Smoke "puffing" out somewhere on secondary burn...

[brider]

My Harmon Oakwood was very hot this morning, stovetop thermo read 650-700 deg, so I closed down the damper to ignite the secondary burn and turned down the air lever to about half.

I was expecting the temp to drop after 10 min or so like it usually does when I close the air down so far, but it stayed hot, and I wasn't really concerned because I had a great coal bed going and a load of dry slab hardwood.

So I told the kids to watch the glass and see the flames occasionally WHOOSH and then die, WHOOSH and then die, etc. But this time, when it WHOOSHED, a puff of smoke came out the back, near the bottom.

What's up with that?! I asked my wife, and she said, "oh yeah, I've seen that before".

Now, I've also seen the "whooshing" during secondary burn, thinking it was a cyclical buildup of gasses that rythmically ignites then goes out. It's kind of beautiful to watch, but I've NEVER seen the smoke before.

My theory: This was a big, hot fire, so the "whooshing" produced sufficient internal force with the damper closed that it forced smoke back out the intake vents.

But then: I thought the intake on my Oakwood was near the FRONT, not the back where I think the smoke came from.

And no, it's NOT coming from the ashpan door, or front door, or top door.

Thoughts?

 

[Brian VT]

I'd guess the smoke comes out the secondary air intake. I get it sometimes, too, but it comes out the griddle top.
I guess what happens is that alot of wood is burning and when you decrease the available air that amount of wood is still trying to burn.
Due to the lack of air, unburnt gasses build up and, eventually, they get enough air to ignite in a burst. It's called backpuffing.
It usually stops happening after the burn rate adjusts to the available air (parts of the load stop burning).

 

[brider]

Makes perfect sense!

Thanks for the sanity check!

 

[EddyKilowatt]

By "damper" do you mean the primary draft control, upstream of the fire... or do you mean a flue pipe damper, downstream of the fire?

From your description, I'm guessing upstream... but if you have any kind of restriction *downstream* of the fire, that could definitely contribute to smoke being forced out your secondary intakes when gas builds up in the firebox and tries to light off.

Eddy

 

[brider]

By "damper", I mean the flue butterfly downstream of the flame. By "air control", I mean the primary air upstream of the flame. The previous poster makes sense; if I have a roaring fire and slam closed the flue butterfly (and force the combustion thru the more restrictive secondary combustion process), I would imagine the instantaneous internal pressure would rise, and when the gas DOES ignite, there's enough pressure to force back out the air intakes. In the future, I'll close down the primary air FIRST, let the flame die down, THEN close the bypass damper.

 

[Brian VT]

I find that lowering the primary air in stages, rather than all at once, alleviates the backpuffing.
Sometimes, though, the load will have been cruising nicely for hours and then it starts backpuffing for a little while.
I guess what causes it during the heart of the burn is maybe the load shifts and more surface area of baked wood is exposed and tries to burn.
I no longer bother with it. I even enjoy a little bit of smokey smell in the house. (it, hopefully, masks the dog smell)
I don't think it's a safety issue for me since it doesn't last long, the griddle releases the pressure safely, and it's never been enough to set off a smoke or CO2 alarm.
I have heard of it blowing a flue connection apart so you might should address that possibility in your system. It sounds like the griddle on your Oakwood must be heavier than the one on my Resolute if it's not lifting during a backpuff. Screw those pipe joints together if they have the potential to get seperated by backpuff pressure.

 

[begreen]

These stoves are fussy about draft. They work best when it's just right. From the description, the backpuffing may be due to marginal draft. From the manual:

"Too much draft may cause excessive temperatures in
the stove. On the other hand, too little draft can cause
backpuffing into the room and/or insufficient heat."

 

[LisaP]

Last night about two hours after we had put our stove into AB we went to bed and began smelling smoke, I came out to check the fire and watched the rolling flames come out of the sealed front door and leave a huge puff of black smoke in the room. We are burning very well seasoned oak, this is the best wood I have ever burned, I only need to get the flu temp up to around 800 and the stove is keeping temps in AB of 4 to 500 for hours. Very concerned about the flames and smoke, any thoughts would be great. Also when I put the stove into AB the wood continues to burn with flames, with all of the other wood that I have burned I would just get glowing coals. Thanks.

 

[begreen]

Can you check the flue cap and top pipe section for restriction? This might be a sign of the pipe starting to choke up. Or it could be a case of the stove air being closed off a bit to much and a bit too early?