# Does A Flue Damper Increase Heat Output?



## linuxrunner (Jul 23, 2015)

We have an old wood stove from the 60's that I have been working with to try to increase the heat output on.  last year we installed a full chimney liner top to bottom and sealed the top and bottom of the chimney to eliminate losing so much warm air. 

This stove has intake dampers but no flue damper, i was told if we install one of those pipe flue dampers it will help slow down the rate wood we go through and will give off more heat. 

Although we replaced the gaskets last year the stove still seems to be very leaky and with both intake dampers closed we still have a nice fire going.

I can see how a flue damper would slow down how fast the wood burns, but how would it increase the heat output?


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## begreen (Jul 23, 2015)

If the draft is strong a flue damper can help flue gases linger in the stove longer and burn better. This depends a lot on the stove design though. It is more applicable to older stoves than new ones. A stovetop and flue thermometer can be helpful guides for this. For example, a flue damper worked well on our Jotul 602, but not so well on a Jotul F3CB on the same chimney. The 602 has a simple baffle with secondary air being introduced at the front turnaround of the flue gases around the baffle. With pipe and stove temp readings you could see the difference. With the flue damper open the flue would run about 5-600F surface temp. With the damper the pipe temp would drop down to around 400F and the stove top temp would go up 50-100F. With the F3CB closing down the flue damper generally meant a bit weaker secondary combustion with little change in stove temp and no notable change in burn time.


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## linuxrunner (Jul 23, 2015)

Interesting, i have a infrared thermometer and the temps of a good burn last year were about 450 16in up the pipe and about 350 in the top of the stove. sounds like it might be worth a shot.


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## begreen (Jul 23, 2015)

Could help, it's cheap to try. Other factors are in play though, like the leakiness of the stove, height of the chimney and dryness and species of wood being burned.


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## Grisu (Jul 23, 2015)

What type of pipe do you have and how tall? When you reduce flue temps through the damper you may also accumulate more creosote in the pipe. When you start to use the damper keep a close eye on your chimney pipe to make sure you don't create an unsafe condition.  

If your goal is to get more heat from your stove then a modern EPA-approved stove would be a wise investment. Some models can be purchased for ~$800.


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## linuxrunner (Jul 25, 2015)

last year i was burning pine that while it had been cut 1.5 years ago was only split about 6 months ago and was not covered well so often got wet with rain.  This year we have fir that's been split and covered for 10 months already so it should be ready come winter.

our chimney is 6in liner in an internal mason chimney (internal as in the center of the house, not on the exterior) about 20ft long.


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## Corey (Jul 26, 2015)

I don't know that a flue damper would offer substantially much more heat than proper control of the intake dampers.  With either one, you're basically trying to slow the airflow through the stove and make sure you only have enough air to burn the wood you need for the amount of heat you want.  If you can achieve that with the intake dampers, then you're set. 

I suppose if you have a leaky stove and the intake dampers are full closed, but still leaking too much air, then the flue damper might be worth a shot.  Though also note, it might be possible to close the flue damper too much and result in smoke spillage back into the house.  You'll also need to open the damper before you open the stove door, or you'll get smoke spillage.  The intake dampers don't have these requirements.


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## begreen (Jul 26, 2015)

Corey said:


> I don't know that a flue damper would offer substantially much more heat than proper control of the intake dampers.  With either one, you're basically trying to slow the airflow through the stove and make sure you only have enough air to burn the wood you need for the amount of heat you want.  If you can achieve that with the intake dampers, then you're set.
> 
> I suppose if you have a leaky stove and the intake dampers are full closed, but still leaking too much air, then the flue damper might be worth a shot.  Though also note, it might be possible to close the flue damper too much and result in smoke spillage back into the house.  You'll also need to open the damper before you open the stove door, or you'll get smoke spillage.


True, it's one more control to manage. Smoke spilling may depend on the chimney, stove design and draft. With our setup on the 602 the draft was strong enough in winter that with a hot stove you could open the door with the pipe damper closed and not get smoke spillage. But on a shallow east/west stove with a short stack one might get quite a bit. The butterfly damper is not solid, it still allows some flue gas to pass when closed. The temp rise is quantifiable by taking the stove top temp and the flue temp above and below the damper. Our 602 was not leaky. I had rebuilt it a few years earlier. It just worked better with the pipe damper. Note that most current EPA exempt 35:1 stoves have a flue damper built in like the Vogelzang Mountaineer.


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