Hot water issue

[Randy]

Here's one for you guys? My engine coolant circulates through the hot water heater whle underway thus making some nice hot water once the hook goes down. Recently, After running engine and turning on COLD water, hot water comes out. I could understand a little hot water, but it keeps doing it. Turn on the cold, a little cold comes out then it turns HOT, then after a few seconds cold starts to flow? It's like a check valve is leaking or something. I can't figure it out. Any ideas?

[EdHouston]

Do you have a mixer valve faucet or independent faucets if its a mixer I think maybe the o rings have failed allowing hot into cold but not visa versa!

Ed

[RayNWanda]

What brand water heater is it?

[RayNWanda]

 One of the water heaters has a bunch of check valves and extra plumbing on it. I pulled one out of our boat because all that stuff just complicates things. There were spring loaded plungers and just a lot of stuff that I felt was unnecessary. I can't remember which brand it was.
We had a perfectly good 6 gal. Raritan that we salvaged out of the other boat. I figured I would be heading off future problems by going ahead and swapping it out.

[Shacklepin]

I have the same problem, When I get back to the Marina I hook into the shore water supply and just flush out the cold water lines untill they're cold. The water in the Tank gets scalding hot so I thought it was just radiated heat from the pipes running side by side. Tell me more please

[slokat]

My heat exchanger develops so much extra pressure and heat that I'm going to install a valve to shut it off after it reaches temp when the engine is used, because the over pressure valve on the water heater vents it to the bilge.

In the interim, leaving the pressure pump off helps for short engine uses.

You might be developing too much pressure for the system?

And the galley faucet is just after the main water manifold from the tanks.

[SV THIRD DAY]

Well it's good to know that I'm not the only one to have hot water coming from my cold water after some periods of long motoring.  I've never tried to fix it honestly, just figuring it was the water expanding and pushing back through the cold water line. 

[Randy]

Thanks guys. Sounds like a common issue. I'll leave it alone. Probably cause more problems if I try to resolve it.

[RayNWanda]

Most of us set our water heaters at home to about 140 or so degrees. When we make hot water using engine coolant, the hot water is going to be right at engine temp, or 180 degrees. It's HOT! And if your engine runs hotter, it will still be at engine temp, whatever that is. Also, the relief valve should be a temp/pressure valve- it will relieve on either. It may not be relieving on pressure- it may be temperature that is causing it to open.