Reducing output of log burning stove

[sophiecentaur]

TL;DR Summary

My stove is too powerful. Should I solve this with firebricks?

My log burner is just too powerful for everyday use. It is spec'd at 5kW and has an area of about 200X400mm. It's a single fuel design with no bottom vent. The air control is quite clever. It has an 'air curtain' supply which comes down from the top and two adjustable slots in the doors to get it started.
Basically, when it's running at low enough power, it tends to die. I was considering putting two small fire bricks at the sides and limiting the burning volume.
Any opinions?

 

[dlgoff]

Summary:: My stove is too powerful. Should I solve this with firebricks?

I was considering putting two small fire bricks at the sides and limiting the burning volume.

I had this problem years ago when I had a wood burner. IIRC, I put several firebricks inside (maybe six?), and they were not small ones.

 

[jrmichler]

Wood combustion occurs in two phases. The first phase is combustible gases driven off and burned. It is necessary to have a hot enough fire to combust those gases. If not, you get smoke and creosote in the chimney. There is no way to avoid making a lot of heat in the first phase and still getting clean combustion. The second phase is red hot coals the burn directly to $CO_2$, with carbon monoxide as an intermediate.

I once built a wood stove with that same thought as the OP in mind. The bottom door is a plate welded to a V-shaped ash pan. Fire bricks inside form a flat bottom V shape ( \__/ ) with slots between the bottom bricks to allow air up from underneath and for ashes to fall down into the ash pan. The top door was a square plate torched out of the front panel with hinges and a latch welded on.

That stove got clean combustion, and made clean ash with almost no charcoal in it. During the first combustion phase, the top door warped open, adding just the right amount of overfire air. That phase generated a lot of heat. The second phase could be damped down, but getting reasonably steady heat required adding small pieces of wood at relatively short intervals.

Fire bricks on each side should help concentrate the fire, and allow a smaller fire to keep burning. My design resource was The Woodburner's Encyclopedia, by Jay Shelton: https://www.amazon.com/dp/0915248085/?tag=pfamazon01-20. A good, and readable, book on wood combustion.

Photo of that stove taken when it was newly installed in 1993.

 

[sophiecentaur]

I once build a wood stove with that same thought as the OP in mind. The bottom door is a plate welded to a V-shaped ash pan. Fire bricks inside form a flat bottom V shape ( \__/ ) with slots between the bottom bricks to allow air up from underneath and for ashes to fall down into the ash pan.

Thanks. That construction looks impressive. What is the box on the flue for?
My wood burner has no slots at the bottom; just a steel floor. You empty the ash after several days of burning. Air falls down from the top (having passed through a channel from a (isolated) vent under the stove, up the back and over the top. The warmed air falls onto the fire over the glass door. Burning is fine and the fire sits on a bed of well burned ash. The logs tend to burn all over their top so you see a red glow under the strange looking bluish flames, once you have adjusted the air vent for best running.
The multi-fuel stoves that were available all had air coming in from the bottom, of course.
You have convinced me to try the fire brick idea. Less burning area should produce lower power. small changes at a time, I think.

 

[jrmichler]

That box is a Magic Heat$^{TM}$ heat recovery unit. Link to a randomly selected retailer: https://www.homedepot.com/p/Magic-Heat-6-in-Bottom-Crimp-for-Wood-Applications-MH-6R/205672412.

There is a lot of heat going up the chimney when a wood stove is burning hot during the first combustion phase. I remember seeing the lower heat exchanger tubes glowing red hot.

 

[sophiecentaur]

That box is a Magic Heat$^{TM}$ heat recovery unit. Link to a randomly selected retailer: https://www.homedepot.com/p/Magic-Heat-6-in-Bottom-Crimp-for-Wood-Applications-MH-6R/205672412.There is a lot of heat going up the chimney when a wood stove is burning hot during the first combustion phase. I remember seeing the lower heat exchanger tubes glowing red hot.

Sounds interesting. Is there a control to turn it off? I've read stories about inadequate flue temperatures and sooting up but it would be good to reduce losses where possible.
I'm in the UK so I will try to locate a local supplier but I have never come across it over here.

 

[jrmichler]

Creosote buildup in the chimney is the result of incomplete combustion, followed by cooling in the chimney. If you have complete combustion, then you can cool the flue gas until water condenses without creosote buildup. Outside wood burners are popular in my part of the world (Northern Wisconsin, USA). These burners are typically loaded with several logs about four feet long, then the fire is regulated by a motorized air damper. The air damper is controlled by a thermostat in the house.
Owners like them because they can load the firebox in the morning, start a fire, and heat the house all day. Owners also like the labor savings of burning four foot unsplit logs. The fire is smoldering most of the time, and the burner emits heavy smoke. Up to 50% of the wood can be lost as smoke. The firebox is surrounded by a water jacket, so the walls are cold. I saw one such wood burner with a full two inches of dry creosote firmly attached to the firebox walls. That wood burner would be expected to also have heavy creosote buildup in the chimney.

Chimney fires are the result of chimney creosote catching fire. Creosote burns much hotter than wood because it is a hydrocarbon with very little oxygen, while wood is about 40% oxygen.

Wood smoke consists of a mix of heavy hydrocarbons, including polycyclic aromatics. Polycyclic aromatic hydrocarbons are the reason tobacco smoke causes cancer.

If you burn wood hot enough so as to not smoke, then you can cool the flue gases as far as you want without getting soot or creosote buildup. the minimum flue temperature is that which creates just enough draft to keep the fire going. The Magic Heat$^{TM}$ has a thermostat so it only runs when there is a fire.

Randomly selected internet photo of an outside wood burner:

 

[sophiecentaur]

That box is a Magic HeatTM heat recovery unit.

They seem to be unobtainable in UK but people seem to like them. Your behemoth of a stove would not fit in my living room as there is an aesthetic requirement too.

I went down the road this morning, to the dealer who supplied the stove and he sold me an off-cut of 'fibreboard' which I cut and fitted in the fire bed. I will see how we get on tonight with it. It should run hotter for a reduced power output, I think.
On the same topic, I have been using a 'Peltier' stovetop fan which doesn't;t perform very well. A new Peltier block has improved things but I have now identified the probable reason as being a tiny ΔT across it. I think a much bigger heat sink would sort things out. More experiments to do.

 

[sophiecentaur]

It should run hotter for a reduced power output, I think.

On a subjective basis, I think the stove now works a lot better. It no longer makes the room too hot. The glowing red / blue flame conditions establishes quickly and the flue temperature is at the low end of the 'optimum' range on the Valiant flue thermometer. Success for an outlay of <£10.
When the parts arrive, I will try to improve the stove fan.

 

[gmax137]

My design resource was The Woodburner's Encyclopedia, by Jay Shelton: https://www.amazon.com/dp/0915248085/?tag=pfamazon01-20. A good, and readable, book on wood combustion.

Wow that really took me back; Jay Shelton was my physics professor (classical mech out of K&K) in 1975. He was very into wood stoves even back then. And frisbees .

Now, back to the thread in progress...