Wow! An awful lot of issues being raised from the armchairs here.
It is a good idea, and it works a treat. We have a 3.6kW WhisperGen DC generator with a coolant loop through the water heater. It heats the water fast, and free. For us as a long range cruising boat this make a lot more sense than using a coolant loop from the drive engine. We have hardly any need for domestic hot water while on passage
, and we use the drive engine rarely while underway, and never while at anchor
While a leak from the stainless steel heat exchanger
pipes inside the hot water heater is theoretically possible, it is highly unlikely, and a "poisoning" is even more unlikely. Ethylene glycol has a very distinct smell and taste, and in addition commercial
coolants have a dye added that is quite obvious even at very low concentrations. Do not run your engine without the correct type and concentration of "antifreeeze." It is needed for corrosion
protection even in the tropics.
You will not cause the engine to run cooler than it otherwise would. There is simply not enough heat capacity in the hot water system to soak up a significant proportion of the engine waste heat.
Someone suggested using the raw water
as the heat source. This is not possible. Raw water
exiting the engine before the exhaust elbow
is lukewarm at best. If it was anywhere near as hot as your domestic hot water something would be very wrong. In addition, this would set up a whole bunch of serious and difficult corrosion
A coolant loop like this will quickly heat the hot water tank to the temperature of the engine coolant. Typically 180F, or there about. This is WAY too hot for safe use from a tap. Every hot water heater SHOULD have a tempering valve on the outlet that bleeds cold water in to control the maximum temperature of the supplied water. Not just for safety
, but it lets you routinely heat the tank much hotter than needed for tap water usage and thereby be able to supply a lot more hot water from the same size tank.