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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
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
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 hasnt been used much over the years.
The clay liner looked pretty clean when I did the install.
Ive 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 Im not home. She may never let me light the stove.
Any advice you could offer would be a huge help.
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. You can read more about
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. Could this be a problem with the stove design??
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. Do the circumstances surrounding your explosions resemble the circumstances in Kerry's situation?
Q: Yes, 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.
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 return the draft setting to medium-low just as soon as the
wood is kindled.
Re: Your comments on "wuffing".
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 Jutol 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.
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!
Sounds like your problem might be gusts of wind pressurizing the chimney, or the phenomenon we call whuffing, which occurs when you turn
the draft control down too far too quickly on a hot fire. If you think it is wind, you can read about the ultimate fix here. If you think it is whuffing, you
can read about how to avoid that situation on this page.
I think I have a whuffer. 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!
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