Thermal Expansion Issue with water lines - Fiberglass RV
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Old 06-01-2021, 07:21 PM   #1
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Thermal Expansion Issue with water lines

Has anyone found a way to deal with excessive water pressure build up in lines when using hot water heater, I checked it today at the faucet and it went from 50psi after the water pump shut off to over 100psi after the water heater finished heating the water (pressure was checked with a gauge attached to the end of the sink faucet) no wonder I had some leaky connections! is there any way to leave the hot water heater on while camping and not worry about leaks?
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Old 06-01-2021, 10:01 PM   #2
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Yes. You simply add an accumulator tank like this one:

https://www.amazon.com/Seaflo-Pre-Pr...94358144&psc=1

Adding two works even better. Place this where it cannot be valved off from the water heater, such as on the cold supply line. Tee it into that line, near the WH.

I used two. The brass Tee between them connects to a Tee in the cold line before the WH. Use a plumbing pigtail, for undersink connections, to connect them.

Adjust the pressure to slightly higher than the "cut-in" pressure on your pump, typically, about 35 PSI, or so. This should limit the pressure rise to about 20 or 30 PSI. it will significantly reduce the stress on the system and it will stop the pressure relief valve from leaking.
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Old 06-02-2021, 09:55 AM   #3
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Appears to me your water is boiling. Check the water temp and if to high, turn down the heat on the water heater.
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Old 06-02-2021, 10:23 AM   #4
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It's not about boiling. It's simple expansion from heating up. You can literally watch the gauge pressure rise as the water goes from 60-140, or whatever the set point is. And it easily reaches the pop point of the pressure relief. A lot of people complain that their pressure relief valve leaks, but it is actually just doing its job. As water heats up, it expands. If you have a pressure regulator feeding the system from the city, the water cannot back up into the city system. If you are running on your tanks, the water pump prevents pressure from backing into the tank. Result: the pressure rises until the relief opens at about 100 PSI. The accumulator tank gives the water a place to expand into against compressible trapped air, which reduces the pressure rise.
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Old 06-02-2021, 11:01 AM   #5
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Excellent solution, thanks for the info!
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Old 06-02-2021, 01:04 PM   #6
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Raspy - Never had a problem with something like this B4. Was news to me. Learned something today. I wonder if he turned the thermostat down on the water heater if it would not expand that much. Thanks for the science lesson.
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Old 06-06-2021, 04:45 PM   #7
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Some hot water tanks incorporate this feature into their design. The tank fills from the bottom up to the outlet near the top. Water is unable to fill to the top of the tank past this outlet. The air trapped inside this space provides the cushion to accommodate the expansion of the water as it heats.
That is why water will run from the hot water tap for a few seconds before the pump cuts in. The partially compressed air in the top of the tank provides the necessary pressure.
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