hot water out of cold faucet- why?

[malbert73]

Hi everyone,
This happened on my old boat as well- and I am a bit mystified.

Especially when hot water tank is freshly hot, my shower and galley sink take a lot of running to get cold water out. I'll have diverter handle on galley sink all the way to cold and it will remain burning hot for 10-20 seconds until it finally runs cold. Then if I turn it off to wash more dishes, same happens. Water temps also fluctuate a lot in shower when turning on and off at shower head. I thought this was the mixer valve but same happens at shower where I have separate hot and cold water knobs. I have a single water pump, and an accumulator tank.

Is hot water somehow backing into cold water plumbing, needing to get flushed out? I'm a bit stumped on how (if at all) to fix this to prevent scalding and improve shower quality. Saving water would be nice also.

[Bohemian17]

Water system runs through the engine room everything gets hot

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[senormechanico]

Hot and cold lines in a common insulated wrapping?
Possibly to prevent freeze damage?

[Sailmonkey]

Check valve on the cold inlet side of the water heater should cure this.


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[Stu Jackson]

Many times H&C hoses are wire tied together. Guess what happens.

[Scott Berg]

ditto on the check valve (there should be one on the cold water going into the heater)

This can also cause water to swing from hot to cold.. they do go bad...

[sailorchic34]

DO you have an accumulator/expansion tank on the water system. What is most likely happening is as the cold water in the tank heats up it expands quite a bit, this with the accumulator will cause hot water to back up into the cold water line. To prevent that. Move the accumulator to adjacent to the water heater. That will keep the expanding hot water local to the heater.

The other think that could be happening with engine heating is a thermo-syphon where hot water rises and cold water falls. Normally that would be on the hot side. But if the water heater is hot and there is a cold water faucet above the water heater, you can get thermo-syphon on the cold side.

Specially with a horizontal heater. Adding a check valve at the heater cold water line will prevent that. Best to add an accumulator between the check valve and heater to prevent excess pressure in the tank.

[MarkJ]

Can you put some heat insulation from the hot water transfer heater outward hose to the sink?

Mine both hoses, hot and cold run touching each other. after the engine is on for a bit the hot water hose heats the cold water hose.

[Ukeluthier]

Yep, it is likely the proximity of the hot and cold lines to each other near the water heater. I had this problem in my old liveaboard cutter until I replaced the original water heater and rerouted the lines. It was exceptionally bad in my case because the lines were copper, enhancing the heat transfer.

[tinkrman69]

Originally Posted by Sailmonkey

Check valve on the cold inlet side of the water heater should cure this.


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this is the common cause. most boats have this check valve, but they do go bad and leak back somewhat. Cleaning or replacing would be in order. I don't go for replacing right away cleaning works quite often. being next to large temp deference will cause calcium buildup. pull it out and soak it in white vinegar for a couple hours and scrub it with a soft brush. don't mar the mating surface of the valve disc and seat.

[malbert73]

Thanks everyone for the input. Definitely not the hoses touching each other. When I mean hot- it is scalding, steaming hot coming out of cold water faucet. Hotter than outside of hot water tubing ever gets.


There is no check valve at my cold water inlet to hot water heater that I can see. There was not one on my old boat either, which had same problem. So I am hearing that hot water leaks back into the cold water line until the pump and then gets pushed out to the faucets?

I have an accumulator tank, though it's not close to hot water tank. That shouldn't matter, right, since backpressure will equalize along water line?

Any other ideas? I will be installing a check valve as soon as I find the right one. Would be amazing if it's that easy.

[CaptTom]

I just added a check valve on the cold (input) side of the water heater and solved the exact symptoms the OP describes.

[capt jolly]

Is your f/w pump cycling? I've seen plastic impellors deform from the heat build up in the water from friction .

[Old Swampy]

A check valve in the cold water line to the hot water tank probably will solve the problem of hot water where cold water should be, but suggests a new problem. As noted, the hot water expands a lot. Your hot water heater should have a pressure relief valve. Mine isn't plumbed anywhere. Where do you want your excess hot water to go?

[tinkrman69]

The accumulator should be the first thing inline from the pump then the "T" that splits off to the cold water manifold and the check valve to the cold water inlet to the hot water heater, then the hot water manifold out of the heater.

When the engine is runing the water will get up as high as 180 deg. depending if your engine is fresh or raw water cooled. raw water cooled engines run cooler.

If you don't have a check valve, then install one. it should be bronze or brass tho, as synthetic ones won't handle the heat over time. you can put hose to its inlet side, but it's preferable to use hard pipe from it to the hot water heater inlet

I use hard pipe al the way to the "T" and hose to that and the cold water manifold