Trying to mitigate chimney effect...HELP!

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lukeg199

Member
Jun 17, 2013
14
Vermont
2 years ago we bought a new house, built in the late 70's, and the first thing we did was weatherize. This was a combination of air sealing the box sills and the chimney chase as well as other protrusions in the second floor ceiling. Not surprisingly this made out house much tighter and more efficient to heat. However, an unintended consequence was that infiltration, presumably due to the chimney effect in extreme cold temps (think -20F, Vermont), took place in our Jotul Fireplace insert when it was not in use. The breeze coming from the insert is very noticeable and makes the living room in the house VERY cold. 5-8 degrees colder despite essentially being and open concept layout with the rest of the first floor (Though the living room is slab on grade about 12 inches lower then the main part of the house.) Additionally the chimney is exterior. To give an idea of the negative pressure we experience, if I want to start up the insert from cold, I have to open an exterior door, prime the flue with a torch and then light a big newspaper fireball to establish the updraft.
Here is an annotated photo of our house (with arrow pointing to the exterior chimney:
1649180911106.png
And here is the woodstove/fireplace:
1649181013078.png

As for how to combat this backflow when the stove is not in use, I can think of two solutions: An Outside Air Kit (OAK) for the insert, or a top mounted flue damper. Each has it's pros and cons AFAIK:
SolutionProsCons
Outside Air Adapter
  • Isolates the supply and exhaust air from the interior space.
  • relatively inexpensive
  • It can leak, allowing outside air to infiltrate, though likely much less than the current rate.
  • I would have to rent a coring drill to create a 3" hole through ~12" of cement brick and mortar.
  • I have read a lot of content (alot on woodheat.org) that argues outside air adapters a not great solutions.
Top Mounted Damper
  • Easy(er) installation
  • should provide a good seal and prevent most if not all outside air infiltration when stove is off.
  • expensive ($400-500 DIY price)
  • it's intended for fireplaces, not woodburning stoves. (to draw the top of the cap down to the closed position there is a SS cable that is supposed to pass through middle the flue liner into the fireplace. This wont work with a woodburning stove as it is sealed and reaches much higher temps...)
  • Would have to find a work around some how to work with a wood burning stove.

If you are still reading, thanks!!!! There is no clear fix that I can think of, but i also have to imagine i am not the only one facing this issue. That's why i am am posting. If anyone has suggestions, please share!!! I love burning wood on the coldest days. we otherwise heat with air source minisplits which are super nice until it gets down to ~-10F and have resistance heat if needed.
Thanks in advance!!!
 
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peakbagger

Minister of Fire
Jul 11, 2008
7,330
Northern NH
Given the higher roof nearby I predict some unwanted aerodynamic effects when wind is going right to left or left to right. If you house is typical in VT its probably not on flat lot and is built on slope and then too can also introduce odd drafts. If you are comfortable on a roof, you could put a temporary stack cap on so that flue gas exits to the sides instead of straight up. It a relatively cheap experiment.

An OAK is only as good as the stove its attached to. If its sealed tight to the flue and the there are not leaks through the ash pan (if there is one), it should improve things. The problem is frequently inserts are not well sealed to the chimney so a lot of air is going around the body of the stove into the former hearth and then getting sucked up the chimney along with the flue gases.

I do not have direct experience with the top of stack dampers and how well the cable survive a wood stove so someone else will have to comment.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,701
central pa
Is the liner insulated? Is there an insulated block off plate? Is the firebox around the insert insulated? It is possible you are having negative pressure due to the chimney effect but also very possible it is just an extremely cold chimney dumping cold air into the house.

As far as mitigating chimney effect you need to identify and eliminate air leaks in the house (mainly on the upper floors)
 
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lukeg199

Member
Jun 17, 2013
14
Vermont
Thank you bholler and peakbagger! Yes the wind whips around here! That said, unless there is an active fire, the flow is consistently in reverse direction, calm or windy. Last year I installed a custom fabricated block off plate (uninsulated) and it helped a little. I have not used any insulation around the stove and agree that would help but it's very evident that the biggest issue is reverse flow through the flexible SS chimney liner (uninsulated). The biggest confirmation of this is when opening the doors and feeling a strong cold breeze in your face!

Also, there is no ashpan, so that's a win on the tightness front (this is a Jotul Kennebec model if I am not mistaken).

Based on this feedback I am thinking next steps might be best spent, in order of impact:
  1. Rent a coring drill and install the OAK. Being very conscious of tight seals at the back of the unit and hole on in the masonry.
  2. replace door gaskets if needed
  3. Air seal second floor with use of thermal imaging camera.
  4. improve seal between stove and flue liner. My liner has a male tapered section the fits inside the female flange on the top of the stove with a few screws that go through the flange and flue liner. I'm sure this is nowhere close to an air tight connection, but given the temps experienced how would one make this connection more air tight, if possible? Are there special, super high temp caulking?
  5. There is a ~1/4" gap between the block off plate and the flu liner. Outside of jamming roxul in this gap, is there another way of tightening this up? Maybe a custom made flange?
  6. Stuff insulation around the outside of the insert

Any other suggestions? Again, thanks so much for the help!!
 

begreen

Mooderator
Staff member
Nov 18, 2005
93,849
South Puget Sound, WA
In some respects it will always be a fight due to the chimney location, particularly if this is on the leeward side of the house. How tall is the flue liner?
At the top of that list: Pull the liner and get it insulated. Add an insulated block-off plate. I would do that before considering an OAK.

chimney location.png
 
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moresnow

Minister of Fire
Jan 13, 2015
1,980
Iowa
Has anyone considered transitioning to Class A at the current termination and adding a additional section, or two, to see if that takes care of the reverse draft issue? Seems plausible with the current low lying, possibly prevailing wind susceptible arrangement.
 
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EbS-P

Minister of Fire
Jan 19, 2019
2,686
SE North Carolina
Air sealing is a win win. If cold air is coming in hot air has to be getting out. As a quick fix. I pulled the trim cover off all 6 ceiling fans an used a cheap tube of some type of white caulking I was about to toss to seal the junction boxes where the met the Sheetrock from the inside. It had to help some.

OAK is good for keeping cold air out of the house but doesn’t address the cause of the reverse draft.

Sealing and insulating the the blockoff plate and stove will put more heat into the house. Insulating the liner


All of that will help. How’s the draft at during coaling stage. How long does it take to reverse draft after the fire goes out? Do you have a CO detector? Thinking safety here.

The solution of not letting the stove go cold does come to mind. how old is the liner and do you have any idea the amount of wood that been burned through it). When’s the last time is was cleaned and or inspected? Insulating should help with draft reversal. Those questions are all relevant.

Evan
 

stoveliker

Minister of Fire
Nov 17, 2019
4,590
Long Island NY
Air sealing is a win win. If cold air is coming in hot air has to be getting out. As a quick fix. I pulled the trim cover off all 6 ceiling fans an used a cheap tube of some type of white caulking I was about to toss to seal the junction boxes where the met the Sheetrock from the inside. It had to help some.
^^That. That's why I spent 6 weekends in the two attics. Sealing all penetrations. (Also think about the power outlets in the wall that provide a direct path to your attic.)

OAK is good for keeping cold air out of the house but doesn’t address the cause of the reverse draft.
In fact, it's good to prevent heated air from being used in the stove (which needs to be replaced by cold air sucked in through leaks), but the stove will still be cold when not in use, and likely more so as there now is a path of flow from outside to outside, and that likely results in much more "air changes" in the stove. A stove that is then a cold air bag with only some sheet metal between that colder air and your room.
All of that will help. How’s the draft at during coaling stage. How long does it take to reverse draft after the fire goes out? Do you have a CO detector? Thinking safety here.
Possibly the most important advice here when draft issues are present.
 

brenndatomu

Minister of Fire
Aug 21, 2013
7,236
NE Ohio
Has anyone considered transitioning to Class A at the current termination and adding a additional section, or two, to see if that takes care of the reverse draft issue? Seems plausible with the current low lying, possibly prevailing wind susceptible arrangement.
Exactly what I was thinking...as soon as I seen the picture I noticed the main house roof is taller than the chimney height, so its acting as the "chimney"...
 

lukeg199

Member
Jun 17, 2013
14
Vermont
OK, lots to respond to - thank you all so much for weighing in.

So the chimney faces the wind on most days, but because it's attached to the side of the house i wouldn't say it's either windward or leeward. let's call it neutral.

The liner is likely 20-25 feet. I've no idea how old it is because we only purchased this house about 2 years ago and it was preinstalled. I have swept it once a year and very little comes out. At most 1 quart of soot/creosote.

Couple recommendations out there for modifications to the flu liner. I can see an insulated liner helping with draw when the stove is on, but that isn't really solving the problem, which is steady backflow when the stove is off and is exacerbated by extreme cold temps. How else might it help when the stove is off? As for adding extensions, that seems like an interesting concept but again, how would that help when the stove is off. Please don't read that as a critique, I am just not well versed in liners.

After the fire goes out i would say that i have coals for a few hours depending on a lot of factors: Air intake lever position, How much wood i burned/how long prior, size of splits. We have lots of CO detectors in the house and thankfully one has yet to go off after 2 seasons of intermittant wood burning. I am keenly aware of this, and quite honestly it scares me (we have a 17 month old). That said, we have never experience smoke or even the smell of smoke in the house following a fire. We have also never experience draw or backflow issues while the fire is going.

To date the best solution i have experimented with is removing the secondary air baffled and sticking newspaper into the flu to block the cold draft. admittedly crude, but this made an appreciable difference in reducing the reverse flow of air.

As for air sealing, the house was weatherized extensively 2 years ago. they removed all the pink stuff in the attic, sealed all penetrations, place insulated hoods over bath vents/recessed lights and sealed them, then 16 inches of blown in cellulose on top of everything. I have gone around to all the exterior wall outlets and tried my best to seal them with spay foam. At this point the next step would be replacing 40 year old windows, but that is incredibly expensive, so for the time being i have done my best to use removable caulking peel and stick in all the windows. I think the infrared camera is the next step for second floor air sealing...

Thanks in advance for all the replies!
 

peakbagger

Minister of Fire
Jul 11, 2008
7,330
Northern NH
I missed the air sealing. There is good chance that you need an air to air heat exchanger. With a tight air sealed house, there is still going to be negative pressure in the house from bathroom and kitchen fans as well as the "stack" effect. By the way, you would not happen to have a radon removal system? They can also make a house negative. In theory when they did the air sealing they would have done a before and after leakage rate. My guess is the house is borderline too tight and without an air to air exchanger, the stove is acting as an air inlet. My guess is if you had an accurate differential pressure from outdoors to indoors you would see a big difference with a door open or closed. An air to air unit is adjsuted so that the differential is balanced with the fresh air being introduced to the bedrooms.

A lot or air sealing contractors skip air to air units as it can be lot of specialized work to retrofit them. They make their money on cheap unskilled labor doing the sealing so they are better off doing sealing.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,701
central pa
Honestly I doubt this is caused by chimney effect from the rest of the house. And very possibly not negative pressure at all. Just a cold chimney with cold air dropping through it.
 

stoveliker

Minister of Fire
Nov 17, 2019
4,590
Long Island NY
An OAK would have this mostly go to the outside, although the box will still be a heat sink as it's not insulated with respect to the room and will remain cold.
 

EbS-P

Minister of Fire
Jan 19, 2019
2,686
SE North Carolina
The insulated liner would just keep it warmer longer so you would have better drafting for a longer period. If it’s still warm it’s still drafting up and out. I would guess that even an insulated liner won’t completely solve the issue but I should help. I recommend a fire every evening. Light it top down. Reload before bed wake up it should still be warm. Toss on some kindling then a couple splits. Repeat that evening.

It’s an exterior chimney it going to get cold the cold air will sink into the house. If you like it live with it. Seems like you have figured out how to use as is. One idea and I’m not saying it’s a good one one (in theory it should work) is to hook up a blast gate you can operate to the air intake (where you hook the OAK. My thoughts are these. You close it. No down draft. Stove stays warmer since no cold air is moving through. Want to light a fire open it. Have an over fire situation you can close is down. In principle it’s a win win. Marking it actually work? That will take some thinking and I’m guessing pulling the surround off several times.

Evan
 

lukeg199

Member
Jun 17, 2013
14
Vermont
The insulated liner would just keep it warmer longer so you would have better drafting for a longer period. If it’s still warm it’s still drafting up and out. I would guess that even an insulated liner won’t completely solve the issue but I should help. I recommend a fire every evening. Light it top down. Reload before bed wake up it should still be warm. Toss on some kindling then a couple splits. Repeat that evening.

It’s an exterior chimney it going to get cold the cold air will sink into the house. If you like it live with it. Seems like you have figured out how to use as is. One idea and I’m not saying it’s a good one one (in theory it should work) is to hook up a blast gate you can operate to the air intake (where you hook the OAK. My thoughts are these. You close it. No down draft. Stove stays warmer since no cold air is moving through. Want to light a fire open it. Have an over fire situation you can close is down. In principle it’s a win win. Marking it actually work? That will take some thinking and I’m guessing pulling the surround off several times.

Evan
I love this idea!!! It's crazy this never occurred to me... I'm a woodworker and have like 10 blast gates that i use every day! Yea, it will be tricky to figure out how to control it, but i think it is definitely worth a shot, and i'm nothing if not resourceful. Certainly easier to try this instead of a OAK and renting a coring drill to create a hole through over a foot of cement. When i do this i will share photos. Thanks for the recommendation EbS-P!
 

lukeg199

Member
Jun 17, 2013
14
Vermont
I missed the air sealing. There is good chance that you need an air to air heat exchanger. With a tight air sealed house, there is still going to be negative pressure in the house from bathroom and kitchen fans as well as the "stack" effect. By the way, you would not happen to have a radon removal system? They can also make a house negative. In theory when they did the air sealing they would have done a before and after leakage rate. My guess is the house is borderline too tight and without an air to air exchanger, the stove is acting as an air inlet. My guess is if you had an accurate differential pressure from outdoors to indoors you would see a big difference with a door open or closed. An air to air unit is adjsuted so that the differential is balanced with the fresh air being introduced to the bedrooms.

A lot or air sealing contractors skip air to air units as it can be lot of specialized work to retrofit them. They make their money on cheap unskilled labor doing the sealing so they are better off doing sealing.
Yea, i have been looking into air exchangers as a possible solution for the future, but i don't think we are there just yet. Maybe after this problem is solved we will. Scary to think about how much fun it will be to retrofit new ducting into the house to make this type of system possibly. does not seem like an easy retrofit...

I DO HAVE A RADON SYSTEM! and totally forgot about this. Not much i can do about it unfortunately, except maybe an air exchanger. Maybe it's time to have another blower door test following all the improvements we've made since out weatherization a couple years ago.

Also, another indication this is likely a stack effect problem is that we have some older windows with storms on both floors. In the coldest weather, despite my best stick and peel caulking efforts, still leak a little. First floor, i see frost form where air leaks in around the frame/sash and clear glass. Second floor, no frost on the frame/sash but fogged glass due to condensation of warm moist air against cold glass.