I am starting this thread after googling and finding very little on the subject. It is frequently demonstrated on youtube etc. that the exhaust or flue gas temperature of a rocket mass heater (RMH) is near ambient and does not smell smoky, which indicates both that the burn is complete and that the energy from combustion is somewhere inside the dwelling, which is where we want it. This is really an easy test to determine efficiency at the high end of the efficiency range rather than worrying about kg of wood burnt, temperature of inside, and all the other calculations. Ordinary slow combustion stoves are obviously inefficient, due to the smoke and creosote produced. In complete combustion stoves, the efficiency is determined by the exhaust gas temperature. If it is not room temperature (or better yet, outside temperature if you are exchanging heat with the incoming air), then there are efficiency gains to be had.
1) Is it smoky? and (If smoky other than during initial start up time, e.g. 5 minutes or so), then obviously we do not want it as it is inefficient, bad for the environment and bad for human health.
2) What is the temperature? Lower is better, provided that the system works.
So, to that end, I'm asking for anyone who has experience to comment on the exhaust gas characteristics (smokiness and temperature) of masonry heaters, and some of the more exotic European style heating devices such as the wood gassification boilers with thermal storage. Thank you.
I asked Dan Alan questions about his own masonry heater here. Apparently the final exhaust temperature gets up to around 200 degrees, which I assume is Fahrenheit and would be 93°C. This seems to be very good performance, but it starts out at only a little above room temperature so with denser thermal mass (e.g. basalt, water) or a PCM such as paraffin wax, it might be capable of maximally efficiency over a longer burn time or be able to hold the heat for longer. Still, I want to emphasize that this is excellent performance compared with pretty much anything else but a rocket mass heater, though I would like to see what the exhaust temperature of one of those is at the end of a burn.
Another design I came across that had exhaust temperatures listed was the Cornish Masonry Stove, where the temperature of the exhaust gas is listed as 350F, or 176°C. To quote:
By the time the air exits the chimney it has cooled to around or below 350 F (meaning no wasted heat).
Obviously there is wasted heat, because 176°C is not the outside temperature. But still, kudos on actually giving out some figures that are likely legit. They have a very interesting modular design.
I have just burned my brand new Masonry Heater for the first time ever... so I can give an example. It is currently 10 deg. F outside, and the interior temperature is about 60. After burning about 25 lbs of firewood in the heater for two hours, the exhaust temperature is about 60 deg. F. I expect this will rise as the mass begins to saturate with heat, but quite simply put I am dumbstruck!
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