I started my original post with a comment about my being paranoid about loosing my engine cooling system. Over the past 19 years of having this boat I have found that if I leave the both valves open then I forget and a few hours later of motoring I have 180 degree water in the hot water tank. One time the overtemp relief valve let loose and filled the galley
with hot water. There was a thread some time back about fitting a temperature regulator
valve to the coolant lines from the engine to keep the hot water tank temperature at an acceptable 110 degree plus/minus. But it got awful complicated.
- - With a small engine and short run times it is unlikely that the hot water tank will get too hot as compared to my big 6 cylinder Perkins. So leaving the valves open might not be a problem - except for - the drain back problem if the water heater is higher than the engine coolant tank. Ideally with an "tight" hose system there should not be any backflow to the coolant tank. But the world is rarely ideal, so I keep the valves closed all the time and only open them when I am about an hour or two's motoring away from an anchorage/harbor. Then during the shutdown, put everything away phase I close the valves.
- - But as I said I am paranoid with my engine's coolant system. Most production boats have hot water heaters with hoses from the engine and do not have any valves what-so-ever in the system. And for those boats everything works just fine. So it is strictly a judgment call on your part which way to go as far as the valves are concerned. And additionally a couple of gallons of coolant stored away would be your guarantee that should you get a coolant draining back to the engine coolant tank you can easily replace any lost
- - If your engine coolant reservoir is above the hot water heater then there is not a problem with back flow from the water heater. Then the valves would only be there for emergency
use should a water heater or hose break. With experience actually operating the engine you need to check the temperature of the hot water heater water after engine shutdown to make sure it is not too hot for safety
. If it is then there are two options - 1. use the valves to turn on and off the hot water heater loop; and 2. partially close the pressure side valve to reduce the flow of engine coolant to the hot water heater until your average temperature is acceptable.
- - I am not a big fan of "pressure caps" as I got severely burned once when one let loose on my car. That kind of accident
would be catastrophic onboard a boat in the middle of nowhere.