Wisdom of Shut off valve in the water heater supply hose

svnalu387

svnalu387

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Dec 15, 2025
9
Catalina 387 Vero Beach
Just replaced all the hoses for the water heater supply loop off Yanmar 3JH4E. Looking for advice on the wisdom of installing a Groco ball valve shutoff in the system to prevent excess heat in the cabin on hot days. Plus our hot water gets blazing hot! Google says I should install valves on both supply and return hoses. That has the advantage of being able to completely isolate the engine from that coolant loop, but that’s not why I’m doing it. Long motoring days gets the galley quite hot. I did replace hard water hoses with Shields 132 hose and put pipe insulation foam everywhere I could so maybe that’s enough to make a difference?
 
Davidasailor26

Davidasailor26

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May 17, 2004
6,222
Beneteau Oceanis 37 Havre de Grace
My guess is most of the heat is radiating from the engine compartment itself, with not that much from the hoses. The insulation will help with that even more. I know my cabin floor above the hoses gets warm when the engine is hot, but I don’t think it’s dramatic - rubber hoses aren’t great for transferring heat, and insulated ones are even less. As for the blazing hot water - the better way to fix that is with a thermostatic mixing valve on the tank outlet that can mix some cold water in. That way you don’t need to worry about forgetting to open and close a manual valve whenever you want hot water.
 
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Stu Jackson

Stu Jackson

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Feb 26, 2004
23,498
Catalina 34 224 Maple Bay, BC, Canada
Before you do anything, you should seriously analyze just how the coolant flows to and through your engine and the water heater. It could be that a shutoff valve would interrupt the flow of coolant not only through that leg to the heater, but for the entire engine's coolant circuit.
 
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M

MFD

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Jun 23, 2016
295
Hunter 41DS Pacific NW USA
For your 'blazing hot water' you want one of those temp-control mixers on the output side of the hot water tank.
Having (diesel engine hot) hot water is awesome for having hot water, the mixer helps with humans not being scalded by it and is a pretty standard device both for safety and comfort.

For shutoff valves on the engine coolant lines to the hot water tank - I have that.
My hot water tank is located some distance from the engine.
I have a valve on each of the two lines to the hot water tank - basically to isolate that entire portion of the coolant system.
Beyond my desire to be able to isolate the hot water tank, I have heard conflicting and weird information about trying to just put a valve on one side side of the auxiliary coolant ports of the engine for something like this. My valves are both within a few inches of the engine auxiliary coolant ports which from the factory I guess just have a plug screwed in. No idea if weird pulses can happen or something else if the water heater line doesn't have full flow and is shutoff on only one leg. I have just heard scuttlebutt on that, and I wanted to be able to isolate the entire segment anyway.
EDIT: I have a Yanmar 4JH4AE.

And YES, as another poster noted - make sure your hot water tank is off auxiliary ports on your engine pump and not part of the full flow cycle of the coolant :)
 
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