Damper vs air control

revdocjim said:

....I know that some people add dampers because their draw is so strong that even with the air-control stopped all the way down the fire can get out of control....

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That's what I use my in-flue damper/butterfly control for....it helps to bring the flue temps back down to a reasonable level. Normally only need this closed for a few minutes. I never leave this closed unattended. This happens if I overload with small splits or extremely dry wood...or if I get my attention diverted after full reload on full open air intake.
I have a stove top thermometer and a flue probe thermometer (which is a must have).

Other than this I leave the flue damper open and use the air-control intake (OAK) to do the main control of the fire.

For me a damper in the chimney (normally just above the stove) is used for additional control over draft when draft is so strong it overcomes the stoves ability, via it's air controls, to slow down the burn. Some also close them to stop/slow down the chimney from pulling air out of the house when no fire is burning. There may be other reasons to but those are the ones that caused me to install one when I had my PE Summit.

On a side note, most time here on the forum I see the abbreviation OAK used to reference an Outside Air Kit not the stoves draft controls.

For clarity of my setup: the OAK (Outside Air Kit) is direct-connected to my stove intake (air control). So, at least to me, it is an integral part of air control and the overall draft. Hopefully there is little if no room air being used to fire the stove.

The damper I use is inline with the black metal stove flue pipe....the pipe then enters the chimney via the thimble unobstructed. So the only air choke points are on my main air control and the flue pipe damper/butterfly.

It is pretty much the same, until you open the door and smoke spills out because you forgot to open the damper. That doesn't happen when adjusting the intake.

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revdocjim said:

Thanks for the replies. I'm a bit surprised more of the seasoned veterans haven't chimed in. As far as I can tell from these responses the effect on the fire is pretty much the same, whether you limit it at the front end with an air-control or at the back end with a flue damper.

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Most EPA stoves do not require a or even discourage them because if used incorrectly they could turn a normally clean burning stove into a smoldering mess. I have 22 feet and 32 feet liners and I live on top of a hill. I have really strong draft in cool dry weather so I use my dampers when below freezing to keep things reasonable. When I use them, I go to about 3/4 shut and it does a good job slowing things down. Dancing flame vs the hires of Hades Reloads and cold starts with damper engaged = bad medicine.

I look at the stove as an air flow system, isolated from the interior of the house except from the thermal
radiation of the fire.

The intake from outside air is metered by the air valve in the fire box, then unimpeded up and through the baffle, into the 6 inch pipe, into the 40+ foot chimney flue via the thimble.

The damper inline with the 6 inch pipe is ONLY EVER used to reduce an over-fire burn in the firebox....it is a
safety item. I never-ever use this as the normal burn rate control mechanism.
Normal operation of the inline damper is full open.

having an insert, and flush at that, i couldnt add a damper to my pipe to solve my severe overdraft problem. So in the offseason i made it so i could adjust my primary and secondary air. To me its the same thing, you are limiting air flow and i feel its better to do it on the front end.

Here's one thought that's been bugging me on this topic: Volume is proportional to temperature, and since the exhaust is hotter than the intake air there's going to be more of it. If we assume a fixed air intake rate, the amount of exhaust will continue to increase as the stove heats up.

So in theory a damper should become more and more effective as the stove heats up, which is not true for an intake control. If you have a stove and chimney setup that's prone to running away, a damper might be the ideal solution.

In practice, I haven't had much experience with a setup that has both a damper and an intake control. It's usually been either/or.

Also to me, you can' shut these EPA stoves down all the way so there is a certain amount on the intake side that you can't control unless you do modify that side of things. If you have a stronger than normal draft, then you are going to increase velocity trough whatever restriction exists on the intake side. To me, that increases burn rates and decreases efficiency by burning too much fuel. So if you can figure out a way to further restrict the intake side, yes that will work. But if it is easier to restrict the exhaust side, then that can be equally effective too. A $9 damper is a pretty easy solution IMO.

revdocjim said:

Thanks for the replies. I'm a bit surprised more of the seasoned veterans haven't chimed in. As far as I can tell from these responses the effect on the fire is pretty much the same, whether you limit it at the front end with an air-control or at the back end with a flue damper.

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If the stove is actually airtight at all junctions and potential points of air entry, then a damper would be the functional equivalent of air intake control. Of course this ignores the inconsequential effect of the slightly lower combustion chamber pressure when damping a fire from the intake side of things (vs. a damper). And, as has been pointed out, no stove is airtight when the door is open!

In other words, an airtight stove doesn't need a damper.

This is very interesting. Myself and another member have just been having a conversation about this exact topic on the "Low Flue Temps" thread....

WoodyIsGoody said:

If the stove is actually airtight at all junctions and potential points of air entry, then a damper would be the functional equivalent of air intake control. Of course this ignores the inconsequential effect of the slightly lower combustion chamber pressure when damping a fire from the intake side of things (vs. a damper). And, as has been pointed out, no stove is airtight when the door is open!

In other words, an airtight stove doesn't need a damper.

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Yes but most EPA stoves have an unmetered secondary air supply so they are not technically air tight. Even with the air control shut there is still air entering the stove.
I have occasional overdraft situations when the outside temps get below zero, which is common where I live. I use a magnet to restrict the secondary air supply. I am considering adding a damper because reaching under a hot stove to cover the secondary inlet is cumbersome. It would be much easier to turn a damper lever.

I only use the damper in the flue pipe as a safety item. I have a flue temp probe...I do all normal burn adjustments with the air inlet control on the stove. BUT if the flue temp keeps climbing with the air inlet control fully closed this is when I'll adjust the flue pipe damper from 0 degrees (full open) to 45 degrees....and if the temps don't come down fast enough I'll go to near 90 degrees (full closed).

NOTE: I typically do not need to use a damper. But the times when I load a stove with small splits to get 'er goin'
AND I forget about the stove or get my attention interrupted....that's when the fire can go crazy. Again, the damper to me is a safety device.

thanks, Joe

The flue damper is absolutely critical to the safe and controlled operation of my stove. I use it every single time that I fire up the stove, which is 24/7. I've got the procedure down to a standard operating procedure now, and that requires me to start by adjusting the primary air rather quickly down to approximately 75 - 80% closed, and follow that by closing down the flue damper incrementally until it is almost fully closed, or perhaps 90% closed. Without this component of the stove I wouldn't be able to operate the stove safely, as I have a spectacular draft (running from the basement) which causes crazy crazy secondary's and constant overfires unless it was babysat at least half way through the burn cycle..... To each his own, however... some people need it and some do not, but for those that do need it, like myself, it is an absolute game changer.... It's nice to have control over the burn....

Laymans terms, air control is like a gas pedal on a car, the more you press it the fast you go, the more air you add the faster your burn. A damper is like a vehicle governor, you set it and it will only allow so much, even if your fully pressing the gas pedal.
Now most new stove don't require a damper, but some members here use them (including myself) to control excessive draft.

All stoves are meant to be operated and were tested within a given draft specification, even if the manufacturer doesn't necessarily state the specification. Damper is needed if draft is drifting outside of spec due to install, weather or burn cycle and air controls of the stove are insufficient to maintain control.

If one wanted to measure and regulate draft in a narrow window with a damper, your air controls could always be set more less the same but wood burning don't warrant that kind of sophistication.

I agree with what most of you with dampers say. However, my comment was predicated on the stove being airtight which, as some of you mentioned, modern EPA stoves are not.

And this is the only problem I have with the EPA regs. They require that enough air be let in at full choke that sufficient temps can be maintained to pass emissions. A knowledgeable operator could improve emissions and efficiency of many modern stove installs if they had full air control and stayed on top of the situation. But not all operators are knowledgeable or necessarily babysit their stoves well enough to make this practical or safe. This wouldn't be necessary if everyone had the same flue, same wood and same weather but, of course, we don't. The good news is that modern stoves are designed around typical installs and modern best practices for flues are designed around minimizing these differences so most installs work just fine.

The problem of chimney fire/runaway draft could be solved with an emergency shutdown (that would tightly seal all air intakes). This would probably be in the regs if not for the unavoidable fact that some ignorant people would routinely use it to shut down their stove when the house was warm enough and save the remaining unburnt fuel for their next heating cycle!

Dampers can be helpful in some situations with stoves that are not airtight (including all EPA stoves) but they can also be severely misused.

But, yes, it would be nice if regulations allowed a competent stove tech to adjust the minimum air supply to suit the particular installation because adjusting intake air is a safer solution than damping the exhaust. Some stove installs could safely use more air supply.

I'm wondering how long it will be before we get to truly "smart" stoves. These would have multiple temperature sensors, a tiny computer and three motorized air supplies and a motorized damper built into the stove. I don't see a reliability problem with this in the same way that all my computerized cars have been far more reliable than any of my old, dumb, carb'ed cars. And they burn cleaner at less expense for more miles.

Agree with your post.

Question: Is the full choke leakage also more of a safety issue?
Meaning, allow some draft to evacuate toxic gases (CO) from the firebox rather than maintaining operating temperatures? FWIW, during normal firing on full choke I'll slowly lose my secondary burn and the particulates (bad emissions) going up the flue will increase.

My experience in woodstove burning is only the last 10+ years. I've learned a lot trolling this forum and so I'm still
learning as I go. Thx, Joe.

WoodyIsGoody said:

I agree with what most of you with dampers say. However, my comment was predicated on the stove being airtight which, as some of you mentioned, modern EPA stoves are not.

And this is the only problem I have with the EPA regs. They require that enough air be let in at full choke that sufficient temps can be maintained to pass emissions. ......

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jhjove said:

Agree with your post.

Question: Is the full choke leakage also more of a safety issue?
Meaning, allow some draft to evacuate toxic gases (CO) from the firebox rather than maintaining operating temperatures? FWIW, during normal firing on full choke I'll slowly lose my secondary burn and the particulates (bad emissions) going up the flue will increase.

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My take on this is that modern stoves prevent shutting off the air supply completely to prevent smoldering burns and the resulting air quality problems. A side effect is limiting dangerous creosote deposits. But it's a "one size fits all" solution which may pass the lab tests with standardized flues and kiln dried fuel but every install is different so some people with flues that have weaker drafts have found they can't use the lower settings. Of course it also varies with the quality of the wood.

I don't think it's a CO poison preventing strategy because, from a CO poisoning standpoint, it would probably be safer to have complete air intake sealing vs. current situation due to the fact that with poor flue draft and/or damp wood the flue can still backdraft with a smouldering fire and yet there's no way to shut it down completely. This won't happen with a proper install and seasoned wood but, as you can see from reading stove forums, not everyone has a proper install and seasoned wood.

My experience in woodstove burning is only the last 10+ years. I've learned a lot trolling this forum and so I'm still
learning as I go. Thx, Joe.

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There's always a lot more to learn but the basics are pretty simple:

1) Install should meet code and manufacturer's requirements. This includes making sure the workmanship is good quality and the flue fits tightly at all junctions and accommodates the hot/cold expansion/contraction without causing small leaks.

2) Wood is properly seasoned (15-20% moisture) and is cut/split to appropriate size.

3) Fire is not starved of air until combustion chamber has come up to temperature. I like to err on the side of burning a little hotter rather than shutting down too early or shutting down too much for the situation.

These are the three basics that would solve 95% of the issues I see here. Of course there's always more to learn to fine tune the experience.