Hot water pressure differential? (How not to scald yourself)

[kev_rm]

I get incredibly hot water for a few seconds when using the valve on my shower head. I assume this is due to a small pressure differential between the hot and cold sides? Any tricks for solving this?

[a64pilot]

I think your probably correct, if you have an accumulator and the water in the tank is cold, then as it heats it builds pressure that has no where to go so it increases, it increased the whole systems water pressure, not just hot water unless there is a one way valve between the hot and cold (unlikely) if you have an accumulator the pressure rise is still there, just lessened by quite a lot.
There are mixing valves that can be bought that will control the temp and not let it exceed the set point by mixing cold with the hot, you see these a lot on hotel showers for instance.

We know it’s there and set the temp so we get a shot of overly warm, but not scalding water. It only lasts a few seconds, oddly on ours it repeats if you leave the shower off, like when you wash your hair, but if the water isn’t set to hot to begin with ours only gets overly warm then quickly returns to warm water.

The mixing valve would fix it, but we just live with it.

[sailormed]

Are you using a thermostatic controlled shower head? If so it can be the that the bi-metal adjusting the flow of hot and cold water for the requested temperature is slow in adjusting. Maybe replace with new?

EDIT: On my hot water tank a overpressure relief valve venting to outside the boat is installed. I thought that this is standard installation for hot water tanks. I have the same in our kitchen at home. ( not venting to the boat :-) )

[Phil Turner]

Put up with scalding water for years. Mixing valve is adjustable and great.

[Wood]

If you install a hot water winterization bypass - it has a one way valve.

https://www.ebay.com/itm/8-RV-Winter...gAAOSwvBpeFmcQ

I did this with a tempering valve as my DHW is also heated off the engine ~ 185*

[ggray]

Originally Posted by Wood

If you install a hot water winterization bypass - it has a one way valve.

https://www.ebay.com/itm/8-RV-Winter...gAAOSwvBpeFmcQ

I did this with a tempering valve as my DHW is also heated off the engine ~ 185*

Where is this one way valve?

I installed a bypass for winterizing, and while I obtained the parts myself rather than a kit like this, it looks to be roughly the same as what I have. But I have no one way valve, nor see one in the picture.

But I'm always looking for improvement! There is a mixing valve on the WH, but temperature still fluctuates when using.

[doublewide]

Water heated by a boat engine is inherently dangerous, somewhere around 190 degrees Fahrenheit. In my opinion, every boat hot water tank should have a thermostatic mixing valve. I installed one on each of my hot water tanks. It was extremely simple to do, and not particularly expensive. Now I can set the temperature of the water exiting my tanks, and guarantee that no unwary person - or especially child - will get scalded. Please consider adding a mixing valve to your tank, too

[Wood]

Originally Posted by ggray

Where is this one way valve?

I installed a bypass for winterizing, and while I obtained the parts myself rather than a kit like this, it looks to be roughly the same as what I have. But I have no one way valve, nor see one in the picture.

But I'm always looking for improvement! There is a mixing valve on the WH, but temperature still fluctuates when using.

Sorry - I provide an incorrect link and I should have provided a better explanation. This is a better link - however you need 2 bypass valves instead of one valve and one Tee.

https://www.ebay.com/itm/Boat-Marine...wAAOSwGvhULbwG

The short brass piece is the one way valve. It is intended to be installed on the hot water out. I reversed this and installed it on the cold water in to prevent flow out of the heater there.

[sainted]

I had a similar problem. My hot water heater came with a thermostatic mixing valve that mixes hot and cold water when the hot water is flowing. However, after you’d shut the water off in the shower using the valve in the shower head, hot water would back up into the cold water side of the mixing valve and you’d get scalding water as a result. The solution was to put a one way valve on the input to the water heater. Doing so solved the problem nicely.

[DoratheExplora]

I have scalding hot water issues. Turn the shower on stand under it, warms up, nice, hot then get out from under it. Reset hot cold ease under it and alls good. I dont see how the mixing set up works? is there a temp controlled mixer of some sort?
Bradley

[misfits]

Originally Posted by DoratheExplora

I have scalding hot water issues. Turn the shower on stand under it, warms up, nice, hot then get out from under it. Reset hot cold ease under it and alls good. I dont see how the mixing set up works? is there a temp controlled mixer of some sort?
Bradley

A mixing valve adjusts the water temp at the source. You can dial it in at say 110 degrees & just turn the hot water side of your faucet on. See attached

[Lepke]

The shower is hot because the hot water expanded back into the cold water lines and (depending on plumbing) you may be getting hot from both sides until the cold pushes thru.
An easy fix is to have a check valve where the cold enters the water heater. Or check valves on the shower plumbing.

In another life, I installed film processors. The water supply had to be within a couple degrees. When not processing film, the water supply shuts off. If the water heater comes on in that time the water expands and the incoming water temp spikes and ruins film. Even with a mixing valve, you need check valves to make sure the water only flows in the intended direction. And an expansion tank protects your plumbing from over pressure.