I suppose, then, that the PAV's were pulling the stack gasses out through the draft hoods, and since the house was 24 Pa negative to atmosphere, there was little risk of stack gasses entering the house...correct?
Originally Posted by mark beiser
Even so, what in your mind would create a backdraft severe enough for rollout to occur with PAV's in operation? A sudden gust of wind with the stack on the windward side of the roof?
I had a separate thought today regarding soffit venting/ridge venting or whirlybird venting near the peak of the roof. I have stood under soffit vents on hot days and felt hot air spilling out of them. I can't recall at the moment how windy the day was when I observed this, but probably not all that high. The house in my memory where I last observed this occurring has a hip roof with soffit vents all around, and whirlybirds near the peak.
I see two causes, and your experience can either confirm or nullify my thinking...
Food for thought, yes?
- With little to no wind, the heated air in the attic expands to the point where it drops out the soffit vents. To do this it would need to draw from the whirlybirds or ridge venting, or from the house interior, unless it merely dumps at any available opening due to sheer expansion. Regarding the house interior, being that it is cooler and the air is more dense (hence a slightly lower pressure), the "reverse stack effect" through ceiling penetrations would conceivably allow attic air to enter the house. The point of make-up air for the attic air as it heats, expands, and dumps out the soffit vents would need to come from somewhere.
- With adequate wind pushing passing over the roof, air could conceivably flow down ridge venting or whirlybird openings and then pressurize the attic, hence causing heated air to dump out the soffit vents. But this same air is also pushing against windward facing walls, whereby air would be encouraged to flow into soffit vents due to pressure difference between windward wall and attic conditions. Hence, it may cause hot air to dump out of soffit vents on the leeward side of the house, and any roof venting on the leeward side of the roof.