Can a wood stove be efficient? Is there a way to increase the efficiency of a wood stove?
Not everything in our house is high-tech. For those cold winter days that are not sunny we have a low-tech method to warm up the house.
Just because a wood stove is considered low-tech it doesn't mean that it should be inefficient. We started with a Morsø 2110 stove that was 71% efficient (test according to Australian Standard 4012) and improved its efficiency by installing a non-standard flue and a cold air intake.
The long and non-vertical stainless steel flue helps to cool down the hot fumes before they fly away, and hence helps to retain more heat from the burned fuel in the house interior.
This is especially noticeable when we start the fire. While the firebox is still cool to touch, the flue radiates plenty of heat.
The increased surface area of heat-radiating elements of the stove increases the amount of heat delivered to the house interior, decreases the amount of wasted heat escaping through chimney and hence increases the thermal efficiency of the stove.
Normal wood stoves draw air for combustion from the room that they try to warm. The result of this strategy is that the hottest air in the room is put directly up the chimney ! If we supply air from outside directly to the combustion chamber - the hottest air around the stove stays in the room to warm it up...
We estimate that with the long non-vertical flue and cold-air intake the efficiency of our wood stove approaches 80%. This efficiency is nearly 4 times higher than efficiency of the electricity production from fossil fuels such as brown coal !!!
Some people argue that there are no benefits for using outside cool air intake for wood stoves and that the best source of combustion air for wood stove is the room in which people live and try to heat. If taking combustion air from where people are present was really a good idea, why combustion air for car engines is not drawn from inside car cabins?
An outside cold air intake in the ash compartment of the Morso 2110 wood stove is installed in a way that we can compare behavior of our stove with different combustion air supply just by opening and closing valves without touching the fuel or altering the fire under various atmospheric and other conditions.
From our experience the cold air intake is amazingly better that a room air intake. There is no comparison. I actually see no reason to use an interior air intake for our wood stove...
Please read the pdf article describing our wood stove cold-air-intake in detail.