Water Heater - more efficient to turn off or leave on?

[epistosaurus]

You have not said how you heat your boat?

Energy loss goes up like a square of temp difference (If I remember correctly). So leaving your heater up with the attendant temp difference between water and surroundings will keep it draw more energy. The energy loss goes to heat up the area around the water tank, and part of it to heat up the livable space on your boat. How much escapes the tank is a function of insulation, obviously, and the temp difference. How much of it becomes useful to your boat - reducing boat heating expenses - depends on where the tank is on your boat.

So not all energy you save by switching off your tank will be a pure gain, as some of it would have contributed to heating your boat. So, better insulation, absolutely, and then better heat conduit from the area around your tank to the boat's living space, and then if you heat your boat by other means than electricity, then comparison of energy cost between that and electricity.

rafal

[sailorchic34]

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.

The engine 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.

[rjdives]

I have gone in two directions on our cat. Removed the old tank type electric water heater and replaced it with a on demand electric for use when we are on shore power at the dock or on generator power offshore. They draw a little more power but over all use much less power in the long run. You don't have to wait for hot water and you don't run out of it either. I have also installed a gas on demand water heater to meet my needs while offshore. The two water heaters take up less room then the old one and there was a small weight savings to. The cost of the on demand heaters have come down a lot in the past few years and can save 25-50% over the tank type.

[SVNeko]

rjdives, would you mind adding a bit more info? What models did you use and basics of installation?
Thx