Hot Water Tank Fitting

There is a cylindrical metal T fitting on the output side of my friends hot water tank. The hot water flows directly through the T. The bottom of the T is attached to another cylinder that has the engine coolant going from the engine and into the tank. Antifreeze is dripping from the slot between the cylinders.

I think this may be a thermostat valve that will stop engine coolant from circulating through the tank if the output hot water is too hot. I expect that the engine coolant can get much hotter than the pressure relief valve.

Any ideas on this? There are no markings that I can see.

You have it correct...

Not sure about the pressure relief valve.. Some valves operate on pressure only and others operate on temperature and pressure. Check your valve as it will say on top what it is. I would think in a marina application, a pressure only valve along with an properly sized expansion tank would would be appropriate.
But yes, it appears to be a temperature operated cut off valve for the coolant. I might have to install one of those on my water heater. Let me know where you source it!

"I might have to install one of those on my water heater. Let me know where you source it!"

They do not work very well.

Toss it and install an Anti Scald valve in the domestic hot water circuit.

Faster easier cheaper and 1000% safer.

I believe they have two separate functions. Anti Scald will save your skin and the other one will eliminate the possibility of water in the tank getting hot enough to build pressure and activate the pressure relief valve. If that happened while underway you could dump your entire water supply in to the bilge.

and the other one will eliminate the possibility of water in the tank getting hot enough to build pressure and activate the pressure relief valve. If that happened while underway you could dump your entire water supply in to the bilge. __________________

The thermostat bypass never shuts down the coolant flow totally , so on a long run the domestic hot water will be the same temperature as the coolant.

High temperature (with an expansion tank installed) will never raise the water pressure to cause a release from the HW tank pressure valve..

I am having an issue where the water heater contents get too hot and drain into the bilge, so this thread is interesting. I don't know what type of valve is on my water heater (pressure or temperature or both) but it definitely is allowing hot water into the bilge.

I have been turning off the water pump and relieving the pressure from the hot side when I can remember to do so.

I was not aware of an available thermostat valve for the coolant loop side. It sounds like I should consider one?

I don't want to turn off the coolant loop valves as we pull cabin heat from the same loop.

Suggestions welcomed, please.

"I don't want to turn off the coolant loop valves as we pull cabin heat from the same loop."

If the thermostat stops most of the coolant running thru the HW box , what do you think will be the heater output?

A parallel coolant loop is required so the heater gets mucho hot water and the HW heater can close off with no problem.

Except after a good run the domestic HW will be coolant temp.

I would look and see if the expansion tank is plumbed in the HW line where it can work.

I have never liked those water heater loops using engine coolant. One failure on that loop and you're putting the engine at risk.

I say this having taken ownership of a boat that did just that during delivery. Poorly supported line eventually caused the metal fitting on the engine to fail and it puked twenty gallons of $25/gal antifreeze all over the engine compartment. Thankfully we caught it quick before the engine roasted itself. Plugged that loop and just use in electric element in the tank. Only takes about 5 minutes or so to bring it up to decent temp for showering.

If you decide to keep the setup, be sure to make it a regular part of your maintenance inspections. Hoses and water heaters are MUCH cheaper to replace than a blown engine.