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Sweep's Library: Whuff's the deal?

Q: I find your site very informative, but still have a question. Last year I was experiencing puff-backs, particularly on cold evenings, when I set back the Vermont Castings Defiant for the evening. The chimney is very tall and oversized for the stove. I intend to install a liner. Will the liner solve the puff-back problem?
Jim Cunningham, Ticonderoga NY

Sweepy Hi Jim,

Thanks for the kind words! Chances are your back-puffing episodes aren't being caused entirely by the lack of a properly sized liner, although the extreme updraft that can be created by an oversized flue could certainly be a contributing factor. For this and other reasons, you should install a liner that's the same size as the exhaust opening on your stove, extending all the way to the top of the chimney.

When you have a rip-roaring woodstove fire going, your chimney is charged with rising superheated exhaust gases, so the air flow through the firebox is considerable. If you cut down the supply of air too abruptly, the fire instantly consumes the available oxygen, creating a powerful vacuum inside the stove. If strong enough, this vacuum can reverse the flow inside the chimney, pulling a "gulp" of air back down the flue into the firebox. When this pocket of air hits the fire, a mini-explosion occurs, and the resulting sudden extreme pressurization inside the firebox forces smoke out through the draft control, door gasketing and other tiny openings that exist in even the most "airtight" woodstoves. This brief period of pressurization is followed immediately by extreme depressurization (because the explosion consumes all the available oxygen in the firebox), and another gulp of air can be pulled down the chimney, causing the process to repeat. We call this "whuffing", due to the accompanying sound of muffled explosions. In extreme cases, these repeated explosions can cause the stove to actually dance around on the hearth!

Essentially, a whuffing woodstove is doing the same thing as a valveless pulse jet engine (read more here.)

Although whuffing usually only occurs for a short time (until the starved-for-air fire dies down, reducing the vacuum effect), it should be avoided, as the repeated pressurization inside the stove caused by the mini-explosions could fill the house with smoke, blow the door open, break the viewing window, disconnect the exhaust pipe, or damage the stove.

Whenever you've had a hot fire going and want to "bank" it down for the night, care must be taken not to cut down the air supply too suddenly. Adjust your draft control to, say, half throttle (you'll have to experiment a bit to find the setting that works best for you) for a few minutes, then, as the flames diminish, continue to turn it down in gradual stages, so the fire can quench down slowly. If the stove starts to whuff, open the draft control just a bit to supply more air to the fire for a few minutes, then resume your gradual reduction of combustion air until you reach your all-night burn setting.

Another cause of whuffing is when you've been burning the woodstove with its door open and then close it while the fire is still burning briskly. The best preventative measure in this case is to NEVER BURN AN AIRTIGHT WOODSTOVE WITH THE DOOR OPEN. If you've already gotten yourself into this situation, let the fire die down before trying to close the stove door. When you do close the door, make sure you open the draft control all the way first, to avoid abrupt air starvation to the fire. Once the fire is under control, commence gradual reduction of combustion air with the draft control as outlined above.


Q: I recently installed a Jotul 400 Castine woodstove in my fireplace. I used a flex pipe that extended up to the clay liner. After loading the stove for the night, I sat back with my wife to reflect on how beautiful our stove was. Just about then, the glass blew out in an explosion, spraying wood, coals and shards of glass six feet across the carpeted room. WHAT THE ****!

A few observations:

My chimney is 15 years old and hasn’t been used much over the years. The clay liner looked pretty clean when I did the install.  I’ve been using the stove for a month and never saw event a hint of draft problems.

I had no concept of whuffing and commonly closed the door abruptly on a raging fire. When starting fires, I commonly used the ash pan door to stoke the kindling (again, closing the door abruptly).

On this occasion, the stove had been burning all day. I had just filled the stove, piling large split logs onto very hot coals.  My wife actually commented that I had overfilled the stove and might put the fire out.  I left the door ajar with the damper fully open to rekindle the flame.  Once I had aggressive flames, I closed the door, leaving the damper open.

My block off plate was blown out of place. Most of the fiberglass used to seal around the edges of the plate was blown onto the hearth.

After the ensuing nightmare of a fire drill (I was proud of my wife, she only ran one full circle in the kitchen before grabbing the fire extinguisher), I sat dumbfounded trying to figure out what happened. I came across your site and learned about whuffing.

This can never ever ever happen to me again. I want to take all of the necessary precautions.

Should I run a stainless pipe all the way to the top of the chimney? Did the exposed fireplace firebox filling with flue gases contribute? My wife is thoroughly freaked out. She may never light the stove again while I’m not home. She may never let me light the stove.

Any advice you could offer would be a huge help.

Kerry Kearney
Plymouth, MA

Sweepy Hi Kerry,

Sounds like you had a BIG whuff, alright. But I think with a little care, you'll be able to avoid a recurrence.

First, there should be no reason to leave your stove door or ash drawer cracked open to kindle a fresh load when you already have a bed of coals. Too much combustion air can create a "blacksmith's forge" environment inside the fire chamber, which we call overfiring.

Any attempt to reduce the air supply to an overfired stove can easily result in the creation of an extreme vacuum in the fire chamber which pulls a big gulp of air down the chimney, causing an explosion when it hits the flames.

Extending your stainless liner to the top of the chimney would be a big help, as an oversized fireplace chimney that's been heating up all day can create WAY too much updraft, contributing to the likelihood of overfiring. Another thing you can do to avoid overfiring the stove is add new pieces of fuel one at a time over the course of a few minutes instead of all at once, to keep your fire under control. Finally, to prevent future explosions, avoid any abrupt reduction in combustion air: when you want to cool down a hot fire, turn your draft control down in stages, a little at a time.


Q: I read [the letter above], and am concerned. I too have a new Jotul 400 castine and have had several scarey explosions blowing out the gaskets and ashes all through the room with the doors shut. I open the bottom ash door to get new logs started on a bed of coals; after the logs catch I shut the door and thats when the stove blows up. Quite freightening and very dangerous. I went to Jotul website but they have no contact info to report this problem. Could this be a problem with the stove design??

Sweepy Hi Andy,

The scenario Kerry described above would tend to cause violent whuffing in almost any airtight, so your stove being the same model could be mere coincidence.  You said you read the letter above, but did you read our response? You don't want to leave the ash drawer open when you've just added a fresh load to a hot bed of coals. The resulting uncontrolled burst of combustion air will cause exactly the kind of overfiring that leads to explosions when you abruptly close the ash door.

In the future, open the draft control, not the ash drawer, when you add a fresh load, then gradually return the draft setting to medium-low just as soon as the wood is kindled.


I have a bad habit of opening the ash door to get newly inserted logs hot and have had many stove "explosions" as a result. I use a Jotul F3 CB and feel lucky that the glass door hasn't shattered as a result.

I wasn't aware of how widespread this problem was until doing a Google search on it and finding the discussion on your web site. The information was very useful to me and I won't be using the ash door again to quick start the stove.

Ben DeIpolyi


I have a Englander free standing wood stove I have used for 29 years. It burns 24/7 when it get cold. The last couple of years it has started to puff out a little smoke out the front dampers. It happens when it is choked down. It will puff about every 10 min, but not all the time very irregular. I clean the stove pipe and masonry chimney every fall. My wife is ready to shut it down if I can't fix it! After reading your website, I am glad to find out it is just operator error, I can fix that. Thanks for your words of wisdom. I am sure you have saved my marriage and hundreds of thousands of dollars in electrical bills!

Thanks again,
Gary Cate


I just wanted to say thank you for your forum and puff back information. I now know I caused it and the firemen learned what one was. While waiting for them to get here I googled what happened and your site came up. By the time they got here the flames were subsiding and I thanked them for coming and read your information to them when they said they never heard of the term. My mistake was putting in a large very dry piece of wood which flamed up to much for my liking, so I closed the wide open damper most of the way to slow fire down. Sat down in easy chair and the explosion was violent enough to blow out the insulation on all 3 sides. My next thought was smoke would fill the room from the sides so I quickly opened the damper all the way. Had I not done that it would have done it again. I could not believe how long that piece of wood burned hot and kept thinking something off of house must have fallen into fire. I did figure out that the wood had sat underneath my grill for years and most likely had gotten charcoal lighter on it more than once. So the motto is, be careful that nice old wood has not been exposed to some chemical. No more wood that looks like it has oil on it either, as I have burned those and sparks would fly everywhere. I am a 63 year old female that lives alone with 3 cats. You have pointed out other mistakes I have been making that could have been lethal. I am going to be so much more aware of safety issues now.

Sandy Becker


Thank you for your thoughtful and well-articulated responses re: back-puffing in wood stoves. We've been experiencing this on and off (but increasingly so) for the past couple of years. This website was the first place where we heard our situation described precisely and an appropriate solution was prescribed. In can be really hard to find such information on the vastness of the internet but we're really grateful to have stumbled onto your website. Thanks and please keep it up!




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