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Chimney Draft Problems; Excessive Updraft
Q: I read [your] discussion on drafting. I am told that I have the opposite problem - too much draft. I recently had a new Lopi Liberty woodstove
installed with a stainless flue liner. When I fill the stove for a long burn, I have trouble reducing the temperature - it shoots to over 800 and
completely shutting down the air supply doesn't cool it until much of the wood is burnt. What are your opinions and possible solutions?
First, we should note that non-catalytic EPA approved woodstoves like your Lopi Liberty reburn the exhaust gases in a secondary burn chamber
near the top of the firebox at approximately 1200 degrees, so stovetop temperatures of 800 degrees from a freshly loaded fire are not unusual.
Second, it is a fact that today's EPA approved woodstoves aren't as adjustable as many of the pre-EPA airtights. In years past, we've had
experience with woodstoves that had air intake controls which could be closed all the way, resulting in a smoldering fire, excessive creosote
formation in the flue, and a thick plume of black smoke belching into the atmosphere. EPA approved woodstoves are designed so the fire can't be
smoldered: even when the air control is closed as far as it will go, the fire still gets enough air to burn efficiently.
That said, some EPA approved woodstoves are less responsive than others, and your complaint is one we hear frequently from owners of
non-catalytic woodstoves with extra-large fireboxes like your 3.1 cubic-foot Liberty: lack of controllability, resulting in hot, short duration burns and
the need for frequent refueling. We believe the problem most likely arises from XL-firebox stoves needing an air control that remains particularly
wide open even when turned down as far as it will go, to keep the larger load burning briskly enough so the stove will meet EPA emissions
requirements. Pacific Energy has addressed this problem by incorporating the unique EBT technology to their XL models, but that patented
technique is not available to other manufacturers.
When you've got a chimney flue with an extra-strong updraft attached to a stove with an air control that doesn't turn down far enough to
compensate, it can be very hard to control your rate of burn. If your Liberty is raging out of control, and you're unwilling to trade it in on a more
controllable model, the only remaining avenue of relief is to attempt to reduce your chimney updraft.
There are two techniques we know of to reduce chimney updraft, and both involve some attendant risk. Barometric dampers, often used in
conjunction with oil-burning furnaces, are installed in the stovepipe and have an adjustable, weighted flapper that is drawn inward by the updraft,
allowing room air to enter the pipe to reduce chimney updraft in much the same way as the thumb slide on a vacuum cleaner hose reduces suction
power below. The problem with barometric dampers is, the reduced updraft might adversely affect the secondary burn, reducing efficiency and
increasing emissions. Further, the intrusion of room-temperature air into the flue cools the flue gases, causing increased creosote formation.
Finally, if the increased formation of creosote leads to a chimney fire, the resulting extreme updraft will pull the barometric damper WIDE open,
and could allow the chimney fire to rage out of control.
The other technique is the manual stovepipe damper. A manual damper is a metal disc inside the stovepipe, attached to a handle on the outside.
Manual dampers reduce air flow through the stove by mechanically blocking the flue. By turning the handle, the disc may be oriented parallel with
the flow of exhaust (no resistance), perpendicular to the flow (maximum resistance), or at any angle in between. Manual dampers also cause
problems: reducing the airflow through the stove can adversely affect the secondary burn immediately, and will almost certainly do so later on, as
the fire burns down and the resulting cooler exhaust slows the chimney updraft. The price paid can be reduced efficiency, increased emissions and
excessive creosote formation in the flue.
Note: if you attempt to reduce your chimney updraft with either type of damper, you do so at your own risk. Only a few of the woodstove
manufacturers we know of recommend the use of stovepipe dampers, and then only in cases of extreme overdrafting: some manufacturers
specifically forbid their use.
Q: My compliments on your site... it is a true public service.
Last year I had puff-backs when the wind came from a certain direction. You sold me one of those [Vacu Stack] chimney caps that looks like a
Burger King crown and I had it professionally installed last spring. It seems to have resolved the problem...thank you. Hopefully you can assist me
again on a much more serious problem.
While he was here the chimney / stove professional also replaced the door gaskets, ash pan gasket, and gaskets around the glass windows of my
Encore Defiant stove (approx 9 years old) since last year I noticed some issues controlling the burn .
I started using the stove again for this season about a week ago, and it was perfect. With the control handle slightly more than half way closed, I
was maintaining 450 degrees (thermometer on the griddle top) day and night with virtually no variance.
Suddenly, today, I seemed to be having problems. In order to keep the stove from getting too hot I had to have the handle almost all the way back
(closed). Another 1/16" or so further it would drop below 300. Tonight I smelled that "hot metal smell" and went down to look at the stove. With the
draft damper closed for catalytic converter use... and with the airflow handle almost all the way back, the temperature was 700 degrees!!! Even
pushing the handle back all the way had no effect and within a few minutes it was at 900 degrees.
Next thing..I had a red glow in the stove pipe and sparks coming out through the chimney. I used a chimney fire "flare" which helped. Never the
less, the fire inside the stove was still burning fast . The fire didn't get down under 600 degrees for over an hour.
It was quite windy tonight but I don't think that should be an issue since it was windy earlier in the week and I didn't have the problem.
I don't plan to use the stove again until I know what's going on. I'll be contacting the chimney / stove repairman tomorrow.
SO.. here's my questions:
Could my airflow control be defective / worn?
Besides that control and the gaskets I described above, is there anything else that could cause this?
Is it possible that high winds from another direction over my new chimney cap could create a vacuum that would allow air to be sucked into a
properly sealed airtight stove?
Does the catalytic converter have any effect on this issue; should it be replaced (it never has been)?
Sorry this is so long... but I am so freaked out by this that I wanted to be sure to give you all the details.
Thanks in advance for any suggestions and recommendations.
Thanks for the inquiry, and for the kind words about our website! A Vacu-Stack draft cap creates an updraft whenever wind is blowing directly onto
it, and this draft gets progressively stronger as the wind velocity increases. It is possible that previous windstorms weren't as strong, or didn't blow
directly onto the cap.
It is also possible that your Defiant Encore isn't as airtight as it used to be. Vermont Castings recommends you completely disassemble the stove
and re-caulk all the seams with furnace cement every few years (if you still have your manual, I'll bet you find this requirement in the fine print
somewhere). It could be your stove is due for a rebuild, and the resulting air leaks through the seams are supplying enough combustion air to the
fire to allow it to rage out of control, especially when the wind is blowing and the Vacu Stack is increasing the chimney updraft.
The red glow on the pipe and sparks coming out of the chimney could indicate that you had a chimney fire, which can result from extra-hot stove
temperatures whenever there's creosote present in the pipe and chimney.
While you've got your stove apart for recaulking, you might consider replacing your catalytic converter. After nine years, your catalytic element is
long past due for replacement, especially after exposure to the emissions from a Chimfex flare-type fire extinguisher. Replacement of the
converter will reduce future formation of creosote in the stovepipe and chimney, helping prevent a chimney fire recurrence.
Q: About ten years ago I bought a Defiant Encore and was real happy with it initially. To be honest, I liked the look as it was just a very pretty
stove at the time and it also did burn for ten or so hours as promised. Initially, I could load the firebox before bed, turn the thermostat to its lowest
setting and wake up the next morning to simply re-stock the box.
Three years ago or so the stove became erratic. It would back puff into my house (you could actually watch the mini explosions you talk about and
it was really frightening) and occasionally, the fire would become uncontrollably hot no matter how I adjusted the thermostat. Having read alot,
cracked a window to compensate for negative pressure issues, regasketed everything and replaced almost every possible piece of the stove (not to
mention $3000.00 worth of chimney work)....I'm tired! I still don't trust the stove to burn safely while I am asleep, but having done all this work, it
seems such a waste not to be able to burn.
I am using the stove now, but no longer "fill the box" before bed, as I worry it will become uncontrollable: I put one good size log in it before bed
and wake up to a cold stove. At least then if it burns too hot, I can just open it. If I buy a new stove I am thinking about non-catalytic, but I worry
about the higher temperatures. I also worry I am going to encounter the same problems, as I don't understand why there is anything wrong with
what I have as it is built so simply and everything has been replaced.
My Encore was more than adequate for my heating needs and I don't want anything bigger. Should I down-size if I buy non-catalytic? Could you
give me a suggestion either to fix my issues or on a stove that is comparable to what I have (thinking about the Shelburne)? I do not want Vermont
Castings again as they have been "less than helpful" to me. Or tell me I am an idiot because all wood stoves are going to present these issues and
I should just convert to a gas stove.... Thanks in advance. Your web-site has been incredibly informative.
You mention you replaced several stove parts, but did you take your stove completely apart and recaulk all the seams with furnace cement?
Vermont Castings says you must do this periodically, and your symptoms certainly sound like excess combustion air is leaking into the firebox
somewhere. This is a messy, tedious job, accented by broken-off bolt heads and parts that don't quite fit back together the way they used to, but will
often solve the problems you describe.
If you do decide to replace your stove, rest assured that, although non-catalytic stoves achieve higher temperatures inside the secondary burn
chamber than catalytics, that doesn't mean you don't have control of the fire. The models we sell are all extremely tightly constructed, so you
control the amount of combustion air with the draft control. And none of the models we sell require periodic re-caulking.
When choosing a stove, the physical size of the stove isn't as important as its steady-state BTU output rating: you might want to refer to our
woodstove comparison chart, paying attention to the Average Output and Heating Capacity columns, before making your choice.
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