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We are building three sound insulated classrooms which will be fairly well sealed. There will be no open windows so we're to use mechanical ventilation. I live in a warm country so heating isn't a consideration.

My understanding was that we would have air extracted through ceiling vents by a motor outside the building. Air would be supplied from vents on the ceiling on the opposite side of each room drawing through ducts from an indoor patio using suction created by the same motor.

We contacted two hvac companies who both tell us we should have a second motor to pump air into the rooms which is pretty expensive.

Is this really necessary or do we just need a slightly more powerful single motor?

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I'm not an HVAC expert, but by pulling air out you're creating negative pressure in the room. This means you'll be sucking air in through every other opening, including under the doors, from the cavity space in the walls via electrical outlets and switches, etc. This to my knowledge is usually only used to contain air in the room - eg, in a chemistry lab. The other downside is anything that would be a minor draft is now a major source of incoming air, which can make it harder to heat because now you've actually creating a bunch of cold drafts from outside air. –  gregmac May 29 '12 at 16:01
    
Are you also heating/cooling this room (how?), or is this just ventilation? –  gregmac May 29 '12 at 16:02
    
Thanks for reply. Sorry, should have mentioned that heating isn't a consideration as I live in a warm country (Gran Canaria). –  mark May 29 '12 at 16:42

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up vote 4 down vote accepted

Is it completely necessary? No I don't think so. As a comment pointed out, you are creating negative pressure. This is good for sealing environments (ie: keep vapors from existing the room in a lab), or even help prevent fire from spreading via halls.

However, you will notice some negative effects to. For one, since the air will be pulled in through any cracks under doors and windows, if a door is not latched, it will likely be forcefully opened. Depending on the pressure, this could potentially hurt a young child who is not expecting the door to be pulled open by the pressure when they turn the handle. Having a second fan push air into the room aleviates this by balancing the pressure. It also reduces the strain on the extraction fan which may help it last longer.

Another benefit is that if one fan fails, you still have a second either pushing cool air in or sucking warm air out; it will function with reduced capacity, but it will certainly be better than no fan at all!

My guess is that you will need two smaller fans instead of one large fan, which might also simplify electrical requirements, installation and maintenance.

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Thanks for your advice which makes a lot of sense. –  mark May 30 '12 at 20:18
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Negative pressure can create a condition where moisture is drawn into the space through any void or weakness in the building envelope, which can create mold problems. For this reason, one company I worked for liked to say "our buildings blow, they don't suck," which I took to mean that when you open a door to enter the building, you feel the air from inside the building blowing on you, which demonstrates positive pressure, and less chance of moisture/mold issues. Obviously lab spaces (especially bio labs) require lots of exhaust ventilation, and so these buildings tend to "suck". –  mac Nov 29 '12 at 21:48
    
Thanks for your comment which I just read as I am unfortunately revisiting this subject. In the end we did install two fans, however ill-advisedly we put the air supply fan above one classroom. We were told with sound insulation we wouldn't hear it much but this is not the case. When you refer to air being drawn through any void causing mold problems, would this matter if the air were being drawn only from inside the building. Also, we live in a warm climate with high humidity in the summer and very low rainfall. Would this be a factor to consider? I'm considering this week turning off the –  mark Feb 17 '13 at 9:05
    
second fan and replacing the first with one more powerful to do the firsts work as well as its own. –  mark Feb 17 '13 at 9:09

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