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My shower valve leaks. It started leaking a couple of weeks ago and I replaced the cartridge. I had to re-use some of the old parts because the new ones wouldn't fit. The valve stopped leaking for a while but it now leaks again, just like it did before.

Two or three months ago my water heater started leaking just a bit. I was going out of town, thought it was just that the water heater was old, and replaced it. Then I checked and it was only 8 years old, making me think that age was not the issue.

I'm thinking the two problems are related. I think either my water pressure from the city is too high, and I need to put in a pressure regulator. (I haven't checked it yet but plan to later today -- it's 2:00 am now.) Or I need to put in an expansion tank to take some pressure off as the hot water causes the pressure in the pipes to rise.

Am I right in my thinking that one of those two problems caused the water heater to fail prematurely and now is causing the shower valve to leak? If so, I suspect both those jobs are beyond my capabilities, and I'll need to call in a professional. And to do it as soon as possible, before a pipe springs a leak somewhere beyond just at the shower valve.

Anything more than checking water pressure from the city that I can do to isolate the issue?

UPDATE: I bought a water pressure gauge and tested it. During the day it was about 110 psi. During the night it apparently peaked at 140 psi.

The fix thus seems to be to put in a pressure regulator and/or an expansion tank. I've looked into that and I doubt I can do it myself. Looks like I'll have to hire a plumber.

UPDATE 2: I talked to a guy from the city. He said they have made no changes to the water pressure in my neighborhood. Since we are at the bottom of the hill our water pressure has always been at about 110 psi for the 30 years we have lived here. He said it shouldn't be a problem unless we don't like the high pressure. He also said that there is no backflow preventer between our house and the water main.

I also talked to a plumber. He seemed happy to put in a pressure reducing valve if we want one. (No surprise there.) He also said that if we put in a pressure reducing valve we should also put in an expansion tank.

But I was surprised that he was not that concerned about the high water pressure. Maybe the leak in the water heater and the leak in the shower are just unrelated problems that happened coincidentally rather than as a result of high water pressure, as was suggested in the first comment here.

I'll think about it some more.

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  • Could be pressure. Could be chemicals in the water causing excessive corrosion. Could be coincidence.
    – manassehkatz-Moving 2 Codidact
    Jan 4, 2022 at 12:14
  • Good points. Any idea on how to find out which? I'm going to buy a gauge to check the water pressure coming into the house from the city. I'm not sure what else to do.
    – Daanii
    Jan 4, 2022 at 15:58
  • Since you already are looking at pressure, start with that. If it is chemicals in the water, others will be having the same problems - in my area (Maryland) this was a big deal 20 - 30 years ago - everyone was getting pinhole leaks due to reaction of copper pipes to some chemicals in the water, and I have heard of similar sorts of problems affecting rubber seals, etc.
    – manassehkatz-Moving 2 Codidact
    Jan 4, 2022 at 16:29
  • Just as a guideline, a good range for domestic water pressure is 65 to 70 psi. It can be higher than that and still not be a problem but once you get north of 100 psi you can start expecting problems.
    – HoneyDo
    Jan 4, 2022 at 18:14
  • As my update above says, it looks like my water pressure is above 100 psi. I think that is probably the problem. We'll see what happens. Thanks for the helpful comments.
    – Daanii
    Jan 5, 2022 at 2:23

2 Answers 2

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Since you have city water any change in volume caused by the water heater cycling will simply cause a little water to move in or out through the main. There won't be any significant change in pressure as a result.

Adding an expansion tank won't reduce the pressure supplied by the city. Only a pressure regulator installed on the main will do that.

This page provides a sizing guide for expansions tanks for use with water heaters. A pressure reducing regulator won't act as a check valve, but backflow won't occur until the pressure in the residence exceeds that main pressure. They suggest that a 2-gallon expansion tank is a good starting point for use with a 50-gallon water heater, though a larger unit may be needed.

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    Is this true even if there's a backflow preventer?
    – spuck
    Jan 5, 2022 at 21:48
  • @spuck A backflow preventer will, as it's name suggests, prevent water from flowing from the house back to the main. AFAIK they are not commonly installed between municipal water systems and residences. An expansion tank would be needed to handle changes in volume caused by water heaters, boilers, ... in a system isolated by a backflow preventer. I've updated the answer since an expansion tank is also appropriate with a pressure reducing regulator.
    – HABO
    Jan 6, 2022 at 4:25
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    My city confirmed that there is no backflow preventer between the city water system and my residence.
    – Daanii
    Jan 7, 2022 at 22:30
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As I mentioned in Update 2 to my question, neither the city water guy nor my plumber seemed to think a pressure reducing valve was essential. And though it took me a long time to finally fix it, the shower leak turned out to be due to a rubber washer that had worn out.

Given all that, I decided not to install the pressure reducing valve right now.

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