming in to the AHU is too hot, then it turns on the compressor. When the air starts coming in cool enough, then it turns of. That's it. With a variable volume system, the compressor runs whenever any of the rooms need cool air, even just one, but the controller takes care to close down the dampers for the rooms that DON'T need it, and perhaps open the bypass dampers if necessary, as well as telling the AHU fan to run at a low speed, since only one room needs the cool air. As more and more rooms start needing cool air, the system controller tells the AHU fan to speed up, and it opens the necessary dampers to direct the cool air where it is needed. If all rooms need it, then the controller open all the VAVs fully and tells the AHU to go to maximum fan speed. With a top-of-the line system that has VAVS with pre-heaters (or re-heaters), it can even cool one room while warming another, if needed.

The heart of the system is the controller, which sounds complicated but really isn't: it's just a small box with a computer inside, connected to a screen for setting things the way you want, and wires running to each VAV and sensor. Something like this:
SIMPLE SINGLE-ZONE CONTROLLER:

MEDIUM MULTI-ZONE CONTROLLER WITH SENSORS AND THERMOSTATS:

FULL MULTI-ZONE CONTROLLER WITH MORE SOPHISTICATED SENSING AND CONTROL:

The top one is a very simple single-zone controller, the middle is a simple three-zone controller, the bottom one is for a more sophisticated multi-zone system. The best ones also come with this:

HVAC--Cell-phone-central-controller.jpg (25.27 KiB) Viewed 58326 times

HVAC--Cell-phone-central-controller.jpg (25.27 KiB) Viewed 58326 times

So you can see what every zone is doing from your cell phone, and make any adjustments directly from there.

The controller can also adjust the flow through the ERV: there's no point in bringing in boatloads of hot moist air when there's almost nobody in the rooms: so it will tell the ERV to slow down the fan to the lowest speed, and only increase that when it detects that the CO2 levels are rising in the rooms: This saves you a LOT of money, since every cubic foot of hot, moist air that you bring in from outside will still need some cooling, and some dehumidifying: even the best ERV's are not 100% efficient, so you still need to cool that, and dehumidify it. In fact, where you live, you might also need a stand-alone dehumidifier, placed after the ERV, to deal with the extra humidity that made it through.... and your system controller could run that dehumidifier as well, if you add humidity sensors....

Even with a sophisticated controller, VAVs, and sensors, I suspect it would probably still be cheaper to do a single centralized AHU, rather than having many individual systems spread out all over the place. It would certainly be cheaper in operating costs! The controller greatly increases efficiency, since it only supplies what each room needs at any given time, based on occupancy.

So, according to my thinking I would need four units – three identical for the three control rooms and 1 larger for the big room.

And the offices? Reception area? Corridor? Bathrooms? Waiting room? Storage rooms? You live in Texas: it gets hot and humid there. If you don't condition all of those other spaces too, then they will be at conditions close to that of the outside world: ie. hot and humid. And if you ever plan to sell the place and leave, it would be a definite disadvantage to the prospective buyers to have no HVAC in those areas: people tend to want buildings where all of the air is conditioned, so they don't have to worry about investing in new HVAC systems.

But my LARGE room is 12,250 cubic feet! That would have to be a powerful mini-split. And one mini split would be dumping the cool air only in one spot in the room – right? Or, do they somethings route the cool air to several places with “internal” ducts?

For a large room like that with one single mini-split, you would end up with an arctic gale blowing out of one side of the room, and mid-day in the Sahara desert on the other side.... OK, I exaggerate a bit, but close to the AHU it would be cold and breezy.. and noisy! That one single unit has to move the entire volume of all of the air in the whole room, through itself, every ten minutes (because you want six room-changes per hour: one every ten minutes): In fact, for a live room like that, I would recommend 8 changes per hour, rather than six. So one single unit would, indeed, be blowing out a very noisy gale. Not exactly what you want when you are attempting to track instruments with sensitive mics...

One more thing here: mini-split systems usually only have three fan speeds: low, medium, and high. There is no infinitely variable flow control, like you get with a VAV system and a controller. You only get "breeze", "wind" and "hurricane". For a single unit in a large room, my guess is that it would be on "hurricane" most of the time, in order to move enough air.

On the other hand, with a ducted system, you could have several registers spread around the room, providing smooth, even coverage, with low air velocity and high volume, to move enough air through the room but at imperceptible rates, and silently: no noise. So you wont have some people with an icy blast on their necks, while others are sweating and gasping.

Ductless mini-splits are fine for small home studios with one or two rooms, but professional facilities with multiple rooms really would do better with a centralized VAV system and a controller. Apart from anything, it is more efficient, and cheaper to run. It is also usually a "set it and forget it" system: let the controller do the work of figuring out how much air and what temperature is needed in each room... and the controller also won't forget to shut down each room as people leave! So you won't be wasting money as the mini-split in the live room runs at full bore over the long-weekend, because the last guy out forgot to turn it off.... More food for thought...

- Stuart -

[Soundman2020]

Source of the post Greetings Stuart,
The ducted mini split makes MUCH more sense for a studio application. One question: does the area around the air handler remain open, or does it filled with insulation similar to the rest of the air gap between the wall assemblies?l

The AHU needs open space around it, for installation, cleaning, maintenance, repairs, etc. There are access panels on at least one or two sides, and they need to be able to open. So it needs to be accessible, and the manual will specify how much space is needed around it on each side.

But apart from that, you can install the AHU wherever makes the most sense. For large facilities, there is often a "utility room" for the main electrical panel, telephone exchange, internet connection, servers, and suchlike. The AHU can go in there too, up in the ceiling area above the other stuff, out of the way. For home studios/project studios, it can be in the attic space, or maybe hung from the rafters, or just below the ceiling. Outside of the acoustically isolated area, of course.

Here's a couple of photos of typical small AHU installations:

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HVAC-AHU-ducted-mini-split-installed--in-roof-trusses-ENH.jpg (76.04 KiB) Viewed 58309 times

hvac-split-ahu-installed-under-ceiling-01.jpg (45.16 KiB) Viewed 58309 times

hvac-split-ahu-installed-under-ceiling-01.jpg (45.16 KiB) Viewed 58309 times

The ducts and silencers can certainly go in the cavities between the two leaves of the isolation walls and ceiling. I normally try to do everything in the ceiling area, so as not to take up any space in the room itself. Sometimes that means "stepping" part of the inner-leaf ceiling down a bit lower, which can have acoustic treatment consequences for the room, but it's better than having chunky sections of the walls poking out into the room! By carefully thinking things through, and designing the parts of the system well, it is possible to minimize incursion into the room envelope.

- Stuart -

[SoWhat]

Greetings Stuart,

Thanks for the clarification. Naturally, it brings up another question: Since I need to have a middle leaf in my ceiling because of roof venting (I do have an email out to the company about the alternative trusses you mentioned), would the correct place to put the ducted mini split AHU between the middle leaf and the inner leaf or should it go between the outer leaf and middle leaf?

Thanks very much indeed.

All the best,

Paul

[Soundman2020]

Source of the post Thanks for the clarification. Naturally, it brings up another question: Since I need to have a middle leaf in my ceiling because of roof venting (I do have an email out to the company about the alternative trusses you mentioned), would the correct place to put the ducted mini split AHU between the middle leaf and the inner leaf or should it go between the outer leaf and middle leaf?

In that case, the AHU would go between the middle-leaf and the roof... where you can have access to it.

Once you build your isolation shell, you will seal everything air-tight, and there's no further access to anything between the two inner-most leaves. It's just an air cavity with drywall on both sides, completely filled with light-weight (low density) insulation, usually plain old "pink fluffy", or maybe mineral wool batts. So there's no way of getting into that cavity again, other than breaking down some of the drywall . You can have your ducts and silencers in there, since they have no moving parts, so there is no need for maintenance or repairs, but you should not have anything in there that needs "hands on" attention: so the AHU would go where you can get to it without too much hassle: in the actual roof space. Or, if you have another area in the building that will not be isolated, such as a bathroom, storage room, kitchenette, office, green room, or some such, then it would be best to put the AHU in the ceiling area of that room, perhaps with just a drop-ceiling under it, for simple access.

- Stuart -

[SoWhat]

Greetings Stuart,

Thanks very much indeed. Certainly makes sense to me.

All the best,

Paul

[Marius]

Stuart,

This is awesome. Thank you for sharing your knowledge.

Marius

[shybird]

Again, thank you Stuart! This is hands down the most comprehensive yet easy to understand explanation of studio HVAC design I’ve ever come across. Priceless!

In a climate like Nashville, TN which is extremely humid and hot in the summer (always hits mid to upper 90s and sometimes into the 100s on the worst days) but also a very damp and cold winter (down to the 20s for sure and sometimes 10s), what is the best approach? Would an ERV serve both extremes or does it only work in one direction (you mentioned it can heat up the air going out, thus cooling down the air coming in...but can it do the reverse in a bad winter)? I know the ERV is there to provide the air circulation plus take some of temperature control/dehumidifying work load off the main unit. Just curious if a single ERV can serve this purpose at both temperature extremes.

I know you said heating is rarely needed in studios and I’m assuming that’s due to how well they are insulated in a typical MSM wall system. But at what temperature do you start recommending that heating be necessary?

I was just curious about that. Also the VAV system with a controller sounds great! I like the idea of automation. It’s how I like to mix music too!

Cheers
Trevor

[SoWhat]

Greetings Trevor,

But at what temperature do you start recommending that heating be necessary?

First, the wise-ass answer: When you're cold.

Now, the proper answer: Mini splits also provide heat (although some are AC only). ERVs are for all high-humidity situations, HRVs recover heat, but don't handle the moisture. HRVs are great for places like the Intermountain West where it gets REALLY cold (I used to live in Montana) but stay very dry (think loads of powdery snow).

All the best,

Paul

[Soundman2020]

Source of the post This is hands down the most comprehensive yet easy to understand explanation of studio HVAC design I’ve ever come across. Priceless!

You are welcome! That's my purpose with this forum: make things understandable, in simple terms.

Would an ERV serve both extremes or does it only work in one direction (you mentioned it can heat up the air going out, thus cooling down the air coming in...but can it do the reverse in a bad winter)?

They work in both directions. The energy is transferred through the enthalpy wheel, or through the walls between the flow paths, so it works in both directions. There might be a preferred direction that works better, depending on the design of the unit.

With both HRV's and ERV's you get maximum efficiency when there's a large difference in temperature (and humidity, for ERVs). If it is 23°C outside and you want 22°C inside, then there won't be much heat transfer going on. But if it is -10° outside, and 21°C inside, you can recover quite a lot of that heat in the outgoing air. Ditto if it is 40°C outside, and 21°C inside: you can recover quite a lot of that "cold". Also, if the RH is 40% inside and 42% outside, there ain't gonna be much transfer! But if you have 40% inside with 90% outside (or 5% outside), then yes, there will be transfer.

One other point I forget to mention in the actual article: HRV's might need a condensate drain, if you have large humidity and temperature differences between indoors and outdoors, but ERV's do not need that, since they transfer the humidity between the air streams.

I know the ERV is there to provide the air circulation plus take some of temperature control/dehumidifying work load off the main unit. Just curious if a single ERV can serve this purpose at both temperature extremes.

A good one can, yes.

I know you said heating is rarely needed in studios and I’m assuming that’s due to how well they are insulated in a typical MSM wall system. But at what temperature do you start recommending that heating be necessary?

The recommended climate for a studio is around 20°C (68°F) and around 40% RH. So if it starts getting cooler than that, switch it to "heat" mode. That is often the case if you live in a cold climate and have had everything shut down for a while. You might well need to heat up the studio initially, to get it comfortable, then switch over to cooling once all the gear is on and the people are in, producing heat.

As you said, studios are insulated very well, and also sealed hermetically twice over, and also have a lot of "thermal inertia" in the walls, plus there's also usually lots of gear, lights, and people putting out heat, so once the place has been in use for a few hours, things get warm, even in a cool climate.


- Stuart -

[shybird]

Thank you Stuart!! That makes total sense and answers all of my questions.

I’m very interested to see what the price differences will be in the good units vs. the cheaper ones. I’ll do some searching on this but obviously Ive got some time to decide. I’d certainly love to spend more in the beginning if it saves on utilities in the long run...plus the idea of the system being automated is fantastic. One less thing to think about all the time!

Cheers
Trevor