Rebecca Norman wrote:I asked for recommendations for back-up heat in my little solar-heated house that has plenty of thermal mass. People explained that a rocket mass heater heats up slowly and stays warm for a long time, so it's not the best choice if you are only going to need extra heat, say, quickly for a few hours, or during a cold snap in the springtime. It's the best choice if you are going to be using wood as a major source of heat all winter. I don't know if that addresses your question, though...
Satamax Antone wrote:Jean Paul, may be an all brick Bell is to consider.
Glenn Herbert wrote:I will second Max's recommendation for a bell system rather than the traditional barrel over the heat riser.
If you have a house where you can get good natural draft without priming the chimney most of the time, you may be able to build a barrel-based system with the barrel largely encased in cob, to reduce the direct radiation fraction. A tall chimney completely within the house will be most likely to work this way.
Glenn Herbert wrote:Another point to consider is that any house has the same issue with regard to the fraction of heat output from the barrel, it's just a larger amount on both sides of the balance. So I tend to think that a RMH will perform similarly in a superinsulated house as in most current installations. For one thing, you would probably not do the fire in one four-hour burn, but two two-hour burns or the like.The heat would not build up so much while burning, and if the house materials have mass too (like lime plaster or stone on the interiors), that will absorb immediate radiation and act as additional thermal storage mass to mitigate temperature swings.
Glenn Herbert wrote:Also, you are not getting the heat output through the mass while you are burning, for the most part, so it's not like an additional dose of heat from the barrel, but substituting for mass output. The mass output starts to take over after the burn is done, depending on how long you burn in a session.
Glenn Herbert wrote:Cold days control how much system capacity you need, but there will be many more milder days where you will need half the total heat output. In my house with some passive solar features (but no real solar mass), a cold sunny day can warm the house until after dark with no morning fire at all.
Peter van den Berg wrote:Our home is super insulated, a real passive house in a moderate sea climate. We heat it with a high-mass batch rocket heater, no barrel at all. Direct heat is estimated as 10% through the glass door, all the rest is stored. See http://batchrocket.eu/en/applications#redbell Maximum delivered energy 3.6 kWh, or 12.3k BTU/hour. When applied to your situation the max heat output should be somewhat higher so a slightly larger heater is in order.
Glenn Herbert wrote: and not expensive if you can get used bricks for the inner shell.