Secondary burn, the hottest?

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Smokeless

Member
Hearth Supporter
Oct 20, 2008
35
Northeast CT
When burning a nice full load of wood (or bricks) lots of flame but not a nice full rolling fireball like a secondary burn.

I have a Regency fireplace insert. I can control the fire with the air adjustment (no flue damper control). So my question, do you get the most heat from running the air wide open or by cutting the air down and getting a nice secondary burn? I would think full throttle would give you the most heat, but is it burning so fast that the unburned (more fuel) is going up the pipe? Whereas the secondary burn is catching anything coming off the flame and adding to the output?

Just wondering...

I've experimented and it seems like secondary may burn hotter, but only at the point where the secondary begins. Too much air cut (with still a secondary burn) and the temp may drop a bit because the flame gets smaller...

Almost 3 pallets of BioBricks burned and little over a cord of wood burned so far this year...

Crazy cold is coming this week!

Thanks!
 
If you burn the stove wide open all the time your going to send alot of heat up the chimney, and your going to overfire the stove. Leaving the air open by passes the re-burn tubes. Your stove should be able to get an 8 hour burn time once you start to shut down the air control. If you dont allready have, it might be a good idea to get at thermometer for the surface of the stove. With the thermometer you will be able to determine when is the time to start to shut the air down. Eventuallly you will know the temperature of the stove with out even looking at the thermometer.
 
I think you got it about right, you get more heat into the room with the air wide open but you pay for it by sending a lot up the stack.

Cut back on the primary air some and the temperature in the firebox goes up resulting in a more complete burn but the mass flow rate thru the stove is much lower so the heat exchanger part of the stove has a chance to get that heat into the room before it goes up and out.
 
There does seem to be somewhat of a curve. Excess air just cools the stove down and the flue gasses rush out so fast they don't have much time to transfer heat to the stove. When you cut the air back some - to the point where you have 'just enough air' for what ever combustion is taking place, you eliminate the excess air and the flue gasses spend as much time in the stove as possible. This is max heat.

This is essentially the same as an engine...too much or too little air and you have crappy performance...get the air mix just right and get max performance.
 
I have gone off the reservation and "adjusted" the "non-adjustable" secondary burn in my VC insert. The law of diminishing returns now applies to the amount of air I allow into the insert. I can reach a point where, with increased air and fire, I get cooling and not heating, although some areas of the insert might suffer the effects of overfiring. I can "tweak" the air supply gradually and build up to 600-650 degrees sustained fire over a few hours. Once I get it there, I stop fiddling with the air supply "forever"... actually I recall where the knob was and readjust when needed to that spot. Depending on fuel, I may have to advance or retard that by a degree or two to get the same results. I watch my glass as with the insert that hot, the soot tends to melt off and the glass gets cleaner with time.

I get 6-8 hours on a load of wood, nice coals to start the fire with at the end of the cycle. Now, if only I could get the wood/coal boiler to carry a load of wood that long and make the same amount of creosote and soot on wood as the insert... (will never happen, temperatures don't even begin to approach the airtight insert).

On coal, with a good charge in it, I can get upwards of 8 hours, but not in zero degree weather. Need to redo the chimney piping with a damper, once that is set properly I'll see what happens. Might end up strictly coal in that baby. I like the heat, burn times, and clean chimney.
 
I get more heat out of my stove with the primary air wide open.

It stands to reason that some of the efficency is lost in this case, but when it is cold the object is the heat the house first and worry about the wood pile later.
 
I have the same insert. I usually let the stove warm up for 45-60 minutes after it's lit. Once the fire is hot, you can see the flames coming out of the tubes up on top of the firebox. Then I start to close the air control rod, first to 3/4, then to 1/2, then to 1/4 open. It slows the burn down and and allows good heat transfer. The blower on high really pushes alot of hot air out into the house. We really enjoy the heat and theres nothing like a real wood fire. ( Sorry pellet burners) Try shutting the air down when the stove is nice and hot and you'll see a difference.
 
Thanks for all the input. With the BioBricks I can get closeto an 8 hour burn because I have to throttle down the air or the stove would overheat. Wth hardwood, I can run it wide open most of the time but the burn time really suffers. Once in a blue moon I have to throttle back with the hardwood because the stove gets so hot.

As for thermometers, I have two... One on the face and one on the top. I know they're off (oven test) but they are a great reference for "norm".

I believe I know all the right burn spots on my air adjustment are. There are certain scraps in the paint... BioBricks, almost all the way closed, wood a couple of inches open, etc.

Other factor... When the stove gets to hot and I get that "Uh oh..." feeling, I can shut the air and turn the blower on high to bring the temp down.

So, I want more heat... turn the fan on high? Now the stove temp drops a bit, air doesn't have as long to heat vs. low blower where the air has plenty of time to cook?

Guess I'm looking to fine tune this here wood burning "engine". (Great analogy cozy heat!)
 
30 year wood burner, first season with a EPA stove.

My approach is to establish a stove top temperature that meets your heeting needs ( in the 300-600 range - indicated) and adjust the primary accordingly.

Cloose the primary as far as you can without dropping the stove top too low and leave it.

When the load burns down to coals and the temp begins to drop too far, reload and open up the primary until the temp gets to where you want and then shut down the air.

I think this is the most wood efficient way to go - you get heat from the temp of the stove surfaces not how many flames are dancing.

My biggest problem has been overcoming my habit of fiddling and poking the fire, which also sends heat up the stack.
 
Good point. I have seen some pretty nice temperatures where the fire just seems like it's really not doing too much, flame-wise. It's easy to forget it's about the temperature of the stove mass vs. the rage of the fire. The raging fire "looks" like it's warmer.
 
I have a smaller Regency I1100 insert and I also have found that the secondary kicks in better after I throttle down alittle. Then I will let the fire build up some more and throttle down again. I repeat this until I am at 1/4. Any more than that and the fire just smolders. I have found that I don't get a raging 8 hr burn but I will have hot coals for 8+ hrs.
I also have a problem of always fiddling and rearanging the splits !!! Something I have to work on. I am a first yr burner so still trying to tweak everything.
 
I haven't had more than 2 hours sleep yet this heating season! I even tried just turning the thermotat up to 70 and say screw it, I'm gonna sleep. it doesn;t happen and I throw more wood on the fire, second thought about using the furnace... I should be able to extend my burn times, I think I'm just fiddling with it too muc and giving it too much air. But it's fun! (Zzzzzzzzzzz....)
 
Smokeless said:
I haven't had more than 2 hours sleep yet this heating season! I even tried just turning the thermotat up to 70 and say screw it, I'm gonna sleep. it doesn;t happen and I throw more wood on the fire, second thought about using the furnace... I should be able to extend my burn times, I think I'm just fiddling with it too muc and giving it too much air. But it's fun! (Zzzzzzzzzzz....)

Yes ...I had the same problem when I first started earlier this yr. But then I found out that if you are throttled down below 1/2 you will get the longer burn times. Your house WONT be at 70 the whole night but you wont have to wake up every 2 hrs to reload. When you wake up....just throw some new splits on the hot coals and it should take off quickly. This is a small insert I have for a 1000 sq ft home and my temp will stay in-between 63 and 70 during the night. The window WILL get dirty though but once you get the fire going again it should clear up. Turning up the furnace wont extend burn times !!!
 
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