Negative Stack Effect?

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velvetfoot

velvetfoot

Minister of Fire
Dec 5, 2005
10,202
Sand Lake, NY
It was a little smokey upstairs last night.
I noticed a couple windows were locked but still closed.
I also noticed that when I opened the second floor window, a breeze came in (not windy outside).
Nothing's really changed on the house since last year-nothing producing vacuum was on.
I seem to remember that when the top floor window was opened a little last year the air would blow out.
Temps were a little above freezing.

Is it just nature of the shoulder season?
 
Sometimes your house becomes a better chimney than your woodstove's chimney. I experienced that first hand when I tried to light the stove with the attic access hatch open one fall day. (two story house) Smoke flowed out of the air inlet of the stove. Closed the hatch and all was well.
 
3fordasho said:
Sometimes your house becomes a better chimney than your woodstove's chimney. I experienced that first hand when I tried to light the stove with the attic access hatch open one fall day. (two story house) Smoke flowed out of the air inlet of the stove. Closed the hatch and all was well.
Click to expand...


Yup. I had to go through my house with a caulk gun and filled every hole under every light fixture. It wasn't fun, but it worked. I also stapled weatherstripping around the attic hatch and plastic to the back of the door.

Matt
 
velvetfoot said:
Nothing's really changed on the house since last year-nothing producing vacuum was on.
Click to expand...
Are you sure? As mentioned, something higher up like an attic hatch could give you that symptom. An open fireplace can draft well enough to negate stack effect. Bad masonry on the chimney can draw smoke in. If the smoke entered through other voids, then there is obviously a problem with the building envelope. When it isn't cold enough for stack effect to be at its worst, the chimney or other appliances could create a negative pressure situation. Under certain barometric conditions, the smoke may fall rather than rise after leaving the flue.

If you have a HRV, a clogged intake could unbalance it enough to produce that symptom. Had that happen to me.
 
"I also noticed that when I opened the second floor window, a breeze came in (not windy outside)."


That's the way it should be.

My draft really struggles this time of year; short chimney, not cold enough temperatures, weird barometric stuff,
and still too many leaves on the trees around the house.
 

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As mentioned, something higher up like an attic hatch could give you that symptom.
Click to expand...

I did replace the 2nd floor bathroom fans and put in outlets with exterior dampers-not sure how effective.

Thing is, on the first floor, air came in through the window.
On the second floor, air came in through the window.
There's not much of an elevation change from the ceiling vent out through the soffit.

Your theory is that the air exits the building through the vents or other ceiling openings and that would be enough to draw the air out of the top floor windows? I have to look more closely at those vents.
 
Again, I'll go back to my comment that if smoke is coming in upstairs, there is a leak in the building envelope or chimney. Finding the source of entry is a good place to start. If it's infiltration through the envelope, sealing it and then providing for make-up air downstairs would be good. If air (and smoke) can leak in, warm air can also leak out, carrying with it moisture that could rot the structure.

Many chimneys are not exactly air tight. Heavy smoke that doesn't want to rise or a plugged cap could also cause smoke to leak out of a chimney. A chimney fire could also produce smoke in the house.
 
I have the same issue during the shoulder season. I was going to buy another 2ft or 3ft piece of chimney to help with the draft...
 
When I replaced the smoke detectors upstairs last weekend, or opened a window upstairs this morning, and it's 10*F or so, I still notice air coming in, not going out. This morning was even before the insert was running. No smoke problem, but a little while ago when it was warmer I think the wind shifted and I got a little wood smoke in the basement.
 
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