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We can't run a shower along with any other fixture in the house or it will slow to a trickle.

I've measured the pressure with a gauge and we get ~110psi before the pressure-reducing valve and ~50psi after it. Apparently, that's the recommended pressure.

I turned on the basement bathroom faucet and saw a ~10 psi drop in the internal pressure. Is that expected - or does the PRV keep a constant 50psi even under demand?

This is a 2 storey + basement 3800 sq ft house. I measured the pressure after the PRV using the tap for the washing machine. The house was built in 1986 - although there's evidence it's been replumbed since.

I'm now at a loss for what could be causing this.

It happens on both of the two working showers. For example, in the basement, if the toilet cistern is filling then the shower flow rate is about half. Upstairs, if anything else is running then that shower essentially stops.

If relevant, we also have a shower panel in another upstairs bathroom which is unusable because it is either hot or cold - nowhere in between.

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  • What size are the pipes?
    – crip659
    Commented Mar 6, 2021 at 22:39
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    Is this a new problem, or has it always been like this?
    – Jimmy Fix-it
    Commented Mar 6, 2021 at 23:22
  • The pipes are 3/4" I believe. (The PRV is marked as 3/4", the fittings on the hot water cylinder are marked as 3/4")
    – Sarge
    Commented Mar 7, 2021 at 15:36
  • We bought this house recently so have no information on what the water was like before.
    – Sarge
    Commented Mar 7, 2021 at 15:36

2 Answers 2

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does the PRV keep a constant 50psi even under demand?

It is designed to try to, by opening all the way up. If for some reason it is not opening all the way up, it could be a cause of downstream flow problems. I would replace/repair that first unless you suspect something else as the cause. Any other restriction, including limitations present due to system design (pipe/fitting size), and/or system condition could also cause the problems you describe.

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  • We had the PRV replaced and it improved things markedly. We do still get water starvation but it's better.
    – Sarge
    Commented Mar 15, 2021 at 4:04
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If on city water tuberculosis in the pipes to the main can cause this problem. If on a well you can get sediment build up in the offset between the pump and the house that will also cause this problem. I’ve replaced well line that was almost completely clogged due to sediment and it had the same problem, tons of pressure to start then it fell to nothing once the pressure tank was empty. Basically given time pressure recovers through the restrictions, but when there is increased demand it can’t get through fast enough. On a well a sediment sock on the pump can help prevent this buildup but won’t completely stop it.

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    – Community Bot
    Commented Jul 1, 2022 at 17:26
  • tuberculation, you mean?
    – ThreePhaseEel
    Commented Jul 2, 2022 at 1:42

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