Negative Pressure Draft - Better to Fix or Airtight Damper

Areliven

New Member
Jan 26, 2020
14
Michigan
Hi there! I'm new to these forums, been looking through them the last few weeks, and I probably read every thread on negative pressure and backdrafting, but did not really see an answer to this question: Is it safe/OK to just put an airtight damper on the top of the chimney to stop backdrafting? Or is it a subpar solution because it does not address the negative pressure in the house? Here are my circumstances:

Recently purchased this home built in the 70s. It has an open fireplace with glass doors which is constantly backdrafting. I've tested by taping a square piece of plastic over the opening on three sides, leaving the bottom side untaped. Within seconds the plastic "inflates", and a small but steady stream of air is coming out of the bottom of the plastic. In this room there is a sliding exterior door as well. I open the door, and within seconds the plastic "deflates", as air is now coming in through the sliding door easier than the chimney.

This wouldn't really be an issue for me except it constantly smells like smoke/fireplace in my house now, even 2 weeks after a fire. Some other notes: it's a two story house with a full basement, fireplace on main level. There's a cold air return over 10 feet away, it has a high efficiency furnace but the hot water heater vents through a separate flue right next to the chimney, and they terminate at the same height right next to each other above the house (but I do not smell smoke coming in through this flue).

With the amount of air coming in through the chimney, there must be that much air "escaping" the house somewhere. It's a pretty tight house, and there doesn't appear to be any single point where that much air could be escaping all at once. So back to the original question:

I could cap it with an air tight damper. But let's say I have a fire in the evening and there are still some embers burning at night. I'll leave the flue open all night while it completely dies out, which also means the backdraft will start up again at some point and I will wake up to that fireplace smell. But at least I can close it at that point and smell will probably go away by days end. This seems like the easiest fix and I know many people have done it. But do I risk some other problems if I do not address the negative pressure issue?

Thanks!
 
Last edited:

maple1

Minister of Fire
Sep 15, 2011
10,465
Nova Scotia
Are you saying it only backdrafts when the room is closed up? (Door closed?).

Does it backdraft when your furnace is not running?

Sounds like you might need some HVAC 'rebalancing' - your furnace return is pulling down your chimney.
 
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begreen

Mooderator
Staff member
Nov 18, 2005
82,734
South Puget Sound, WA
Can we assume that:
1) testing is being done with no exhaust appliance running (bath or kitchen fan, clothes dryer, etc.)
2) that gas HW heater and furnace were not running.
3) there are no windows, attic vents etc. upstairs that are open or unsealed
 

Areliven

New Member
Jan 26, 2020
14
Michigan
Are you saying it only backdrafts when the room is closed up? (Door closed?).

Does it backdraft when your furnace is not running?

Sounds like you might need some HVAC 'rebalancing' - your furnace return is pulling down your chimney.
It backdrafts when the sliding door is closed/everything is closed up. When the door is open (or another nearby window) it does not backdraft.

It also backdrafts when the furnace is running AND when it's not running (although it appears to be slightly less when furnaces is not running).

Can we assume that:
1) testing is being done with no exhaust appliance running (bath or kitchen fan, clothes dryer, etc.)
2) that gas HW heater and furnace were not running.
3) there are no windows, attic vents etc. upstairs that are open or unsealed
1. No other fans or appliances were running (tested with furnace on and off, see above).
2. Not sure if the gas HW heater was actively heating during the tests, but I've done the tests several times. I assumed the exhaust from the HW heater wouldn't be enough to case the backdrafting but I could be wrong?
3. No windows in the house are open. The only attic entry point from the upstairs is very small, and inside a closet. But perhaps I should put some plastic over this point as well and re-test?
 

begreen

Mooderator
Staff member
Nov 18, 2005
82,734
South Puget Sound, WA
This definitely sounds like negative pressure. Check the attic entry for sealing. You can temporarily tape off seams with wide blue painter's tape. Is there a radon fan running? Are there a number of recessed light fixtures upstairs? They can also be potential leakage points.
 

Areliven

New Member
Jan 26, 2020
14
Michigan
This definitely sounds like negative pressure. Check the attic entry for sealing. You can temporarily tape off seams with wide blue painter's tape. Is there a radon fan running? Are there a number of recessed light fixtures upstairs? They can also be potential leakage points.
Ah yes, we did have a radon mitigation system installed before we moved it. It pulls from beneath the basement floor and runs though pipe into the attic and out the roof. There is a fan attached too. I haven't actually seen it, but my impression was everything is completely sealed off so the fan wouldn't be pulling air from the attic...I will try sealing off the attic entry door.

There are no recessed lights, but two upstairs bathrooms have fans that vent into the attic (not good I know). Backdrafting happens whether those run or not.
 

maple1

Minister of Fire
Sep 15, 2011
10,465
Nova Scotia
Seems to me since the problem goes away when the room is open, the problem must mostly lie with the furnace return. Unless there is another thing in the room pulling air. Can you just leave that door open?
 

begreen

Mooderator
Staff member
Nov 18, 2005
82,734
South Puget Sound, WA
Seems to me since the problem goes away when the room is open, the problem must mostly lie with the furnace return. Unless there is another thing in the room pulling air. Can you just leave that door open?
The furnace system is a closed loop within the house envelope. It usually does not affect the house pressure unless the system is connected to a separate section of the house like a MIL apartment.
 

begreen

Mooderator
Staff member
Nov 18, 2005
82,734
South Puget Sound, WA
There are no recessed lights, but two upstairs bathrooms have fans that vent into the attic (not good I know). Backdrafting happens whether those run or not.
They don't need to be running. Seal off the vents in these rooms for a few days and see if that makes a difference.
 

Areliven

New Member
Jan 26, 2020
14
Michigan
Seems to me since the problem goes away when the room is open, the problem must mostly lie with the furnace return. Unless there is another thing in the room pulling air. Can you just leave that door open?
I'm in Michigan and it's winter, so leaving the door open would make it unbearable.

The furnace system is a closed loop within the house envelope. It usually does not affect the house pressure unless the system is connected to a separate section of the house like a MIL apartment.
That is my thought too. The furnace does pull air in from outside for combustion and vents it out for exhaust via PVC piping, not using the flue next to the chimney the the hot water heater is.
 

maple1

Minister of Fire
Sep 15, 2011
10,465
Nova Scotia
Yes the sliding door goes to the outside.
Ah, ok, disregard everything I was saying then. Does sound like stack effect, and those ceiling fans would be my first suspect. Temporarily block them by taping paper over or something & check for a change.
 

spudman99

Feeling the Heat
Jan 26, 2018
326
Yardley, PA
I had a similar issue before I installed by insert. Traditional masonry exterior fireplace with a spring top damper. I regularly would have cold air drafting down into the house at all times, sometimes bringing in the creosote odor if the barometric pressure were dropping allowing the heavier air to settle. I have a not so tight house from 1983.

My simple and not so elegant solution was to hang a fleece blanket over the glass doors when not in use. Get your favorite sports team or college and it doesn't look too bad (better than sponge bob). Took the blanket off when I wanted to use the fireplace on weekends. During the summer or if we had guests the blanket comes off. That just about stopped the cold air infiltrating the house during non-use and stopped any bad odor.
 

Areliven

New Member
Jan 26, 2020
14
Michigan
Ah, ok, disregard everything I was saying then. Does sound like stack effect, and those ceiling fans would be my first suspect. Temporarily block them by taping paper over or something & check for a change.
I closed off all bathroom fans and the attic entry door, and turned off the radon mitigation fan...still have the backdraft. I'm seeing a few solutions at this point, call HVAC and chimney company to come take a look and see if they have any advice. Or just put an airtight damper on the top of the chimney. Doesn't solve the pressure issue, but the negative pressure shouldn't really matter anymore with the airtight damper in place, right?
 

spudman99

Feeling the Heat
Jan 26, 2018
326
Yardley, PA
I think you will find an airtight damper on top of the chimney to not be so airtight. It will most likely be metal (top damper) pressing against the terra cotta liner (clay product) and there can be no gaskets or sealing materials. Hard to get a tight seal between 2 non flexible components.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,497
central pa
I think you will find an airtight damper on top of the chimney to not be so airtight. It will most likely be metal (top damper) pressing against the terra cotta liner (clay product) and there can be no gaskets or sealing materials. Hard to get a tight seal between 2 non flexible components.
No most top sealers have gaskets they are pretty air tight if adjusted correctly
 

spudman99

Feeling the Heat
Jan 26, 2018
326
Yardley, PA
No most top sealers have gaskets they are pretty air tight if adjusted correctly
I did not know that and thanks. My spring loaded top damper did not have such a gasket but maybe due to its 20 year age didn't come with one. Question: is the gasket material non-combustible, or is it just not a concern at that end of the chimney?
 

Areliven

New Member
Jan 26, 2020
14
Michigan
I did not know that and thanks. My spring loaded top damper did not have such a gasket but maybe due to its 20 year age didn't come with one. Question: is the gasket material non-combustible, or is it just not a concern at that end of the chimney?
I was thinking something like is: Amazon product
I do eventually want to convert this to a gas fireplace, and this product says do not use with gas logs...so I'm wondering if my negative pressure issue will be more of a concern with a gas fireplace now...
 

rwhite

Minister of Fire
Nov 8, 2011
1,612
North Central Idaho
Is there an exterior ash door? I had one that the edge faced the prevailing wind and until I sealed it, it would catch any breeze and flow up through the fireplace.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,497
central pa
I was thinking something like is: Amazon product
I do eventually want to convert this to a gas fireplace, and this product says do not use with gas logs...so I'm wondering if my negative pressure issue will be more of a concern with a gas fireplace now...
The negative pressure issue would need addressed to use the fireplace for wood or gas
 

Areliven

New Member
Jan 26, 2020
14
Michigan
Is there an exterior ash door? I had one that the edge faced the prevailing wind and until I sealed it, it would catch any breeze and flow up through the fireplace.
No there is not. At this point I think I've ruled out that the air is escaping from any one or two places. There must be a little escaping from many small places.
 

Mech e

Feeling the Heat
Feb 26, 2019
262
NorCal
www.dtengineer.com
The furnace system is a closed loop within the house envelope. It usually does not affect the house pressure unless the system is connected to a separate section of the house like a MIL apartment.
One thing to watch here however is running the furnace with registers open and bedroom doors shut. Closed doors will restrict return air from those rooms and the furnace can start pulling air down the chimney.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
21,497
central pa
I did not know that and thanks. My spring loaded top damper did not have such a gasket but maybe due to its 20 year age didn't come with one. Question: is the gasket material non-combustible, or is it just not a concern at that end of the chimney?
The gasket is usually high temp silicone. It can easily withstand normal operating temps but won't hold up to a chimney fire.
 
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