stoker-man wrote:I've inquired about this efficiency question quite a few times with quite a few people.
Their answer is: It's not an oil system. You don't have inefficiency at start-up or shut-down. All you have is a blower going off and on and the coal burning or not. I have to agree. With my training in the oil end of the business, I know full well about combustion efficiencies. With coal, I don't see it.
If you're talking system efficiency, you are correct. 40 gallons of water is not necessary for a modern house. I keep my house at 70-75 all winter with a 60K wood boiler, heating over 2000 sq. ft. and I don't even fire it that hard. But, with coal, the fire either stokes or rests. I don't see any combustion efficiency improvement by constant running.
I will listen to any opposing opinions.
Combustion efficiency for any fuel is the highest when the correct amount of air, i.e. oxygen is available. In coal appliances there is no provision for adjusting the amount of combustion air except, depending on design, an on/off control. There is a fundamental problem of determining the correct amount of air when the fuel (coal) input is so variable. Current coal appliances have no way to determine this.
Since air contains large amounts of nitrogen and nitrogen is not part of the combustion equation it just passes through the appliance. You want to limit the amount of extra air, because it takes heat with it, by heating the nitrogen in the air. That's one of the purposes of a barometric damper.
Some day when coal becomes expensive, coal burning appliances will be re-designed to measure flue gases, with a sensor similar to an O2 sensor in an automotive exhaust system. That sensor will provide the feedback to adjust the combustion air to the ideal amount. Until then we just waste fuel.