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Right! ... sort of.Marius wrote:Stuart –
About the HVAC Ducts and Silencers…
If Paul (and myself) are doing Mini-Splits – we do not need Ducts and Silencers for that – correct?
Let me clarify a bit.
The Mini-split does four things, basically: 1) It circulates air around the room, 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 of 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 next time it you boil it, 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 also heats up the 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 it does NOT provide fresh air, and it does NOT removes stale air: it just sucks in air in through 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 to move 2,000 x 6 = 12,000 cubic feet per hour. But HVAC is usually rated in "CFM" for "Cubic Feet Per Minute", so you divide that number by 60 (because 60 minutes in one hour), and you get the answer: 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, lets' just look at volume and flow rate: cubic feet per minute.
So 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.
Thus, in addition to your Mini-spilt, you need to bring in 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 right into the room directly, just 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.
In a more extreme climate, that 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, when it goes through the Mini-split, it makes sense to do as little heating and/or cooling as possible. Let's assume that you live in a very hot climate for the rest of this example... somewhere in Texas, maybe...
So the incoming air is hot, and you want to have cool air inside the room. You could just send all that hot air directly into your Mini-split to cool it: put the fresh air supply vent directly above the Mini-spit air intake, so the Mini-split sucks in the hot air, cools it, then sends it into the room. The mini-split would also be sucking in some room air and some hot air, mixing them, then cooling, etc. That would certainly work, dumping the hot outside air direly 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 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.
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 air cools down the incoming hot air. Or more correctly, the incoming hot air heats up the outgoing cold air, and thus gets cooler itself. It 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. 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 34718 times
- HVAC-HRV-Heat-Recovery-Unit-fantech.net.jpg (115.52 KiB) Viewed 34718 times
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 run.
That's where the ERV comes in: it transfers a lot of the humidity from one air stream to the other air stream. In a humid climate, the incoming humid air condenses on cold plate inside the unit, and that plate is then rotated into the out-going air stream, which picks up the moisture from the plate (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.
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 we. But they still save you a lot of money in operating costs, because they do manage to move quite a bit of heat, and humidity.
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.
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.
Right! And Right!But we do need Ducts and Silencers for the fresh air (ERV) unit – right?
And… do we need two for the ERV - one for the duct of the fresh air coming in and one for the stale air going out – right?
Here's a rough diagram of how that works in general, but not totally accurate for the case of a mini-split:
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.
Yes, but not by a huge amount.... This often causes confusion, so let me elaborate a bit more.And it seems to me that the ERV would not be moving as much air as a typical conventional AC system – so perhaps those ducts could be smaller
The reason you bring in any fresh air at all, is so you can stay alive!
You need to breath 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-monoxde, and a bunch of other gasses, as well as moisture. So each time you breath, 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 not enough oxygen. After a while more, that gets worse and your health starts suffering: headaches, vision problems, coordination problems, lack of concentration, eventually you pass out, and in very extreme cases, you die. 
There is a minimum amount of O2 you need to have put back in 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 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 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", which describes how people suffer a higher level of health issues in poorly ventilated buildings, and 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. So, for a typical studio LR that might have ten people in it, that would be 10 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 get in it, and since we already know how to figure the circulation rate that the room needs for cooling/heating/dehumidify (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 rate as your supply of fresh air to the room. This, 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.
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 form the HRV/ERV moves though 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 design specifically for studios, and for the specific room.
Well, to be honest, ANY hole in a studio wall is "huge!so perhaps those ducts could be smaller… we wouldn’t need to: “chop huge gigantic holes in your carefully built and sealed isolation shell.”
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 34718 times
- loss-through-tiny-cracks-and-reduction-effect-of-small-gaps-on-TL.jpg (28.71 KiB) Viewed 34718 times
Yup. that really is 0.01%. 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.
From a different perspective: a tiny crack under the wall, just 1/32" high, and three feet long, 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 manta, 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.
Then there's the issue of flow velocity (speed) again. That's actually what determines how big the ducts need to be. Lets' do the same hypothetical room above: We need 200 CF circulating inside through the Mini-split, and 40% of that for fresh air. So 80 CFM is what we need to bring in. But the 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. So the HVAC register in the wall/ceiling we need a duct that has 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 form 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).
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 pretty much always do not have experience with the very special needs of recording studios, and 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....
Studios are so unlike all of those. They need special design, that takes into account things that are not even on the radar for houses, shops, offices, etc.
Sorry for the long answer! But it is rather important In fact, I think I'm going to take this reply, modify it a bit, and put it up as an article, to help folks understand this whole, very confusing, HVAC thing.
- Stuart -
