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Exhaust-only ventilation in Maine?

Wondering if I can use an exhaust-only ventilation system in a new home constructed of SIP panels that is heated by 1 to 2 wood stoves depending on outside temperature. I have 6" duct work going the two bathrooms and also installed 6" ducts to be connected to an HRV for fresh air.

Recently I have been wondering whether it is cost effective or necessary to install the HRV. It seems as though the exhaust only route would be fine, but I'm wondering if there will be an issue using wood stoves and creating negative pressure with the exhaust fan.

Thanks to all in advance,
Ben

Asked by Ben Geissinger
Posted Sat, 01/18/2014 - 11:37
Edited Sun, 01/19/2014 - 09:26

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3 Answers

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1.
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Ben,
Most homes need only about 60 cfm to 80 cfm of continuous ventilation. Such low air flows shouldn't be enough to cause your wood stove(s) to backdraft.

For more information, see Designing a Good Ventilation System.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 01/19/2014 - 10:52

2.
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Thanks for the reply Martin, I think that i am certainly in that range cfm wise so you are saying that I can probably do just exhaust fans without need for passive or otherwise makeup air. I appreciate the info. I've been looking at panasonic fans that I would fit in the basement since that is where the ducting runs. If I am using this type of set up is there a need for controls in the bathroom to bump up cfm while showering or bathing or should I just set them up to run from a basement control

Answered by Ben Geissinger
Posted Sun, 01/19/2014 - 17:12

3.
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Ben, based on my own experience with a very tight house, I'd suggest that the woodstoves have directly connected outside air kits, to minimize any chance of backdrafting. My woodstove is small and draws just fine after the fire is going and a good draft up the chimney pipe is established. However, I find that if the range hood or clothes dryer is operating then I can't get that draft going when lighting off the stove. I have to shut off those exhausting devices until I get the woodstove going and have the door shut so that it draws from the outside air duct. Then everything runs well together.

If you have exhaust-only ventilation in your house, the depressurization will draw makeup leakage air through every possible opening, and a chimney is a rather large one. While the stack effect up the chimney will be large with hot flue gas in it, and likely large enough to overcome the depressurization of just the minimal 60-80 cfm ventilation air requirement, I suspect that turning on another exhaust blower or boosting the bath fan draw during/after a shower could overcome that draft. Others have suggested elsewhere that any combustion device in a very tight house ought to have its own directly connected air supply to provide a sealed system.

Answered by Dick Russell
Posted Mon, 01/20/2014 - 15:58

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