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Air Change calculation?

2011laramie

Well-known member
Joined
Apr 2, 2012
Messages
161
Location
Central Alberta
How does one calculate the btus required for an air exchange? Ive read that a room should have anywhere from .2 to 2 full air changes per hour. So does that mean all air is just exchanged throughout the building or that the air is exchanged with fresh air and old air is exhausted out?

Im doing my heat loss calcs for my garage with a suite. Using the effective R value of a 2x6 wall then adding R5 for outside styrofoam. and doing windows and doors separately and adding it all together. I get a decent number that makes sense. But when I add a .2 air exchange of the volume of a 44x24x16 building i get way higher numbers. My house is basically the exact same size and has a lennox 66000btu furnace and it does just fine in the coldest of Alberta winters.
 
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bazar01

Well-known member
Joined
Jan 30, 2009
Messages
326
Location
Leesburg, GA
How does one calculate the btus required for an air exchange? Ive read that a room should have anywhere from .2 to 2 full air changes per hour. So does that mean all air is just exchanged throughout the building or that the air is exchanged with fresh air and old air is exhausted out?

Im doing my heat loss calcs for my garage with a suite. Using the effective R value of a 2x6 wall then adding R5 for outside styrofoam. and doing windows and doors separately and adding it all together. I get a decent number that makes sense. But when I add a .2 air exchange of the volume of a 44x24x16 building i get way higher numbers. My house is basically the exact same size and has a lennox 66000btu furnace and it does just fine in the coldest of Alberta winters.

Whenever you let in fresh air make up for the air exchange, you will need to heat the air mass from the entering outside air temp to the final furnace supply air temp and that will explain the higher numbers. Q=mcp(t2-t1). To minimize the heat loss, adding a heat recovery system will help. For a garage, cost of one may not make economical sense.
 
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2011laramie

Well-known member
Joined
Apr 2, 2012
Messages
161
Location
Central Alberta
I figured out my biggest problem. I was using the metric value for the specific heat of Air.

so for a 24x44 bay with 16 ft walls. with 2x6 framing on 24" centers, triple pane windows, outside R5 styrofoam and a R16 garage door. I have a heat loss around 35000btu/hr on a -40 outside value to 70° F inside temp.
 
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