Water company forced me to get installed a dual check valve, secondary shut off and an expansion tank. The installation of the expansion tank doesn't seem correct since it is before the check valve.

I thought the purpose of the expansion tank was to absorb excess pressure from the water heater heating up the water in the cold input. But since its in front of the check valve, the excess water cant get to it right?

Is this installation correct? Here is a diagram of the setup. enter image description here

  • 1
    I agree with both answers (so far) indicating that this is wrong. I'd suggest calling the plumbing company back and asking them to explain what it is that the expansion tank is going to do, sitting in that particular installed position. When the plumber can't explain it, ask him to correct the situation on the company's dime, since they're the ones who put it there.
    – FreeMan
    Aug 12, 2023 at 13:21

3 Answers 3


No, the point of the expansion tank is so the water that is trapped in your house's piping by the check valve has room to expand when it is heating. If the tank was installed on that side of the check valve, it won't help your home's piping at all.

Before you had a check valve, the "expansion" was provided by a little water being able to back up into the supply side. With the check valve stopping that, your pipes will be under increased pressure if they don't have any room for expansion.

  • agreed, l've never seen a set up like that. A double check valve is usually only required if the supply is feeding a sprinkler system. A thermal expansion tank needs to be near the WH. Doesn't matter if it's on the hot or cold side, but def behind any check valves. Aug 11, 2023 at 17:40
  • It sounds like the setup they installed is going to cause more problems for me and my system. Cant believe a legit company did this. This was a local plumbing company and not some craigslist contractor. Frustrating.
    – GER
    Aug 11, 2023 at 18:52

The intent of the expansion tank in that position (since you mentioned it was "forced by your water company") may be different than the usual intent of one as thermal expansion tank on the other side of the check valve.

If your water company requires that setup, you'll need another tank to actually be your thermal expansion tank, on the same side as the water heater (past the second shutoff.)

  • I dont know what setup is technically required. I got a letter saying, "You must install a Dual Check Valve and Secondary Shut Off.... It is suggested that you also install an Expansion Tank as well." I handed that to the plumbing company and this is what they built.
    – GER
    Aug 11, 2023 at 18:55

Check valve must be in line before expansion tank tee. This drawing is wrong. On cold line shut off valve, check valve, expansion tank, then circulation pump if applicable, water heater.

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