Wood Stove Efficiency
Wood stove efficiency is a legitimate issue to consider because it speaks to the conversion of firewood into room heat. More efficient wood stoves burn less fuel to give off the same amount of heat as a less efficient model. Frugal living suggests that you get the most heat from the amount of wood you burn.
Wood stoves aren't very efficient, no matter how you look at them. The main reason is you can't really extract much heat from the stove because the outside isn't an efficient heat exchanger.
There just isn't much surface area for extraction and distribution of heat like you'll find with a radiator or arrangement of cooling fins.
In a gas furnace, a tremendous amount of surface area can be found on the heat exchanger because there isn't the issue of fouling like there is in a wood burning appliance. If you tried to use a highly efficient heat exchanger in a wood stove, it would soon foul up so much that it wouldn't be a heat exchanger at all after only a few days use.
In comparison with a fireplace insert, wood stove efficiency is typically higher. This is because the entire stove is inside the area you want to heat. Fireplace inserts are only partially inside the area you want to heat. The rest of them is inside the fireplace.
With the additional surface area, heating with a wood stove becomes easier because you can make better use of radiation and convection. An insert needs a fan to be effective whereas getting heat from a wood stove doesn't necessarily require a fan, but will benefit from the use of one.
Older wood stoves might give you 40% efficiency whereas more modern stoves reach about 75% efficiency.
Also, the less expensive the unit, the less efficiency you can expect. Lower cost units are generally iron boxes where wood is burned, and not much more than that. They won't give you any greater efficiency than stoves of several generations ago, because they have designs that are similar to those of generations ago.
As a friend of mine says: "...these stoves are just thrown together..." and, in many respects he's right about the cheaper wood stoves on the market. The focus is on making a stove to burn wood, and not on improvements in wood stove efficiency.
More expensive units are generally designed to be good scavengers of fuel, especially the hot unburned gases. Therefore, they tend to burn hotter, cleaner, longer and have far better heat output for the wood used.
The more expensive higher technology stoves are going to give you the best wood stove efficiency because they focus on creating several zones of combustion, and they introduce combustion air in specific places to promote a hot and complete burn. They're more efficient, and they're considerably more expensive too.
One of the keys to wood stove efficiency is heat. The hotter you keep your stove (within reason) the more efficient the stove is at burning the gases that are otherwise sent up the flue as soot.
Heating with a high efficiency model might have you spending about $2,000 for a new stove. I think I would opt for a corn stove before I would spend that kind of money on a wood stove. Corn stoves appear to be effective, convenient and much less labor intensive.
Nevertheless, heating with wood stoves can be made more economical if you don't have to pay for the wood. For some of us, that can make the whole wood stove efficiency issue melt away. I enjoy the thought of heating with wood, regardless of the efficiency of my wood heat appliances. Heating with corn just doesn't appeal to me.
To increase heat output from a stove you can use a fan to blow air across one or more hot surfaces. Blowing air across the stove pipe will also increase the output of a stove, but can promote build up of creosote in the stove pipe as the cool air tends to promote condensation of hot gases on the inside of the pipe.
Use caution anytime you try to extract heat from the stove pipe. A good general rule is to keep the exhaust gases elevated in temperature, and rob heat off the stove box only. This is a good approach since the stove box will be subject to high temperatures that will burn off accumulated creosote quickly before it builds up to become a fire hazard.
Also, when trying to improve wood stove efficiency with a fan, make certain the fan isn't drawing air out of the firebox or forcing air into the firebox at such a rate that it forces some of the air in the firebox out into the room. This will cause combustion byproducts in your living and breathing space, and you certainly don't want that.
High efficiency gas heating appliances can reach 96% efficiency. Don't expect any kind of efficiency like that from a wood stove - ever. The technology just isn't there. Be happy with wood stove efficiency of 50%, and use a fan to improve it's efficiency, or make an investment in your plans for frugal living by opting for a higher efficiency wood stove with a higher price tag.
Done with Wood Stove Efficiency, take me back to Heating with Wood