Quick Answer
To size a rocket mass heater for your living space, determine the total BTU load of your space, then use the heater's efficiency to calculate the required combustion chamber and riser size.
Calculating BTU Load
To calculate the BTU load, determine the insulation level, window size, and desired indoor temperature. For a well-insulated space with no windows, use 10-20 BTUs per square foot. For a moderately insulated space with some windows, use 20-30 BTUs per square foot. Multiply the total square footage by the BTU load per square foot to get the total BTU load.
Sizing the Combustion Chamber
The combustion chamber is the heart of the rocket mass heater. A general rule of thumb is to size the combustion chamber to handle 10-20% of the total BTU load. This allows for a stable burn and efficient heat output. For example, if the total BTU load is 10,000 BTUs, the combustion chamber should be sized for 1,000-2,000 BTUs.
Sizing the Riser
The riser is the pipe that carries the hot gases from the combustion chamber to the heat exchanger. The riser should be sized to handle the maximum flow rate of the hot gases. A general rule of thumb is to size the riser to have a diameter of 1/4 to 1/2 inch per 1,000 BTUs of heat output. For example, if the heat output is 5,000 BTUs, the riser should be sized for a diameter of 2-3 inches. Make sure to account for the riser’s height and any potential flow restrictions when sizing the riser.
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