Ventilation/Recommended Air Exchanges Per Hour

[antstudio]

Hi Folks,

I've searched the forum and googled for terms like CFM, ventilation, exchanges, and I haven't found an answer. Usually I hate to ask such basic questions.

What is the recommended number of air exchanges per hour? And is my calculation correct below?

I have a small sound "isolated" mixing/recording room-in-a-room. Fully sealed with a duct inlet and outlet on opposite sides for air circulation. I'm planning my air exchanger, an external fan installed in an insulated enclosure (plenum), connected via a vent duct, to vent old air out, thus pulling new air in from the enclosing room (garage).

Room is 8'W x 12'L x 7'H. I calculate 8x12x7 = 672 cubic feet. So I figure a fan rated at 72 CFM would require 672 / 72 = 9.33 minutes for 1 full exchange, or 6.4 exchanges per hour. Is that the right calculation?

Is 6 exchanges per hour enough to keep the room comfortable? Do I need to overspec the fan to compensate for performance loss over the duct system?

Mostly it will be just me in the room, although it would be nice to record a guest performer once in a while.

I was hoping to use a small, quiet computer case type fan (or maybe a pair of fans), but I'm not sure this is realistic. Really want to keep the noise down at all costs, since the room is close to silent right now (e.g., the computer lives outside the room).

..ant

[Rod Gervais]

Originally Posted by antstudio

What is the recommended number of air exchanges per hour? And is my calculation correct below?

I have stated before and will state again...... calculations based on air exchanges per hour just do not make sense.....

Tke a room 10 x 14 with an 8' ceiling - based on your calculations that room would have 6 exchanges oer hour for a total of X cubic feeet of air per hour (the actual number is not relevant)

Now - if we decide to make the ceiling 18' high - we suddenly need more than twice as much air?

Why?

We won't fit one more person in the room than we could before - and they are the ony thing we need to worry about dying if they don't get enough air...........

Fresh air supply should be provided based on the maximum number of people who will be in the room........ and then that number should be multiplied by a factor you feel is reasonable for the purpose.

As an example of this - the Internation Mechanical Code sets a minimum cfm per person in a music room of 15 cfm.

They have different rates for dfferent occupancies - but this is what they set for music rooms.

I have a small sound "isolated" mixing/recording room-in-a-room. Fully sealed with a duct inlet and outlet on opposite sides for air circulation. I'm planning my air exchanger, an external fan installed in an insulated enclosure (plenum), connected via a vent duct, to vent old air out, thus pulling new air in from the enclosing room (garage).

I do not know where you are from - but in the US this is a HUGE code violation.....

Fresh air is just that - fresh air - and it is required to be pulled from OUTSIDE the building - it is not allowed to be pulled from anywhere inside -

Mostly it will be just me in the room, although it would be nice to record a guest performer once in a while.

So around 30 cfm is what you really need......

I was hoping to use a small, quiet computer case type fan (or maybe a pair of fans), but I'm not sure this is realistic. Really want to keep the noise down at all costs, since the room is close to silent right now (e.g., the computer lives outside the room).

Computer fans will not CANNOT get the job done......

Although they might move air in the volume you are looking for - they do not have any capacity to overcome static pressue - picture that computer cases do not block air flow at all - what do you have - 15" or so from the front intake to the rear exhaust - usually only a few inches one way or another above one another.

Nope = you are going to have to decide what you want to supply for volume,,, then determine how much backflow pressure exists with your ductwork the way it is - then size a fan according to those criteria - and look for the best rating in sones that you can find.

Rod

[Rod Gervais]

Originally Posted by moracspace

Critically controlled enviroments need air changes only when the CO levels exceed 800 ppm.BOCA code requirement

Although the above it a true statement (with the exception of the critically controlled part - you can actually do this everywhere) - it takes a fairly expensive system to place into operation - and in the end- when the room is fully occupied - it ends up providing equilibrium to the room (which is actually about 1,100 ppm for each 15cfm............... (Equilibrium is the difference between the indoor air and the outdoor air - which is (in typical rural areas) around 300 ppm for normal air conditions.

Now, I did not bother to get into this becaue of the fact that the system is not something that one would generaly install in their home studio.

In fact - I would not install it in a pro studio - only because - I would also require seperate co sensors operating an alarm system just in case there was failure of either the HVAC CO sensors or (as is more common) a problem with the BMS (Building Management System) .......... it would be a bitch to have CO alarms going off because of a fresh air failure in the middle of a perfect take...........

This is not as simple as just installing a CO sensor in line with your fan,,,,,,, it takes a BMS (a computer operated system) to gather the data, analyze it in real time and make the necessary adjustments to turn on fans - operate motorized dampers, bringing everything to full as it again begins to reanayze the data to determine at which point it can turn everything either off or throttle them down.......

These systems are anything but cheap - and are actually pretty hard to justify in anything other than failry large buildings.

For example - I have been involved with construction of hotels for the last 20 plus years - and have not been able to justify installing them in any building less than roughly 300 guest rooms - and even THEN not for the guest rooms themselves - only for the public spaces........

In fact - the last hotel was 21 stories - and we could not justify (even in that large a building with over 400 guest rooms) using the system for the guest rooms themselves - and thus have continuous fresh air and exhaust provided to the rooms 24/7 whether they are occupied of not.

Just way too much cost - hassle and long term maintenence to make it worth the while.

So although I thank the poster for the information - I do not feel it's necessarily going to mean much in here......... and without it we are back to where we started.....

Rod

[antstudio]

Thank you! Basing the ventilation on headcount rather than room volume makes perfect sense. I will plan for 30+ CFM, maybe even 45+, plus some overhead to adjust for loss. Good advice on requiring a fan that can actually pull air against backflow pressure. Also, it will be easy for me to set it up to vent directly from outside, rather than the surrounding room, thanks for the tip.

Now I just need to go find a quiet but powerful enough fan.

..ant

[tonymite]

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