Why does shower pressure drop when a second fixture turns on?

[All Shore Plumbing NY]

Static pressure throughout the house seems fine, but the moment someone turns on a second shower or even just a faucet, the pressure at the first shower drops much more than I'd expect. The house was built in the 1970s and still has what I believe is the original supply piping. I'm trying to figure out where you'd normally start looking in a situation like this. Would you suspect aging piping, the PRV, the main shutoff, or something else causing the pressure drop under load?

 

[Reach4]

Does the pressure at the first shower seem to stay reduced while the second shower or faucet is still on?

 

[Sponsor]

 

[Weekend Handyman]

I went to a friends house to help her figure out figure out a situation where a sink would run then suddenly stop. No were no other issue in the house. I switched out the faucet, angle stops and flushed the pipe. Nothing would work. She got a plumber in and they changed the mixing valve (at the boiler), which also serves every other fixture in the house. There is not way this should have resolved the issue, but did. All that is to say, I have seen mixing values do things that just don't make sense.

 

[Storm rider]

I would start looking for an obstruction or restriction somewhere. Maybe a shutoff that isn't fully open. If there is a filter or softener in the system I would look at them. There have been several previous discussions on the forum about pressure falloff vs. volume falloff.

 

[RetiredInGueydan]

Static pressure throughout the house seems fine, but the moment someone turns on a second shower or even just a faucet, the pressure at the first shower drops much more than I'd expect. The house was built in the 1970s and still has what I believe is the original supply piping. I'm trying to figure out where you'd normally start looking in a situation like this. Would you suspect aging piping, the PRV, the main shutoff, or something else causing the pressure drop under load?

What is the pressure and volume at the washing machine and water heater?

 

[Sylvan]

Possibly an obstruction, such as galvanized piping or the rubber used on the water heater nipple to prevent backflow is restricting the flow as it is blocking some flow