Ventilation BrAiN sPasM

[chrispire]

Despite many hours of reading on this forum and trawling the internet for information (engineeringtoolbox.com for example), I still have some serious gaps in my understanding of ventilation requirements for studios. I'm grateful to have acquired a good understanding here about subjects such as duct sizes, air volume requirements, noise-in-duct calculations, recommended air flow rates, and silencer box construction, but some basic concepts still give me a brain spasm.

I understand that a healthy room requires at least 6 air changes per hour. This means that a control room of 100 cubic metres requires 600+ cubic metres of air per hour, or 167 litres per second.

Q1:
WHY so much air? That seems like so much for a room with just 1 or 2 occupants.

I've also read that fresh air requirements are typically 10 litres/sec/person.

Q2:
Does (A) all of this air (167 litres per second) have to be fresh air, or (B) is most of this air simply being recirculated, with an additional fresh air component of 10 litres/sec/person being added?

If (A) then to my uneducated mind that seems like overkill. It's so much more air that a person actually breathes. It also makes me realize why ERV would be so essential - without ERV/HRV on a cold day you'd be replacing your studio air every 10 minutes with cool outside air.

If (B) then what does re-circulating the air actually accomplish?

I feel like there's something really basic here that I'm totally missing.

Q3:
I see that some people use a reverse cycle system for basic heating and cooling, and then a separate system for ventilation and energy recovery. Is this an advisable way to go, or are there ERV/HRV systems available that do it all - cooling, heating and ventilation too?


Cheers everyone,
Chris

[Soundman2020]

I understand that a healthy room requires at least 6 air changes per hour. This means that a control room of 100 cubic metres requires 600+ cubic metres of air per hour, or 167 litres per second.

Q1:
WHY so much air? That seems like so much for a room with just 1 or 2 occupants.

These are recommendations put out by ASHRAE and health authorities, regarding how much air flow a room needs to stay comfortable and healthy. If the air flow is too low, the room starts to feel "stuffy" and unpleasant after a while. In reality, you can probably get by with quite a but less than that, but I still prefer to go with what the experts recommend.

Q2:
Does (A) all of this air (167 litres per second) have to be fresh air, or (B) is most of this air simply being recirculated, with an additional fresh air component of 10 litres/sec/person being added?

You need enough "make up" air to replace the amount of oxygen used by the occupants, and more importantly, to replace the amount of excess CO2 removed in the exhaust air. About 20% of each inhaled breath is oxygen. About 15% of the exhaled breath is oxygen. But it's not just the need to have high enough oxygen concentration in the air. Far more important is to remove the CO2 build up. You can actually die in air that has nearly the right percentage of oxygen, if the CO2 concentration is high enough. The average inhaled breath is about 0.04%,CO2, the average exhaled breath is about one hundred times higher, at 4% CO2. Once the CO2 level in the room rises from that nominal 0.04% level to about 0.09%, things start getting "stuffy". At 0.1%, light drowsiness starts, and by 0.2%, heavy drowsiness sets in. At 0.5%, you start suffering health issues, and at 1% you are getting into serious stuff, such as headaches, feeling of intoxication, vision problems, hearing problems, nausea, confusion, gasping for breath, pounding heart, muscle tremors, hyperventilation.... At 3% you can just barely manage to stay conscious by panting extremely deeply just as hard and as fast as you possibly can, because that's the physical limit of how well your lungs are able to force out the excess CO2 into an atmosphere where the partial pressure of CO2 is at practically the same level as the partial pressure of CO2 in the lungs. If you suck in your entire ling capacity really fast then expel it really fast, as hard as you can, then you can remain conscious at 3% CO2 concentration. At that level your entire body is focused totally on just one thing: Breathing. At 5% you are unconscious, and at 10% you are dead. Add to that the fact that CO2 is heavier than O2, so with insufficient air flow in a room, once you pass out and fall from your comfy mix position chair onto the floor, that's where the highest concentration of CO2 is, so you die even faster....

If (A) then to my uneducated mind that seems like overkill. It's so much more air that a person actually breathes.

The range is more like 7 to 70 l/m, depending on a number of factors, but mostly body weight and activity level. Musicians jamming hard need a lot more air than the producer sleeping on the couch at the back of the control room!

It also makes me realize why ERV would be so essential - without ERV/HRV on a cold day you'd be replacing your studio air every 10 minutes with cool outside air.

Right, but probably an HRV is more useful than an ERV for most situations. An ERV is only useful if there are large differences in humidity.

If (B) then what does re-circulating the air actually accomplish?

It keeps the air moving! It keeps the fresh air flowing around the entire room, to the places it is needed, such that the fresh air is well mixed with the room air. It keeps things feeling happy and comfortable, and pleasant. If you really wanted to, you could just have a mask hanging from the ceiling that provides the right amount of fresh air for just you, but that doesn't seem like a studio that would be very usable...

I see that some people use a reverse cycle system for basic heating and cooling, and then a separate system for ventilation and energy recovery.

The systems are not separate: they work together. Even with a mini-split on the back wall and vents in the ceilings but in physical connection, they still work together. If there is an HRV in the re-circulation path, then that, too, is part of the system. All the parts work together. Thinking of them as separate is like saying that the engine in your car is not a complete unit: the spark plugs do one thing, but they are not connected to the fuel injectors, which do something very different, and the air filter does something much more "separate", while radiator is wildly "separate" from all of the other parts, and so is the oil pump. Yes, they are all separate things, and they are not even connected together in any way, but they all function together as an "engine". The same with your HVAC "system". There are lots of parts, but they all work together to keep you alive and comfortable.

Is this an advisable way to go, or are there ERV/HRV systems available that do it all - cooling, heating and ventilation too?

No. An HRV cannot add heat to air that is tool cold, not can it remove heat from air that is too hot. It cannot keep the humidity constant, even if it is an ERV, and even if the humidity levels are different enough between inside and outside. All that the HRC does is to recover SOME of the thermal energy that would have been wasted otherwise. Either it recovers some of the heat from the outgoing air and puts it back into the incoming cold air, or it recovers some of the heat from the incoming hot air and puts it into the outgoing cool air (depending on your climate). But it is not 100% efficient, and it cannot add more heat than is in the air already. An HRV is a supplement to the HVAC system: it saves you money, that's all. The system would work just fine without an HRV, but adding one allows you to use a smaller system and to save money on energy costs.

- Stuart -

[chrispire]

Thanks Stuart. Illuminating as always. Those stats on CO2 really puts things into perspective.

So just to confirm using an example, if you were assessing the ventilation requirements for the above mentioned room (100 cubic metres) you would plan to recirculate the air in the room at least 6 times per hour (167 litres per second) while adding somewhere between 7 - 70 litres per minute per person (between 0.12 and 1.2 litres per second per person) of fresh air from outside?

If so, then how much air from within the room would you evacuate completely and vent to the outside (per minute)? Would it be an amount equivalent to the fresh air component being added?

I'm just trying to get a understanding of (a) how much air is recirculated, (b)how much fresh air is added, and (c)how much is evacuated.

Originally I assumed that a HVAC system that feeds 167 litres per second into a room would be supplying 167 litres of fresh air and then, by necessity, completely evacuating the same amount to the outside, but I think that was my big mistake (and greatest source of brain spasm).

[Soundman2020]

So just to confirm using an example, if you were assessing the ventilation requirements for the above mentioned room (100 cubic metres) you would plan to recirculate the air in the room at least 6 times per hour (167 litres per second) while adding somewhere between 7 - 70 litres per minute per person (between 0.12 and 1.2 litres per second per person) of fresh air from outside?

Right.

If so, then how much air from within the room would you evacuate completely and vent to the outside (per minute)? Would it be an amount equivalent to the fresh air component being added?

Correct. You need to get rid of the same amount of stale air as the fresh air you are bringing in.

- Stuart -