Marine space heaters are not typically rated for efficiency by any approved method, and Internet folks are forever commenting on what is efficient or not efficient, without any engineering basis or understanding of the math. But it isn't complicated and engineers have long known you can calculate everything you know from the stack conditions. After all, what goes up the stack didn't go into the room.
I described my heater installation and early experiences, but I only estimated the efficiency without rigorous measurement. Time to fix that.
Stack Temperature. Obviously, cooler is better. Since the stack temperature on my Dickson P9000 is 185F on a cold day (40F), that gives me a starting point efficiency of 88.5%. However, since the Dickson unit preheats the combustion air with flue gas, this gives a 4% increase to 92.5%. So little, you say? In effect, I'm simply moving the exhaust temperature 200F. It is worth doing simply to keep the hull fitting cooler and to reduce burn potential on-deck. More importantly, while dropping the temperature of the exhaust 200F is not a big deal in terms of efficiency, it means the P9000 exhaust can't damage a nylon rope. That is a big deal.
Excess Oxygen. However, the easiest way to get a cool stack is plenty of excess oxygen. A few percent is desirable, but the rest is energy wasted heating air. I get a reading of 5% on the Dickson heater, and that costs me 2% on heater efficiency; down to 90.5%.
Ambient Temperature. It is also necessary to do a correction for ambient temperature, compared to the base temperature used int he first graph (70F). Colder inlet give the impression of higher efficiency. By observing performance at 40F vs. 70F I gave myself some false credit. Subtract 1%. Down to 89.5%.
Overall Efficiency. Just start at the top and add them up. 88.5% + 4%(preheat) - 2% (excess O2) - 1% (testing temp correction) = 89.5% thermal efficiency.
AFUE (Annual Fuel Utilization Efficiency) Rating. This rating, found on every home furnace, is a bit different, as it includes losses up the stack when the unit is not firing; for example, fire places can be great heat wasters since warm air goes up the stack even when not in use. However, all of this is irrelevant to a simple marine heater such as the Dickon P9000 (it runs steadily without cycling--it can be turned down), I have not considered it. However, for the purposes of comparison, the AFUE is generally about 15% lower than the thermal efficiency. However, since the Dickson is sealed (all combustion air comes from outside) only a minimal 5% penalty is applied. Electric heat is assigned no penalty, so I am being conservative. 84.5% estimated AFUE.
Bottom Line. So how does this "stack-up?" This little marine furnace rates as mid-efficiency. Not too bad for something the size of a toaster and much better than more primitive predecessors.