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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 Mar 6 at 22:39
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    Is this a new problem, or has it always been like this? – Jimmy Fix-it Mar 6 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 Mar 7 at 15:36
  • We bought this house recently so have no information on what the water was like before. – Sarge Mar 7 at 15:36
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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 Mar 15 at 4:04
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There are one or more restrictions - the PRV itself might be one of those restrictions, or it might be one of a variety of pipe/fitting problems. My classic "lots of pressure, almost no flow" was traced (when I got irritated enough to rip the pipe run apart to find it) to a fitting "almost, but not quite completely blocked with solder" - plenty of pressure until you opened the valve.

Some PRVs have strainers (coarse filters, more or less) built in that might need to be cleaned. Then again, it might need to be replaced, possibly with a different model that flows better. Your pipes could have partial blockages or restrictions other than vastly excess solder, that just happened to be my blockage tale.

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