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We have replaced all the check valves from the road to the house.. The city replaced its valve at the road. We continue to have 90psi pressure after shower or dishwasher use. I have to flush the toilet to bring the pressure down to 47PSI. The city says it may be our hot water tank. The tank is 10-12 years old. Whenever the pressure starts to build my kitchen sink faucet leaks and I have already replaced the flow meter in one of the toilets that blew. Is it our hot water heater?

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    Why did you post this twice? You can edit questions. Duplicate of: diy.stackexchange.com/q/88219/43874 – JPhi1618 Apr 7 '16 at 15:41
  • Do you have an expansion tank on the cold water supply line feeding the water heater? – Tester101 Apr 7 '16 at 16:36
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    Interestingly working check valves (you noted in the other question that they were replaced) are essentially what cause this problem, your water heater is raising the temp of the water in the system (its expanding) and it has nowhere to go, it cant flow back toward the street to equalize pressure thanks to the backflow preventing check valves, so it builds. This is exactly what hot water expansion tanks are meant to mitigate. You either don't have one or yours is malfunctioning. – Jeff Meden Apr 7 '16 at 17:05
  • @JeffMeden You may want to convert your comment into an answer... – Lynn Crumbling Apr 7 '16 at 17:59
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Interestingly, working check valves (you noted in the other question that they were replaced) are essentially what cause this problem, your water heater is raising the temp of the water in the system (it's expanding) and it has nowhere to go, it can't flow back toward the street to equalize pressure thanks to the backflow preventing check valves, so it builds.

This is exactly what hot water / thermal expansion tanks are meant to mitigate. You either don't have one or yours is malfunctioning. They are typically made of a tank pressurized with air and a bladder inside to let the water push against it as the pressure builds, since air compresses relatively easily compared to water. If there is a leak on the air side of this tank, it will fill with water and not react at all to the changing pressure.

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