Using a "mixing valve" on the potable water circuit to maintain temperatures at the spigot/shower head
is possible but a total waste of money
and introduces more complicated machinery into a cruising vessel. Water temperature at the spigots/sinks/showers is regulated by the human who opens the hot and cold water faucets. If the human is too stupid to regulate water temperature while taking a shower or washing
in a sink then they should live on land where all the government
agencies make sure you cannot hurt yourself due to your own stupidity.
- - The danger
of overheated water in a cruising vessel's hot water tank is present when you are using your engine's coolant loop to supply heat to the hot water tank. Diesel
engines run at coolant temperatures varying from about 160F to 200F depending upon engine load and age of the engine plus a hundred other problems with keeping the engine from overheating
- - If while engine coolant is being supplied to the hot water heater - the cruising vessel gets involved in a situation involving the need for prolonged high power / emerg power then it is highly likely that the coolant temperatures will start to climb to and possibly exceed the temperature relief valve setting on the hot water heater. This will open the valve and vent very hot water into and around the area where the hot water heater is located. In my case that is in the Galley
area which has happened twice.
- - Subsequently I re-plumbed the relief valve to discharge down a sink drain hose. But still even with the danger
of scalding water now diverted to a drain, there still exists the loss of potable water from the tanks
down the drain. The amount depends upon whether the potable water pressure pump is left on or is off. The third danger is to the actual plumbing
pipes/hoses/tubes around and downstream from the water heater. High temperatures above 140F will harm "plastic" type hoses and significantly weaken them. In the case of polybutyl it takes about one year for the fittings to "cook" and crumble resulting in a lot of water being dumped into the boat's interior
before you can turn off the water pump.
- - So the most critical consideration of using the engine coolant system to heat the water in the hot water tank is regulation of tank water temperature. This is done automatically when the AC or DC power is used with the electric
heating elements. A non-electrical metering or shut-off valve in the engine coolant supply to the water heater would be a good solution to this problem when using the engine to heat the hot water tank.
- - Additionally, I installed two ball valves on the engine where the coolant is tapped off to go to the hot water heater. These valves can be used to turn off the supply to the hot water heater for temperature protection and - more importantly - to shut off both coolant lines to the hot water heater should a leak or burst hose threaten to drain all the coolant out of the engine. This would - of course - only happen when you are in a emergency
situation and need the engine to get your "ass out of a bind," Losing the engine due to a burst coolant hose going to/from the hot water heater would definitely ruin your day.