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Depressurization

Well, I recently built a new home, (its of sip construction) and it would appear that my attempts to ensure it airtightness have been quite successful. I haven't actually tested or measured its airtightness so my opinions are solely based upon observation. I didn't install a HRV although technically the code called for it because my experience with operating HRV's has been mostly negative. On one hand the building code penalizes the owner for having too many windows by requiring increased insulation standards (dependant on window/wall area ratio's) and then asks the owner to ensure "X" amount of air changes per hour through an HRV with a rather pathetic efficiency rating in a climate where -40 F occurs all too frequently in winter. That being said however, I failed in designing my house to consider negative pressures and their effect on my airtight woodstove. The easiest (cheapest?) solution which I'm currently undertaking is to simply leave a window open slightly and for the most part it works fine. With a rather small window opening (approximately 14-28 square inches) I have no problems ensuring good draft for the wood stove and still allowing enough air flow to fulfill the needs of either 80 CFM bathroom fan (2 bathrooms 2 fans 2 30 min timers). However, two fans or one fan and the dryer, or the kitchen exhaust fan can, depending on atmospheric conditions, cause the woodstove chimney to operate in the wrong direction>> much to the dismay of my wife!! Obviously the easiest solution to the negative pressure problem is to simply open the window more when increasing exhaust volumes and this is actually what we have been doing>> don't get me wrong it is effective but to my mind there must be a more "hands free" solution. I just haven't really discovered one yet as it would seem many air tight homes don't rely on wood heat or at least wood heat utilizing interior draft.

The open window seems to have some benefits and some problems. The flow of cold air across the floor towards the wood stove is bothersome. Having to change window opening dependent on exhausting requirements is tiresome. It would appear that the "stack effect" causes increased air leakage from the upstairs bathroom fan vent with the open window being the lowest point in the house and the fan vent at the highest (2.5 storey) -this is based on frozen condensation amounts at exterior exhaust vents . I can't really say the wind is having any real effect otherwise I think I would notice the change in air speed through the window opening during gusting. We have no humidity problems in fact we prefer the lower humidity levels inside in the winter. Air exchange levels seem ok as odours dissipate fairly quickly.

What I want is a way of varying the amount of "makeup" air to supply exhausting requirements automatically. Hopefully without adding to heat losses caused by "stack effect" pressures at the upstairs bathroom vent. I would also like to try and preheat the intake air maybe by introducing it to the area behind the wood stove or via a flue powered heat exchanger. This can't be a new problem and surely the interest in wood heat can only increase as energy prices soar - look at the price of propane this winter. Unless wood stoves all begin to take combustion air from outside adding them to modern airtight homes should create the same problems I'm experiencing.

I like the idea of mechanically controlled vents which increase intake area based on the activation of a particular fan but wouldn't a wireless controller for each exhausting device be better than a on/off switch retrofit (which eliminates timer control). Also, vent size amounts could be programmed based on which channel was active or how many (e.g. channel A is the kitchen exhaust fan and that requires 60% open whereas channel B is the bathroom fan and it requires 20% etc..) Does someone make such a thing? Does the HRV solution solve this problem? How can it be balanced if I'm constantly altering the amount of air I'm exhausting? So far I haven't found a solution specific to my particular problem >> or at least in line with the solution I desire >> variation in the size of intake air vents as determined by on/off state of exhausting devices. Perhaps some one "out there" has found a reasonable solution? Care to share? Thanks.

Asked by Glenn Thornborrow
Posted Fri, 02/28/2014 - 16:47
Edited Fri, 02/28/2014 - 17:03

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Helpful? -1

Glenn,
There are lots of articles on GBA on this issue, as well as several Q&A threads. Here are some links to get you started:

Why Is This Wood Stove Misbehaving?

How to Provide Makeup Air for a Wood Stove

Providing Outdoor Combustion Air for a Wood Stove

Wood stove backdrafts

All About Wood Stoves

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/28/2014 - 17:07

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