Where is the water going?

Couple of points to keep in mind.

When you are not on the boat, never leave the fresh water pump on or have a hose hooked up to a water line.

Second, always keep in mind that the water heater should never be turned on if there is any chance it is not full of water. If it comes on with no water in the tank it will quickly burn out the element.

For me, final checks before leaving the boat: water pump turned off on breaker panel, no hose connected to water supply, water heater breaker turned off.
 
Didn’t read all replies…

my money is on water heater pressure relief valve. Mine currently leaks and will lose a half tank if the pump is left on overnight
 
If that much water is being lost and it is not in the boat the heat exchanger in the hot water heater that connects to one of the engines is the likely leak.
That would make sense, and he being in Ontario, improper winterization might cause a crack in the heat exchanger lines and the pressurized water would flow into the lines and back through the engine cooling system. that would be hard to detect because the water would be going out underwater.

@Rigby the way to test for that is to make up a bypass for your hot water tank. Basically connect the tank inlet blue line to the tank outlet red line. You will need some piping to do that. Then when you leave the water pump on, if your water loss stops, then that is your issue.

Here is what I use when I bypass my water heater for winterization. Its a short length of 1/2 garden hose with hose clamps on each end.
ADA5A612-C72C-4054-A30D-51979E11C2F0.jpeg
 
That would make sense, and he being in Ontario, improper winterization might cause a crack in the heat exchanger lines and the pressurized water would flow into the lines and back through the engine cooling system. that would be hard to detect because the water would be going out underwater.

@Rigby the way to test for that is to make up a bypass for your hot water tank. Basically connect the tank inlet blue line to the tank outlet red line. You will need some piping to do that. Then when you leave the water pump on, if your water loss stops, then that is your issue.

Here is what I use when I bypass my water heater for winterization. Its a short length of 1/2 garden hose with hose clamps on each end. View attachment 131005


You guys make a lot of sense on this. I am going to try this bypass. Unfortunately I am not back at the boat for about 10 days but I am anxious to try this.
 
it does not.
if the water pump is turned off, the fresh tank retains its level.
Check the shore water pressure regulator. You may have a leak here when the tank is pressurized. I had one do this on a trip. Ending up removing the regulator and plugging the line until we were back home.
 
Someone correct me if I am wrong, how can it be "heat exchanger in the hot water heater that connects to one of the engines is the likely leak." if it only occurs when the system is pressurized? That runs from the engine and then back to the engine, raw water.
 
Someone correct me if I am wrong, how can it be "heat exchanger in the hot water heater that connects to one of the engines is the likely leak." if it only occurs when the system is pressurized? That runs from the engine and then back to the engine, raw water.
It's not - that coil in the water heater is normally the engine's coolant (antifreeze). If it leaked the fresh water system being a much higher pressure would lift the engine's expansion tank cap and overflow the expansion bottle and then be all over the bilge and a real mess with engine coolant. Maybe some have it hooked up to raw water but it wouldn't be correct.
My 400DA had that feature and was integrated with the Starboard engine's cooling. I disconnected and abandoned the coil as I thought the risk of losing engine coolant to great at the worst of times.... The water heater on my 52 has a coil for water heating from the engine but was never utilized - the engines are too far away...
 
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Yep you are correct, I meant raw water if you don't have closed antifreeze cooling. Either way, it's water from the engine that's heating up the hot water tank, not fresh water from the water tank. I personally wouldn't look here

I think you know your answer at this point, any system that requires pressurized fresh water is your source, trace it from the water pump to that system.
 
Someone correct me if I am wrong, how can it be "heat exchanger in the hot water heater that connects to one of the engines is the likely leak." if it only occurs when the system is pressurized? That runs from the engine and then back to the engine, raw water.
The raw water heating lines run in coils inside the water chamber of the heater. So if those coils fail and leak, the raw water is connected to the house water inside the tank. Then when the water pump is on, the pressure in the tank will force water out into the heating coils and into the engine cooling system. Many (most??) gas great lakes boats are raw water cooled, not closed cooling.
 

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