Don, when you reinstall, consider putting in a small expansion tank on both the engine loop that contains coolant and also on the cold water side of the fresh water supply if they aren't already there. Tanks can also fall apart because of corrosion, especially with ship's ground and shore ground differentials, so one other thing you also can do is to install a dual pole switch to disconnect both the hot and the neutral when not in use. You can do this through a simple two pole relay for about $30.
It's pretty low, and you can certainly argue that the coolant tank and pressure relief cap will handle expansion, but only if there is a direct flow of engine coolant through the water heater. If an exchanger is used to transfer coolant temperature to a coolant loop through the water heater, without an expansion tank all the expansion will go to the loops which can cause premature failure. If an engine mounted exchanger is used, the expansion tank should be considered essential, IMO. If not, the tank is simply a cheap $65 backup, so why not?
Does the coolant in the engine flow through a heat exchanger that transfers heat to a separate loop through the water heater? If so, you have no expansion tank on the system and might want to think about installing one. With the problem you experienced, it sounds like a hole in the coolant loop through the heater forced fresh water directly into the coolant of the engine, which would suggest you don't have an engine mounted exchanger, so other than exposure to a water tank failure causing engine overheating, you're correct, the coolant tank on the engine probably is sufficient.I'm curious about the expansion tank on the coolant loop. Don't most marine engines have a fairly large expansion tank built in? I know the Lugger 1066 in my boat has a big one that should always have an air gap in the top (your finger should just be able to touch the coolant).
I recently had to replace my hot water heater for the opposite problem that Don had. I was getting fresh water from the heater into the coolant loop of the engine (so I found coolant all over the floor of the engine room).
In my case it was really caused by too much city water pressure being applied to the fresh water system by the PO.
Well, yes. Like this one: SK UNI302631CN Heat Exchanger Universal / Medalist by Seakamp Engineering
Bingo.
A separate pump isn't really needed, as convection does fine. In fact, in the 40 odd acres of greenhouse I built and operated, the hydronic systems we installed needed electric shut off valves to interrupt the convection, otherwise you were always circulating hot water from the boiler. Most water tanks come with a mixing valve option - the Isotemp I just installed had one, as did the Solaris it replaced, so too much heat isn't really a problem.
As noted, I don't think you need a pump. The water will move through the loop from convection, if slowly, but that doesn't matter since you're just transferring engine heat to the fresh water in the tank. The isolation ball valves are a good idea, methinks...
You're right, there is the 2' of hose from the engine to one side of the exchanger and its return, and those could fail. However, it is visible in the ER, which the hoses in my system that run from the heat exchanger to the hot water tank most certainly are not. But that is my system. If everything is in the ER and you use good quality hose no problem. However that still doesn't tell you what's going on where you can't see it, like the exchanger loop inside the water heater. These fail as well, so all in all, I prefer isolation, and if isolated, you need an expansion tank.
Or, you use the all purpose Vise Grip tool for emergency valving on rubber hose. Been there, done that.
