Well in my case the water heater was in the lazzerette so no heat gain to the living space. If the heater is located in an interior
space away from the hull
, you would get some small amount of heat gain from the heat loss from the water heater jacket. But if the water heater is located against the hull
, below the water line, say in the engine
compartment then most of the heat gain will excape through the uninsulated hull.
is a much larger heat source with about 38 percent of the energy used being radiated as heat. Even after running the engine for hours the net BTU gain to the living space is probably not measurable. That is the cabin
in fall or winter will still be cold. The water heater is a much smaller heat source, so again the waste heat, unless the heater is sitting in the open in the cabin
will be practically zero to the living space.
Based on some insulation numbers off the top of my head
, your probably looking at maybe 400 BTU per hour loss from the jacket. All that BTU's in an engine compartment that may be at 50 degrees and have 100 to 400 SF of walls. Only a small portion shared with the living space. Lets say 10 percent. So if the wall temperature was the same through out, then the living space would see roughly 40 BTU's spread out along 10 SF of surface area, or 4 btu's per SF.
But if the water is against the hull is cooler then the living area, which it usally is in the US and EU for example, then almost all the BTU's will escape to the water. So while it is possible that the water heater heat loss is contributing to the living space heating, for most the heat is just wasted to the sea. So says my HP50G calculator anyway.