Is there a way to calculate depressurization values from blower door test results?
OK, this question is for you building scientists and certified blower door testers . . .
Given a theoretical house size, volume, and airtightness result, is there a way to convert a flow rating into a pressure rating?
Example: Two story house, 30′ x 30′ footprint, 20′ wall height (this would include sealed crawlspace and floor joist area)
* Surface Area = 3,300 sq. ft. (Walls and Ceiling)
* Volume = 18,000 cu. ft.
* Blower Door Test Result = 0.77ACH50
What is the effective depressurization if the bathroom and kitchen exhaust fans are pumping out 150 CFM?
I know this is a very simplistic assumption. I know that it depends on the inside / outside temperature difference, outside wind velocity / direction, stack effect, etc. All of you realists, please humor me and assume that all those other variables go away for a moment! 😉
Can you estimate the depressurization at 150 CFM exhaust? Is it 3Pa? 5Pa? 10Pa? I understand the rules of thumb regarding the conversion of ACH50 to ACHnat, but it seems like this question is more definitive.
This question relates to natural draft appliances (read woodstoves) and airsealed houses. It’s not a standard, but the Canadians have used 5Pa as a house depressurization limit for buildings incorporating a naturally drafted woodstove. I’m trying to determine if there is way to calculate depressurization based on known active exhaust fans (given the specs of a certain house), to see the effect on a woodstove.
I recently read several articles discussing the question of outside air supply for woodstoves. It seems their is no consensus. The Canadians have been studying this very question for 20+ years – scientists in the US have too. Codes have swayed back and forth. “Expert” opinions vary. The fact is we don’t have much data . . . and it’s all very confusing.
There’s no question in my mind as to whether or not direct vent appliances (gas water heaters, furnaces) are necessary for airtight houses. But I have yet to see a woodstove that couldn’t spill or backdraft – whether it had an outside air supply or not. The saving grace? We ignore our water heaters and furnaces for months at a time, but we actively tend (or massage, or watch, or fuss) our woodstoves. That and our sensitivity to woodsmoke . . .
Builders of “tighter” houses need to be more careful. That’s what I’m trying to do here – get a better understanding of this whole game – at the planning stage.
I know some will question the need of a woodstove in a well sealed, well insulated house. That’s OK. I’m not against hearing those ideas too 🙂
Thanks for your input!
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