Just had a leak fixed on the main water pipe coming into my house. The repair man suggested I get a pressure gauge with two needles so I can check current pressure along with previous pressures over a period of time. I have never known what the maximum pressure, or spikes, was before. I was really surprised to find that I was getting spikes not only during the day, but overnight as well, up to 110 psi. I didn't think it was the pressure regulator because other than the spikes indicated by the red needle on the gauge, the water stayed between 40 and 55 psi. I started doing some tests with my water heater (which has no expansion tank). When I selected the VACATION or a little less than the WARM setting on the heater, there were no spikes over night or during the next day. When I turned it up to the HOT setting the day after, I watched the pressure gauge I have connected to the faucet on the main line to my house climb from 45 psi slowly up to 110 psi. It would stay there until I opened a faucet inside my house, and I think I know the reason. In the lower settings the heater would not have to come on to heat the cooler water, but in the HOT setting the heater would come on and heat the water to a temperature (I don't know how high. I have to check) that would start the pressure expansion. To me this seems as though I need an expansion tank on the heater. I would appreciate your thoughts on this as well. Thanks a lot, Bob Williams

  • What temperature is your water heater set to, should only be 130/140 F if that much. Recommended is 120F.
    – crip659
    Commented Feb 23, 2021 at 0:15

2 Answers 2


Yes you are dead on. You need an expansion tank connected to the discharge side of the water heater. When water heats, it expands considerably. When water goes from 212* to 213* it expands 1700-times its volume at room temperature. Also check you pop-off valve. If it is over 7-years old, replace it.

  • 2
    Couldn't the expansion tank be on the cold water supply to the tank? It could be anywhere in the cold distribution piping on the house side of a back-flow prevent valve. You must have something like that or the city must have one either inside or next the water meter. Commented Nov 12, 2017 at 3:00

The reason I recommend the place I did to install the expansion tank, Bob clearly has a problem with the heating of water in the water tank. The closer we can get that expansion tank to the problem spot and the clearer the piping path between the tank and the problem spot the more effectively we mitigate the problem. The cold water heat trap nipple does restrict flow to a minor degree up the line toward the street. I'm trying to avoid that issue. The hot water side heat trap is designed to trap heat and still let the water flow freely, thus the tank on that side. I know that there are other ways of doing this and most of them probably effective. I just gave him my way.

  • Paul, you seem to have posted two answers here. You may want to combine and delete.
    – isherwood
    Commented Jul 29, 2019 at 19:05

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