Downdraft/negative pressure issue

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rhanauer

Member
Apr 18, 2019
16
Oxford, MA
I have a Harman insert that was professionally installed 2 yrs ago (replacing an old Whitfield insert). The old SS flue liner was inspected and found in good shape to reuse. The stove runs great. Now it's summer and I'm growing plants under lights in the unfinished basement. I needed ventilation for them, so I connected a duct fan to a second unused ceramic-lined flue to push air up the chimney (there was an old 6" connector in the basement where a previous owner once had a wood stove). This seemed like a convenient solution since the casement windows don't open and there's no other easy way to exhaust the basement air. At first everything seemed great, and the basement air smelled much better! But the next day I started to notice a funky smell upstairs. I realized I probably created negative pressure in the house which is pulling the basement exhaust down the adjacent flue up at the chimney down to the fireplace with the pellet insert. Is there an easy fix for this situation? TIA for any ideas!
 

rhanauer

Member
Apr 18, 2019
16
Oxford, MA
Open an upstairs window
Yes that would likely help if not solve it, but I’m trying to avoid that since I have asthma and need the A/C on. House was built in 1984 and is pretty tight. One approach I read is to raise one flue 12-18” higher, but there’s a stone cap preventing this. Another is to seal the dormant flue, but I think that may be difficult with the Harman unit installed. So it seems wherever I vent the basement air intake air needs to be provided to prevent chimney downdraft.
 

SidecarFlip

Minister of Fire
Close the vent plate on the fireplace.
 
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Jan 29, 2021
145
VA, east central
I have a Harman insert that was professionally installed 2 yrs ago (replacing an old Whitfield insert). The old SS flue liner was inspected and found in good shape to reuse. The stove runs great. Now it's summer and I'm growing plants under lights in the unfinished basement. I needed ventilation for them, so I connected a duct fan to a second unused ceramic-lined flue to push air up the chimney (there was an old 6" connector in the basement where a previous owner once had a wood stove). This seemed like a convenient solution since the casement windows don't open and there's no other easy way to exhaust the basement air. At first everything seemed great, and the basement air smelled much better! But the next day I started to notice a funky smell upstairs. I realized I probably created negative pressure in the house which is pulling the basement exhaust down the adjacent flue up at the chimney down to the fireplace with the pellet insert. Is there an easy fix for this situation? TIA for any ideas!
Get a greenhouse. Or do something with the casement windows so you can open one or more and keep the unconditioned air down in the basement. But if you're bringing in hot/warm outside air into your basement, you could end up with a musty basement and mold growing if you have cool spots for moisture to condense on. Unless you live an a region where it's low humidity.
 
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rhanauer

Member
Apr 18, 2019
16
Oxford, MA
Close the vent plate on the fireplace.
I'm not sure I can do that with the Harman insert installed as there's no damper control knob/rod above the fireplace. Edit: I just remembered having seen the damper in the garage, so it must have been removed when the original insert was installed (apparently a damper isn’t needed with an insert).
 
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Don2222

Minister of Fire
Feb 1, 2010
8,810
Salem NH
Hello
I always run a separate 3” diameter aluminum liner to the air input connection on the Harman frame. The chimney top plates around the liner seal the chimney from any rain or down drafts. Therefore the only air coming down the chimney goes right in the the Harman Air Adapter and right into the burn pot.
The larger flue top has a wide metal band which is a downdraft preventer. This also makes any pellet stove 10% more efficient and saves pellets!!
Also by code the flue for the smoke must be 12 inches higher than the top the takes in the fresh air so no smoke will come down the chimney! I always recommend this installation. :)

InsertFlueAndFreshAirLiners.png
 
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rhanauer

Member
Apr 18, 2019
16
Oxford, MA
Hello
I always run a separate 3” diameter aluminum liner to the air input connection on the Harman frame. The chimney top plates around the liner seal the chimney from any rain or down drafts. Therefore the only air coming down the chimney goes right in the the Harman Air Adapter and right into the burn pot.
The larger flue top has a wide metal band which is a downdraft preventer. This also makes any pellet stove 10% more efficient and saves pellets!!
Also by code the flue for the smoke must be 12 inches higher than the top the takes in the fresh air so no smoke will come down the chimney! I always recommend this installation. :)

View attachment 280391
Thanks for the info, Don! A few details: My installater (from Higgins Energy Alternatives in Barre, MA) said I didn’t need an OAK, and the Accentra 52i TC has been running well, although the house is pretty tight so maybe it would run even better with the air kit. In any case, I don’t know if they did a separate air intake liner such as you described. As mentioned, the stone chimney cap doesn’t seem to allow staggering flue heights (see pic); I’m not sure it’s worth making expensive modifications up there. I called Higgins and explained my issue; a guy there suggested that the ceramic lined flue I’m pumping basement air into could have masonry issues and might allow crossover into the fireplace flue lower down, as opposed to a downdraft up top. This assumes the pellet stove flue’s SS liner isn’t sealed to the masonry near the fireplace. It makes sense, as I’ve never smelled basement air near the fireplace when running bathroom exhaust fans. If that’s the issue, I could seal around the SS pipe (if permissible) or just exhaust the basement air out a window instead of into the chimney flue.

E377264D-CEB3-4C8D-92C3-CA8FAB245EFE.jpeg
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
25,498
central pa
Open a window upstairs or down. You can't force air out of a house without replacing it and expect it to not cause issues.
 

Don2222

Minister of Fire
Feb 1, 2010
8,810
Salem NH
Thanks for the info, Don! A few details: My installater (from Higgins Energy Alternatives in Barre, MA) said I didn’t need an OAK, and the Accentra 52i TC has been running well, although the house is pretty tight so maybe it would run even better with the air kit. In any case, I don’t know if they did a separate air intake liner such as you described. As mentioned, the stone chimney cap doesn’t seem to allow staggering flue heights (see pic); I’m not sure it’s worth making expensive modifications up there. I called Higgins and explained my issue; a guy there suggested that the ceramic lined flue I’m pumping basement air into could have masonry issues and might allow crossover into the fireplace flue lower down, as opposed to a downdraft up top. This assumes the pellet stove flue’s SS liner isn’t sealed to the masonry near the fireplace. It makes sense, as I’ve never smelled basement air near the fireplace when running bathroom exhaust fans. If that’s the issue, I could seal around the SS pipe (if permissible) or just exhaust the basement air out a window instead of into the chimney flue.

View attachment 280392
Chimney design has changed dramatically over the years and unfortunately that design of having one top over multiple flues in the picture that was easy for the masons to install has now been deemed a bad design because it is notorious for creating and exacerbating down drafts!!
The latest design is a separate top for each flue with the wide downdraft metal band.
Updating it to the current design would be best for safety and efficiency and to meet any new fire codes. :)
 
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