Do I need a flue damper

[brianbeech]

I've recently hooked up my older model Jotul 118. I have 6' of black stove pipe going to a chimney with an 8X8 flue. I get a GREAT draft from this chimney...I mean GREAT. When I start a fire, it takes just a bit of kindling and the draft does the rest as it pulls air front to back and really heats up the stove. Fully loaded, I can go to bed and in the morning have enough hot coals at the back to set a small split or some kindling on and get a fire quickly. I can get my stovetop to around 600F when I have dry wood and my flue top temp runs about half. This is single walled pipe with a magnetic thermometer on it. I have a 'feeling' that I'm able to control the fire quite well with the air control on the stove. If I open the air up too much, the heat goes right out the chimney and my flue temp is the same as the stove.

My question is: if I'm able to control the fire as mentioned above with the air controls, do I really NEED a flue damper?

 

[shawneyboy]

quick answer no but..... you knew there was a but.... If it is really cold or really windy, you may need one. I personally would spring for the 15-20 bucks and have one just in case.

 

[begreen]

Ideally the flue thermometer would be reading around 3-400F, so yes, I'd add one, unless the air control can still be closed off more. They are cheap. If you are reading 600F on the surface of the flue pipe, it is much hotter inside the pipe.

 

[soupy1957]

Single wall flue pipe that reads 600ºF is hotter inside? I figured it would be the same????

I considered a damper in the flue pipe, but in the end, the damper that is built into our stove does just fine.

If it ain't broke, don't fix it.

-Soupy1957

 

[begreen]

There is no outlet damper built-in, just a spin wheel intake control. The air controls doesn't shut off completely and gets leakier with age as it loosens up.

 

[oldspark]

Sounds fine to me as you state, 600 stove top and flue (surface temp) half that, perfect.

 

[shawneyboy]

Sounds fine to me as you state, 600 stove top and flue (surface temp) half that, perfect.

Thats what I read.... then I reread.... and indeed... As I said short answer is no but........

 

[oldspark]

Sounds fine to me as you state, 600 stove top and flue (surface temp) half that, perfect.

Thats what I read.... then I reread.... and indeed... As I said short answer is no but........

I agree I did not put one in but my draft is not super strong, I will put one in if I change my chimney.

 

[pen]

I think you guys miss read this line

I can get my stovetop to around 600F when I have dry wood and my flue top temp runs about half.

So long as your flue pipe stays under about 450 to 500 on the surface as measured 18 inches up you are in the just fine category. By what you are saying your stove is at 600 and the pipe 300, which shows me that you are not over drafting and that thing is working just fine.

I have a damper on mine and only have used it a few times over the past few years. For less than 10 bucks it's another option for controlling the stove so I choose to have one even though I really don't need it the vast majority of the time.

Has it been a long time since a stove was hooked to this chimney? Get a good spotlight (like ones for looking for deer at night) and shine down that chimney to make sure all the flue tiles are in tact and none are cracked. If you have cracked tiles (which unfortunately many masonry chimneys do) then you wouldn't be fully protected in the event you had a chimney fire.

pen

 

[brianbeech]

I think you guys miss read this line

I can get my stovetop to around 600F when I have dry wood and my flue top temp runs about half.

So long as your flue pipe stays under about 450 to 500 on the surface as measured 18 inches up you are in the just fine category. By what you are saying your stove is at 600 and the pipe 300, which shows me that you are not over drafting and that thing is working just fine.

I have a damper on mine and only have used it a few times over the past few years. For less than 10 bucks it's another option for controlling the stove so I choose to have one even though I really don't need it the vast majority of the time.

Has it been a long time since a stove was hooked to this chimney? Get a good spotlight (like ones for looking for deer at night) and shine down that chimney to make sure all the flue tiles are in tact and none are cracked. If you have cracked tiles (which unfortunately many masonry chimneys do) then you wouldn't be fully protected in the event you had a chimney fire.

pen

Yes, when I'm running around 600-650F my surface temp on the flue pipe is right around 300-350F. I thought it was working fine, but someone suggested that at one time and I wanted to be sure I wasn't missing out on something good. I just hate to add a variable that I can screw up and begin running poorly.

Stove was hooked up to the chimney a few weeks ago. I didn't hook it up, had a local Chimney Sweep come do it and he got up there and checked it out. Said it didn't look like it had ever been used. I've got two chimneys and 3 flues, so it wouldn't surprise me if this one wasn't used.

I suppose for safety sake, I need to 'see' that my flue is in good condition for myself.

 

[Battenkiller]

From your description of the way the stove is burning for you, I would not put one in. You have a short run of 5" pipe dumping into a 8"x8" tile flue. When those gases expend inside the main flue, the temps will drop by about half. 300ºF external pipe temp? That's about a 600º internal gas temp dropping to about 300º almost immediately as it enters the tile flue, with temps falling off further along the way to the exit. If they drop below 200º, the water of combustion and the tar droplets in your smoke will condense on the flue walls at some point before they exit the top of the stack.

There is a lot more water involved here than you might think. For every pound of well-seasoned wood (20% MC) you burn, about 3/4 of a pound of water exits the firebox. To make matters worse, since nearly all of the hydrogen in the wood goes into the outgassing smoke, most of this water goes up the flue during the first part of the burn. Therefore, the rate of water production is much higher until the coaling stage has been reached. After the wood has turned to charcoal, no more water is made by combustion, and all of the water initially present in the wood has been driven out as well. That's even more reason to run higher flue temps in the beginning. That's the time you really need to get all of that water out.


With that stove, you will get significant creosote buildup in the main flue when that happens. I ran that basic stove in a nearly identical setup for almost 20 years. Over all that time, the air control was always sufficient to snuff the fire out completely if I closed it all the way. There is no evidence I have seen that would warrant needing a damper to control that stove. I put one in merely to try to extend the burn time, and in retrospect, it was not a good thing to do. I can see no advantage of trying to lower your flue temps beyond what you are now able to control them. High pipe temps do not necessarily mean all your heat is going up the flue, there is the question of volume per unit of time that is more important. With the intake narrowed sufficiently, there is not a large volume of air coming into the stove, so there is not a large volume of hot air leaving it. That is the principle of the air-tight stove - to control the air volume at the intake.

 

[brianbeech]

I see what you mean BK. So, I think I'm hearing that I should run a higher flue temp until I get to the coaling stage? Or should I first inspect the flue and see if there has been significant creosote build-up before doing that?

 

[Battenkiller]

I see what you mean BK. So, I think I'm hearing that I should run a higher flue temp until I get to the coaling stage? Or should I first inspect the flue and see if there has been significant creosote build-up before doing that?

Well, it kinda happens automatically as you reach the coaling stage, so there is no need to try to lower it at that point, just run it on the high side in the beginning and it will be drop on its own in due time.

IMHO it's always a good idea to check your flue soon after you start using it for the first time. Even very experienced burners might be surprised at what they find (or don't find) after a couple weeks burning in a different setup.

 

[Battenkiller]

Single wall flue pipe that reads 600ºF is hotter inside? I figured it would be the same????

I considered a damper in the flue pipe, but in the end, the damper that is built into our stove does just fine.

If it ain't broke, don't fix it.

-Soupy1957

Soupy, the way smoke travels up a flue, the hottest gases are in the very center of the gas column. Also, just like the internal temps of your stove are always much higher than the outer surface temps, the external temps of the pipe will be lower than the gas temps right next to the inner wall. You also have to remember that temperature is not the same as heat. You can't hold a lot of heat in air compared to how much you can hold in metal... even thin metal like stove pipe. In order to raise the temp of the metal, there needs to be an extremely efficient mode of heat transfer to get the pipe temps to come close to gas temps. Even then, the pipe is constantly losing heat to the surrounding air, so high temps are hard to maintain. Finally, there is almost always at least a thin coating of soot and fly ash on the inside of the pipe. This has an insulating effect, further impeding heat transfer.

The interesting thing about dampers is that they disrupt the gas flow inside the pipe by creating turbulence. This cause the gases to mix better and have more even temps inside the pipe. The pipe temp will rise, but the mean internal gas temp will drop. It only take a few degrees of rotation to accomplish this. Back in the days of trying to get every last speck of heat out of the pipe, this was considered a good thing, but the current trend (which I largely agree with) seems to be toward keeping cooler pipe surface temps (usually through insulation) and hotter gas temps, and letting the stove do the actual heat transfer instead of the pipe. Makes for a much cleaner flue and a more powerful draft.

 

[begreen]

I did mis-read it. 300F surface flue temps are not bad. But wait til next year when you have super dry wood .

 

[brianbeech]

I did mis-read it. 300F surface flue temps are not bad. But wait til next year when you have super dry wood .

Am I going to be looking at super hot pipe temps. Okay, what's a safe limit?

 

[begreen]

About 450F or when it starts to glow in the dark. When it starts to glow in daylight, a few hail marys might be in order, it's way too hot!

 

[Battenkiller]

About 450F or when it starts to glow in the dark.

???

No way does anything glow in the dark at 450ºF. If that was the case, my whole stove would never stop glowing. Try about 950º.


Hint, Brian.... don't let your pipe get to 950ºF, your thermometer will get demagnetized and fall off if nothing else.

 

[begreen]

450F surface = ~900F inside which is what the pipe is tested at for continuous use.

 

[oldspark]

About 450F or when it starts to glow in the dark. When it starts to glow in daylight, a few hail marys might be in order, it's way too hot!

Wont glow at 450 surface or 900 internal as that temp is internal, I can assure you nothing glows with about a 700 surface temp flue.

 

[oldspark]

My bad I missed BK's post about glowing in the dark.

 

[Battenkiller]

I don't care what the test rating is, in my universe it won't glow until about 950ºF.

 

[begreen]

Folks, I said 450F surface OR if it glows in the dark. The pipe should not be run continually with much over 900F internal temps. I know what temp metal glows. Sheesh.