Back puffing

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Deiswerth

New Member
Nov 27, 2021
16
Pa
Hello all, new to the forum. I occasionally have a problem with back puffing. I have a wood furnace with automatic draft that is controlled with a thermostat upstairs. The furnace is very tight, holds great all night long even on the coldest of nights. The problem seems to occur when the draft opens when the thermostat is calling for heat, it gets burning hot, blower kicks on, heats the house to the setpoint of the thermostat and then the automatic draft closes. Then I get this blow back of smoke that blows the draft door open and couple times and blows smoke out the pipe joints! What can I do to correct this?? Would putting some air across the top of the firebox help? I do have a slide gate I can open on the feed door. The draft is on the clean out door. Thank for any help!
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
Draft on the cleanout door...sounds like a wood/coal furnace...how long have you had, or run this furnace? What kind is it?
Open that upper air control a bit...if its closed and the lower draft closes then the fire is basically snuffed out suddenly...but the wood is still hot, so it is still smoking and off gassing...that builds up and eventually the fire reignites just from the pent up heat...when that happens its basically a small explosion, which people refer to as a backpuff.
The trick is to open the upper air enough to stop the backpuffs, but not make the wood burn too fast that it overheats the house...sometimes you can find the happy spot, but often you end up with a compromise that doesn't backpuff "too much", but also doesn't overheat the house "too much"...but does an exceedingly good job of making creosote in the stovepipe and chimney...not good.
 
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Deiswerth

New Member
Nov 27, 2021
16
Pa
It is a wood/coal/oil furnace. Made by Marathon. I've had it for years, since we moved in, it was already here. Couple things I've changed is put in a stainless steel 6" liner and it had a barometric draft, which I took off because I was told it was for use with oil and I don't use any oil. Didn't remember any puff backs before then...I clean my chimney religiously ever 3 weeks. It has 2 pipes coming out the back, top one has a draft but I don't ever close it because the wood holds all night. The stove pipe is 7". But where it goes through the block wall it is the 6" stainless, so I neck down there.
 

peakbagger

Minister of Fire
Jul 11, 2008
6,820
Northern NH
Keep an eye on the stack, furnaces that control output by cranking down the air are creosote producers. That is why there are so few made.
 
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brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
I clean my chimney religiously ever 3 weeks
A good idea...probably needs it.
It has 2 pipes coming out the back, top one has a draft but I don't ever close it because the wood holds all night.
What's the second one for? Most people with a furnace in the basement have a tall chimney and high draft...and that needs to be controlled somehow, either with that baro damper, or a manual damper...but to do that well you need a gauge...a manometer...Dwyer makes a decent one called the Dwyer Mark II model 25 that works well for wood/coal chimney draft...you can find them on ebay NOS or lightly used for $25-40.
You might try opening that upper air control a bit though, see what that does for ya...
 
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Deiswerth

New Member
Nov 27, 2021
16
Pa
The top pipe has the dampner in it. If I close the top one, it makes the smoke go down a "chamber" to the lower pipe which has no damper on it. Was told it is to extract more heat from the smoke...but I never close the top one cause the fire holds well.
 

Deiswerth

New Member
Nov 27, 2021
16
Pa
It seems to have a really good draft. When I open the little slid gate on the top door and put my ear to it I can hear it sucking air also
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
If I close the top one, it makes the smoke go down a "chamber" to the lower pipe which has no damper on it. Was told it is to extract more heat from the smoke
Hmm, that sounds like something I'd try...if it's meant to burn the smoke, that would clean your chimney up some, if it works...I'm gonna need to look this thing up now...have any pics of it?
The new wood burners burn almost all the smoke...smoke is fuel, sending it up the chimney is a waste and just craps up the chimney (and can be dangerous)
 
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brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
Found this...not much help, but it tells (part) of what happens when the top air is closed.
The other thing I just thought of, in addition to opening the top air a little, is to see if the bottom damper can be adjusted, or modified so that it doesn't open as far...less of a roaring fire, that way when it does close, its not such a dramatic change...I'd guess with the top air open some, the bottom only needs to open a tiny bit to stoke the fire...that's how it is on the Kuuma furnaces, they get generous amounts of air up top (secondary air) and the primary air (not under grate like yours) is open/closed a tiny amount during the first half of the fire...only starts to open up more later on to burn down the cellulose and coals...
 

SpaceBus

Minister of Fire
Nov 18, 2018
6,446
Downeast Maine
Found this...not much help, but it tells (part) of what happens when the top air is closed.
The other thing I just thought of, in addition to opening the top air a little, is to see if the bottom damper can be adjusted, or modified so that it doesn't open as far...less of a roaring fire, that way when it does close, its not such a dramatic change...I'd guess with the top air open some, the bottom only needs to open a tiny bit to stoke the fire...that's how it is on the Kuuma furnaces, they get generous amounts of air up top (secondary air) and the primary air (not under grate like yours) is open/closed a tiny amount during the first half of the fire...only starts to open up more later on to burn down the cellulose and coals...
My cookstove is like this. It has a primary slider that controls under fire air, an air wash slider, and an uncontrolled secondary manifold (of course the oven damper as well, but not relevant). The tiny bit of air that seeps around the slider is enough to keep the fire going once established.
 
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Deiswerth

New Member
Nov 27, 2021
16
Pa
This is what it looks like

20211128_120929.jpg 20211128_120859.jpg 20211128_120811.jpg
 

SpaceBus

Minister of Fire
Nov 18, 2018
6,446
Downeast Maine
How tall is your flue? This looks like a Vermont Castings Defiant I copied into a furnace shape. I think the intent with the multiple flue exits is to create a downdraft stove function by closing the top damper. You stove probably didn't back puff before because the barometric damper was keeping the draft down. Now when your stove gets a call for heat it probably gets that fire going a lot harder than necessary due to the high draft. Then the flap closes and you have a giant smoldering mass of wood just waiting for enough O2 or heat to combust, sending smoke into your basement. The downdraft system is supposed to burn any smoke that makes it out of the firebox OR maybe it was just a simple heat recovery system. I don't know enough about the stove to tell for sure.
 

Deiswerth

New Member
Nov 27, 2021
16
Pa
You mean how tall is the pipe? If so it's about 28', almost 5' out the roof. So maybe put another barometric damper in you think? I used to burn oil, which from what I read, was the use for the barometric...but stopped years ago cause of the cost of oil, plus wood is free, I like to cut it, and we also put in a heat pump. So when I had to replace the stove pipe, I did away with it...but come to think of it, I don't remember getting and back puffs of smoke!!
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
The downdraft system is supposed to burn any smoke that makes it out of the firebox
Yeah I'd have to see inside the back of the firebox to know for sure, but I'm betting its a downdraft burner and the top exit is just the bypass for loading...if so it would be much more efficient and clean burning with the bypass closed...I've never run a downdrafter like that, but I think the way it works is the red hot coals are pushed to the back to where the exhaust exit is when the bypass is closed, then the smoke/gasses light off when they contact the coals...this thing was way ahead of its time if that's the way it works!
 
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Deiswerth

New Member
Nov 27, 2021
16
Pa
Well I'll try closing the top damper and see how it goes. Maybe I'll have to put the barometric damper back in also...last night I did open the vent on the top door some to. One thing I forgot to mention is I DO NOT get much creosote build up in my chimney, but I do clean often. So I don't think that is causing any issues.
 

brenndatomu

Minister of Fire
Aug 21, 2013
6,787
NE Ohio
One thing I forgot to mention is I DO NOT get much creosote build up in my chimney, but I do clean often. So I don't think that is causing any issues.
You must have good dry wood too...