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 20psi Pressure Drop When a Fixture is Open
Author: Stoopalini (TX)

I'm looking for assistance in trying to diagnose an issue where my house experiences a 20psi pressure drop whenever a single fixture is opened. This can be a sink, an outdoor spicket, or a flushing toilet, etc ...

The house was built in 1998, is slab on ground, has a 3/4" PVC feed from the meter box to the house. The house then has 3/4" copper main lines with 1/2" copper branches to each fixture.

About 10 years ago, I had a water softener system installed in a small shed behind the house. To plumb it, we intercepted the water main line at the front, and created a loop. I installed 3 3/4" ball valves at the intercept point, so I could bypas the softener loop if there was even an issue with it.

I made a diagram to try and help explain the issue, and what I've tried to resolve it. Points C, D and E are outdoor water spickets. C and D are on the "send side" of the softener loop. I had these installed because I wanted a couple of outdoor spickets that were not soft water. Points A and B are just points where the city took pressure measurements.

Last week, the PRV went bad and was leaking horribly. I had a plumber come out and replace it with a new Watts brass unit. I asked him to set the pressure to 70psi, which he said he did. After he left, we noticed the pressure wasn't as good as before the repair, so I bought a gauge and checked it. It was at 60psi. So I adjusted the PRV to set it to 70, and then checked with a sink flowing, and the pressure dropped to around 50psi. I then increased the PRV to 80psi, and checked again, and the pressure is dropping to 60psi when a single sink is open.

So I called the plumber and explained the issue, and he recommended I call the city. They came out to troubleshoot and took some pressure readings. At point A on my diagram, the pressure was 84/78 (84 static, 78 running). At point B, the pressure was 84/65. So they decided to replace the meter.

After the meter replacement, I still had the 20 PSI drop at the house.

To troubleshoot, I bypassed the softener loop to send water direct to the house, and took measurements at point E, and still see the 20psi drop. I also have taken measurements at point C, using point E to flow water, and still see the 20psi drop. I did this to rule out the house plumbing.

So I called the plumber back, thinking the PRV may be bad. He pulled the PRV, the check valve, and the cutoff valve, and we inspected everything. They all seem fine, with no buildup or corrosion. So I asked him to replace the PRV anyway, which he did. After the 2nd new PRV, nothing changed and I'm still experiencing a 20 psi drop when a single fixture is opened.

This makes me think there's either an issue with the city supply, or with the pipe between the meter and the 3 valve bypass, or possibly a system design issue.

The city supply is showing a 6psi drop, measure at point A. Would this translate to a 20psi drop due to distance of pipe to where I am taking measurements? The main feed to mu house is a 3/4" copper line, which they tell me connects to a 4" main about 100 feet away.

I've read that a PRV needs a delta of 10psi between the incoming pressure and the outgoing pressure, to function properly. So I backed the PRV down to 50psi and tested. At 50psi, the drop is no longer 20psi, but is around 10psi.

We really need a min of 60psi due to a recent master bath remodel, where we installed multiple shower heads. So to get even close to 60psi when the shower heads are in use, I've set the PRV to 80psi; but I would prefer to be able to set the PRV to something lower, like 70, if I can get the system to maintain that pressure when a fixture is in use.

Any help with this would be greatly appreciated!

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 Re: 20psi Pressure Drop When a Fixture is Open
Author: hj (AZ)

A "good" prv should not drop the pressure more than about 5 psi maximum, and there is NOTHING that says a PRV "needs a 10 psi differential to operate". You do not apparently have any means to test the pressure immediately after the PRV, so we really do NOT know where the pressure loss is occurring.

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 Re: 20psi Pressure Drop When a Fixture is Open
Author: Stoopalini (TX)

The 1st replacement PRV was a "Watts LF N55B M1", and the 2nd replacement PRV (the one currently in use) is a Wilkins 70XL.

Isn't the system pressure uniform throughout the system? You're right, I do not have a gauge installed on the PRV outlet, but thought since the pressure at points C and E are the same, I could expect those same readings to be between the PRV and check valve if I had a gauge there.

I'm wondering if the issue may be with the city's feed to my meter. It's a 3/4" pipe, which has a 90* bend, and runs for another 100' or so after the bend, where they tell me it connects to a 4" main.

I assume I didn't notice the pressure issue before this because the original PRV most likely failed a while ago, and was probably allowing full pressure to pass. I did have 3 hot water heater tank failures (internal tank ruptures) in the past 5 years. I've since replaced one tank with a tankless, and added an expansion tank to the other.

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 Re: 20psi Pressure Drop When a Fixture is Open
Author: Stoopalini (TX)

Here's a closer look at the softener bypass:

So for testing purposes, I opened valve 1, and closed valves 2 and 3. Then tested with a gauge on point C, and opened the spicket at point D, and still saw the 20 psi drop.

This tells me it's not related to the house plumbing at all, and has to be either the city supply, or something to do with the piping from the meter box to the bypass.

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 Re: 20psi Pressure Drop When a Fixture is Open
Author: Paul48 (CT)

Knowing how many gpm you can get at "A" should tell you what you need to know.

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