Hot water in cold water lines

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Dann38

Dann38

Newbie
Joined
Jun 10, 2016
Messages
4
Location
United States
Vessel Name
Pelagic Pelican
Vessel Make
Cheer Men Marine PT38
My PT38 now has scalding hot water in the cold water pipes going to every faucet and toilet. If you run the cold water tap for about 60 seconds, then it becomes cold water. Leave it sit for a while (even when engines are not running) and the cold water becomes hot again.

I am wondering if there is a check valve that has gone bad? It seems a bit too hot to be just a hot water line laying against a cold water line, but I suppose that is possible too.

Any other ideas?

Thank you,
Dann
 
It does sound like the inlet check valve in the cold water line has gone bad
 
Hot water

"It does sound like the inlet check valve in the cold water line has gone bad" :thumb:

That is the only way it can happen. Start looking near the supply to water heater for the check valve.
 
Thank you. I will see what I can find.
 
Do you have a handheld shower (either in the head or the swim platform)with an on/off control near the shower head?

If yes, is the valve turned off?

If no, turn it off and the problem should go away.
 
Feel the tube/conductor either side of the check valve. Both sides hot...bingo. You will figure out real quick if that is the problem. If everything was good...look simple.
 
Thank you all. There is a hand held shower head in the bathtub. I will check that as well. I looked at the mfg specs of the water heater thinking there might be a built-in check valve but there does not appear to be one. There is no check valve on the cold water input to the heater so I suspect y'all are correct and that's the problem. I ordered one from West Marine (they had none in the store) and will put that inline this week. The heater is only a few years old so I suspect that the previous owner had replaced the heater and simply forgot to replace the check valve, or they thought it was part of the water heater.

As for scalding, yes it is way too hot. The engine runs at 190 degrees so that's what it gets heated to. I think it would be prudent to add some anti-scalding device but I'm not clear on what that would be, or where to insert it. Any suggestions as to what to use for this?

Thank you!
 
If this was never a problem keep looking for the cold water supply to hot water heater check valve.
 
I have completely turned off the loop from the engine to the heater. The water still stays hot as some must leak through but it's not "scalding."
 
You have 2 issues then.
Hot in the cold lines which you resolve as Bilge says.
Scalding hot lines that are remedied by not using the engine loop or installing a thermostatic mixing valve downstream of the tank before any fixtures.
 
Last edited:
It could also be the expansion tank failure. As water heats up, it expands in volume, and if all the hot water faucets are closed, there is nowhere for the hot water to go except back up the cold line. If the check valve is stuck open or not there, there would be nothing to prevent the backflow.

If the check valve was there and working, then the over-pressure would either cause the hot water faucets to leak or a pressure relief valve to vent the pressure, usually in the bilge.

As hawgwash says, a tempering valve would make sense to add to the hot water tank outlet, so it mixes cold with hot and prevents the temperature of the water from getting too hot. Home Depot has them as well as many other sources, but you need to find the hot water backflow problem first.
 
"I think it would be prudent to add some anti-scalding device but I'm not clear on what that would be,"

Beware , there are "tempering" valves and "Anti scald valves".

A tempering valve will cool the HW to the extent that there is enough cold water, not enough cold water? You get scalded.

The anti scald valve will shut off the hot water if there is not enough cold supply.

Install the valve so every user gets the protection.

Beware , when installing a check valve in the HW heater supply , be sure an expansion tank is in the heated water line , so the heat expanded water has a place to go.
 
Last edited:
Dann38 said:
Thank you all. There is a hand held shower head in the bathtub. I will check that as well. I looked at the mfg specs of the water heater thinking there might be a built-in check valve but there does not appear to be one. There is no check valve on the cold water input to the heater so I suspect y'all are correct and that's the problem. I ordered one from West Marine (they had none in the store) and will put that inline this week. The heater is only a few years old so I suspect that the previous owner had replaced the heater and simply forgot to replace the check valve, or they thought it was part of the water heater.

As for scalding, yes it is way too hot. The engine runs at 190 degrees so that's what it gets heated to. I think it would be prudent to add some anti-scalding device but I'm not clear on what that would be, or where to insert it. Any suggestions as to what to use for this?

Thank you!
Click to expand...

0206003-5.jpg


0559116 - Watts 0559116 - 1/2" LFMMVM1-UT, Lead Free Threaded Mixing Valve

Easy to install and should be required, does not change hot water capacity and allows you to adjust temp to boat by blending cold water in stream via valve and controlled by thermostatic proportioning balance valve.
 
Last edited:
Are you sure that the water heater in not staying on? Could be a stuck tsat causing the water heater to over heat the water ,then it is migrating thur the cold water side. Check with meter.
 
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