I’m having a difficult time wrapping my head around some wood stove concepts.
This is the wood stove we have used for our cabin the last two winters. Terribly inefficient and no way to turn it down after loading it up for the night other than using whole logs for a longer burn at night. What I do like about it is it warms up our house quickly. It has no firebrick or secondary burn. I have went through 5+ cord of wood in our little 400 sq ft house.
After building the evaporator and adding a secondary burn, I was able to realize the efficiency of wood burning while maintaining a boil.
Here’s my dilemma, If firebrick is used in a wood stove, is it used to store heat or insulate? My guess is the top of the stove and a single walled stove pipe inside the house is the only source of heat from a wood stove? If that’s the case, a blower fan stripping heat and pushing it around the room makes sense. What is the best way to utilize the heat from a wood stove?
My observation is that efficient wood stoves produce the most heat when up to gasification temperatures and damped down. I believe that makes combustion most efficient, and increases the gas temperature and residence time in the stove and nearby stove pipe allowing more heat exchange. Also, if the stove doesn’t have a cold air duct from outside, less draft limits the air infiltration through the building envelope. For maximum efficiency and comfort i recommend a designated air duct to feed the stove.
In another post I read a while back Steve U had mentioned that the fire brick is meant to improve combustion / gasification efficiency at the point of combustion. Hotter original fire with less draft needed will volatilize the wood better, and less air flow to achieve it does as described above.
5 cords could heat a 3 bedroom bungalow here, with an efficient stove.
Simple wood stoves are deceiving huh. You’d think us guys building advanced gasifiers could figure this simple technology out. haha. This is on our agenda now too, Im now working on scaling the small rocket stove up, to an in house heat system and also a boiler system version. What will make us different is we will pass primary combustion and pyrolysis gas through a carbon bed and then secondary combustion directly after. This should crack tars better resulting in better emissions from the stove while producing gas composition that is closer to what our gasifiers typically produce. So as stated above the intake air will be throttle back while still producing higher core temps and a cleaner secondary combustion process.
Look into the TARM or similar downdraft gasifier stoves, very simple (in principle), and very robust design.
Strawman and I were discussing fluidized bed combustion of small material, should work for waste sawdust without any additional processing, or wood pellets or chips up to a certain size.
I had a tarm boiler in my last house use it for 5 years. They are very efficient I was burning about 5 cord of wood and 300 gallons of oil with a regular wood that had a catalytic converter in it so about 80% effect and a regular oil boiler. I installed a tarm boiler and a cold start oil boiler in the house and heated the entire house and got my hot water with 4 cords of wood and maybe 100 gallons of oil at the outside burned on the few times I was away for a weekend or something like that.
But I will tell you if your tarm boiler gets cold like below 140 degrees f it is he!! To start the boiler pulls soo much heat out of the fire box that it is too hard to get it to burn.
I think I will try to build one of the boilers in the link below here I am try to figure out if I can find enough information to actually build it. I don’t feel confortable with just the photos below.
As to the op. Two things. First that stove looks like what I would call a pot bellied coal stove not a wood stove I know alot of people who use them for wood but it is hard to get a good clean burn in them.
Now to fire brick. A couple of things there. First it is about safety and stove life. The old steal stoves will burn out and develop holes. Depends on the wood and how hot you burn them but it isn’t a fun day when your fire falls out of the bottom of a steal stove. That happened here when I was a kid lucky for us the stove was in the basement on a cement floor.
But yes the second part of fire brick is about getting a hot burn. The hotter you burn wood the cleaner it burns and the more complete the combustion which means you turn that tar and crap that wants to plug up your chimney into heat for your house. The issue is ofcourse you don’t want all that heat at once you want a little heat all the time to keep your house warm 24 hours a day. This is where the very old concept of masonary heaters or Russian fireplaces comes in. Tarm gasification boilers work along the same basic lines with water as storage. But hack to masonary heaters. They have a core of firebrick with a outer body of brick or stone to make a big thermal mass. The idea is you burn the fire at upwards of 1800 F and the chimney path inside the heater is long enough that the smoke leaving the house is only 300 f or less. The heat is slowly absorbed by the brick and radiated out of the stove. I have a soapstone stove here which works on a similar theory being a high mass stove and it is much nicer then the steal stove I have in the kitchen. Now the steal stove will make heat way faster but the soapstone stove will provide heat for hours after the steal stove has gone out and is not providing any more heat. I have to keep a small fire burning in the soapstone stove all the time to really keep heat in the stone and in the house. This is the difference with a stove and a masonry heater a the masonary heater is more efficient and more importantly you fire it a couple times a day and that is it. The firebox is designed to hold enough wood to provide a full 12 hours heat in one loading and the brick work is sized to store that heat from a quick fire and slowly release it into the house.
So the short answer is to get good clean burning of wood in a stove you need to burn it hot. With enough heat and air flow the tars will burn up and the exaust will be clean your chimney will be clean and the stove is safer because there is less chance of a chimney fire. But you need some sort of mass be it stone brick or water to absorb all that quick heat and store it for later.
I would add some practical observations to the above. In sub zero climate, thermal mass isn’t a significant design issue, you want steady heat anyways, at times up to the limits of the stove design (ie minus 35). Where people run into trouble with wood stoves is when they are run at minimum load, ( spring and fall), slow smouldering fires. Chimneys tar up, causing dangerous issues, and obviously the wood combustion efficiency is much lower going up the chimney as particulates and tar.
I have to say that I hadn’t thought about the warmer climate scenario requiring cycled high efficiency burning and thermal storage. But I agree with the logic. In Bill’s case it doesn’t really apply too much, unless using a stove too big for the dwelling, which could easily be case given the small footprint.
Garry we can easily see -25 to -30 f here with day time temperature of only single digits. The advantage of a higher thermal mass isn’t lost with cold temperature. The system just needs to be designed based on how much heat you plan on needing on the avg winter day with 1 or 2 burns. My tarm worked best when I filled it twice a day with a 700 gallon storage tank. It would burn out the full load of wood dumping the heat into the storage tank. On very cold days I would fill it 3 times a day and early in the heating season I might only fire it every other day. That was the part where the tarm design comes up short because if you let the boiler get too cold it doesn’t start good. In my case I would pre heat the tarm from the cold start oil furnace. I understand what you mean about needing alot of heat I have an old farm house now which needs tons of heat. But the higher mass stove is better because I can go to sleep and know I have a few hours after the wood burns out before my soapstone stove will stop pushing out heat.
I agree with your analysis and the operating principle of the high efficiency boilers. The opposite of that efficiency is what gave the older outdoor boiler stoves such a deserved bad reputation. I have observed them close up.
I am assuming that as you go further south the weather and heating requirements fluctuate a fair bit, not so much the case further north.
I hadn’t thought of the issues with the TARM system getting up to efficient temperatures, but it makes sense. Really from my point of view that makes the case for designing the combustion process to run as close to steady state as possible, that will give the greatest efficiency.
Excellent posts already, so I won’t repeat it all. Two points I wanted to make:
First, any stove can be efficient if you feed it enough air. The hard part is trying to slow it down. “Airtight” stoves are NOT more efficient. They just slow things down better.
Second, and related - a too big stove will always need to be slowed down. Thus it will always be inefficient compared to a smaller one. Garry is right. That coal stove is probably too big for your tiny well-insulated space, at least in the milder months. If you can’t run it full blast, it’s too big.
Chris that is why I like the idea of some sort if thermal storage then you can burn full bore but only for a short time.
I am actually looking at doing some solar thermal with a wood boiler backup here to remove the need for oil in my mother’s house. She doesn’t want a wood stove in the house because she can’t run it at her age but solar would be ok. There are two houses here so I think I can setup a combined system maybe even for a shop and have one big solar hot water system on the barn that is backed up by a boiler. Just a concept right now. I point this out here only because what I loved with the tarm was that the hot water tank heated my house and provided my hot water for the house. So I got rid of the gas hot water heater. Just something to think about the down side to thermal storage is you need power to pump the water. But you can use any thing that can heat water to heat your system. That is why I like masonary heaters no added electric power needed.
Garry I was just reading this again and something I forgot to mention hit me. I think the Russian design for a wood boiler that has the water coil in the second bell of the chimney would eliminate the startup issues with the gasification boiler. My Tarm boiler and the other gasification boilers I have seen had fire brick in the bottom part of the fire box and metal exposed on the top probably 2/3 to 3/4 I would guess of the firebox. The metal was fire box on one side water jacket on the other side of the plate steal. So it really sucks up heat. That masonary boiler is a complete fire brick fire box so the heat exchanger coil being in the chimney bell beside the fire box won’t effect the startup the same way. Somone linked to that page has a write up where he talks about running both systems and he saids the same thing about startup and that the Russian design is much easier to start which makes perfect since. Now for the record my tarm would relight by just the heat in the stove for about 10 to 14 hours between filling depending on how cold it was and it took about a day and a half or so before it was down below 140 where it started hard. But from 50 to 140 in the basement for the first startup of the season that is no fun at all.
5 cords or face cords? Hard or soft? We heat 1300 square ft on 5 face cords but it’s well insulated. My take is the bricks radiate heat back into the primary combustion zone increasing the burn temp. I think most of the heat generation is coming at the secondary burn and through the top steel before it exits. Some heat leaks through the bricks once they saturate with heat. Secondary burn and insulation allow you to slow down how much air is required to maintain good burn temperature. In older stoves you could not turn them down or they would start throwing out creosote. You burned them wide open so lost tonnes of heat to the chimney. My personal read on them. I’m interested as to what Steve thinks.
Dave good points. The number I was mentioning is in cords of mixed hard wood about half oak. I am not sure what the difference between a cord or face cord is i was tough stack 4 feet by 4 feet by 8 feet is one cord of wood.
A face cord is 1/3rd of a cord. 16 inch by 4 ft by 8ft…
I know you all won’t agree with what I’m about to say. but I heated solely with wood for 40+ years. I bought or built every kind of burner there is; ciculators, cast iron, outside boiler, outside forced air, and more. The best wood burners hands down I have used are the sotz 55gal barrel kits, single for in a house, double for in an out building. They start making heat as soon as you light a piece of paper in them. If you keep water from coming down the chimney they will last for ever. The last one I used the barrel was in use every day for 15 years, and was still good when I took it out. I used less wood with it than any thing else.
Hey I’m with you Al
I made my first drum heater in 1975 . The top lid where you fed the heater was round and the same diameter as a 5 gal bucket . I could insert a 5 gal bucket of water down in the fill hole and the surface being heated was almost the whole outside surface of the bucket . Would heat water in a hurry .
I’m thinking for your tiny little place a rocket mass heater would be just the ticket. I remember seeing a video in which they had switched over to one and burned a fraction of the wood, and were more comfortable too.
I have no problem with rocket stoves. My only problem is when they pretend they are getting something for nothing. Even at 100 percent efficiency you are still getting roughly 7000 btu per pound. It might feel warmer but that is hard to quantify… I would rather incorporate the mass into my house in terms of double drywall thick slabs or cement board under tile instead of loosin floors pace to the RMH. Mass might be your ticket bill.
Best regards, David Baillie
Thanks for all the helpful info guys!
I am well aware of this current stove I have is a coal stove. It was all I had at the time. It really has served us well. It is my go to for reheating meals and keeping the coffee pot hot. The sad fact is that it is too inefficient. It gets so hot in here we end up opening windows in the Spring and Fall. I feel if I am able to turn it down, I would be able to regulate the heat better and use less wood.
I’m thinking I should be only going through 3 cord of wood in the winter for this little house.
A mass rocket stove heater won’t work in this house because the house is up on concrete pillars with a wood floor. I do like the thought of the stored heat to carry us through the night.
I kinda look a ty wood stove like a deisle engine. Air intake is open, output is controlled by fuel supplied, not only amount but size and surface area just like in a gasifier. And the throttle being the chimney damper. What kind of deisle don’t have a flywheel from he!! ? That is your thermal mass. David B listed some great examples for every home. With the mass and weight evenly distributed and awsome fire protection to boot. Not to mention the sound protection and structure al