Water Pressure Imbalance

[Beneteausailor]

My faucets began varying between hot and whatever it was set to every few seconds. After conducting a few experiments, it seems that as soon as the water pump kicks out the water pressure begins to drop and the water rises to a scalding hot temperature. This seems to be an issue with all of my faucets, but the faucets in the head and the shower seem to have a far exaggerated issue. I’m looking for suggestions on possible causes.

[Lepke]

Without a check valve, hot water from the heater can back up the cold inlet when the water heater heats and expands the water. So when you turn on a faucet, there's an amount of hot water already in the cold pipes. When the pressure drops (there is less pressure in the cold pipes), the hot backs out of the water heater via the cold inlet pushing more heated water into the cold pipes. And you get the hot plus the unintended hot water in the cold pipe and no cold until it runs long enough to flush the hot.
It depends on how quickly your water heater heats and if there's an accumulator in the system.

[Beneteausailor]

Attachment 241210

Originally Posted by Lepke

Without a check valve, hot water from the heater can back up the cold inlet when the water heater heats and expands the water. So when you turn on a faucet, there's an amount of hot water already in the cold pipes. When the pressure drops (there is less pressure in the cold pipes), the hot backs out of the water heater via the cold inlet pushing more heated water into the cold pipes. And you get the hot plus the unintended hot water in the cold pipe and no cold until it runs long enough to flush the hot.
It depends on how quickly your water heater heats and if there's an accumulator in the system.

Thank you for the advice.

There is now a check valve in the system preventing backfeeding from the water heater, but the issue is persisting. I'm attaching a diagram of the current water setup.

[Beneteausailor]

I think I have it solved. Thanks!

[Capt Ricky]

I'm curious what you learned. When I read your post I immediately thought of a missing check valve, too. But you said it didn't fix the issue? What ultimately fixed the problem.

[Beneteausailor]

Originally Posted by Capt Ricky

I'm curious what you learned. When I read your post I immediately thought of a missing check valve, too. But you said it didn't fix the issue? What ultimately fixed the problem.

It appears to be an accumulator issue. I'm still tinkering with it, but that has alleviated a great deal of the issue. There still seems to be a large pressure variance between when the pump is on and when it is off. Unfortunately, the PSI it kicks in is not adjustable.

[Lepke]

If you're not happy with the pump start & stop pressures, you can add an adjustable pressure switch and bypass the one with the pump.

[Beneteausailor]

Originally Posted by Lepke

If you're not happy with the pump start & stop pressures, you can add an adjustable pressure switch and bypass the one with the pump.

I’m definitely going to look into that. It looked like a sealed system but I’ll take another look.

Thx!

[Lepke]

I use two of the type shown on a couple pumps. Switch is made in China and seem to be reliable. Good for about 10 amps. To run a higher amp motor, use a relay. You can find 220, 110, and 12v versions on ebay. I use a 110v and a 12v. The 12v one I have doesn't have psi, just some communist measurement system. I have to use a table to program. Hopefully they make a psi version now.
You can set any pressure on and off to 1 psi or the communist equivalent. When running, the switch shows the current pressure. Threads are 3/8" pipe. One of my switches comes with a commie to US threads adapter.

There is also a switch available that turn on the pump as soon as pressure falls, maintaining uniform pressure for showers, etc. My pressure pump has one of the traditional 40/60 switch and one that maintains pressure. I can switch between the 2 switches depending on what going on in the boat.

[dfelsent]

By measurement system do you mean bar. Not psi?