Mikael Long wrote:Hi Erik
Do you know if there is a calculation to determine the amount of heat retained in a burn? Something like temp at the burn vs temp at the end of the chimney?
I'm sure there is. however I'm not an engineer. But for simple layman's use, I
think a measure such as you discuss is reasonable approach. I would add one element in front of that, which is the efficiency of the burning of the
wood and the exhaust (smoke, etc). I wouldn't get too wrapped in trying to capture that information however, because most of us cannot. If you can afford an analyzer that's a different matter.
Otherwise, I would use burn temperature as a corollary/analogue. If one really wants to get a good idea of what is going on, I think the easiest way is to embed thermocouples. Another option is to make tubes into which thermocouple probes may be inserted. The expensive route is to replicate how temperature is measured in kilns. I've seen ceramic tubes which are inserted into the kiln, and then the probe is threaded into that. This is one option.
Or, given thermocouples are relatively cheap, cast them directly into the build. They last as long as they last.
Or, build in copper tubes (or something similar) into which the thermocouple probes may be inserted. If these are filled with a fluid,
water comes to mind as safe and readily available, they will conduct heat through the interior of the tube pretty well I
should think. (I do not think this will work in core however, where temperatures are at their highest.)
Another important criteria will be fuel. It's moisture content should be measured and each piece should be weighed before being burned. Then the theoretical energy of the fuel can be calculated, and one will know the theoretical BTU (or kilowatt hours) of heat energy that is produced.
If one knows the energy content of the fuel, the temperatures at which it is burning (so percentage of conversion into heat may be estimated), and the temperature drop across the system, to the entry of the chimney, I would expect to be able to arrive at a reasonable estimate of efficiency.
But there may other methods that some of the older hands and experts can share. I'd be interested to hear more about how people determine these figures. And of
course, one can always run an Internet search and see what stove manufacturers do to test their efficiency. There's a
thread either here or over at Donkey's forum that shows part of this process. As best I recall it involved identifying the species of wood, weight and moisture content, and then temperatures at certain points in the system. It also included a gas analysis, which will be beyond most of our abilities to measure. From this data they arrived at efficiency figures.