20 PSI Pressure Drop with Zurn NR3XL PRV Normal?

[Slicker]

Hi everyone. Moved into a new house a few years ago and had somewhat mediocre shower pressure in the upstairs bathroom. I adjusted the PRV (a Zurn NR3XL 3/4") at the water main so that we had 60 PSI static pressure measured at the tank-less water heater cold intake service port a couple of feet from the PRV. This appeared to solve the shower issue for the most part except if any other faucet in the house was turned on. Then the pressure in the shower was weak again.

I went ahead and read the pressure (again at the cold water service port) with a single faucet open (dynamic pressure?) and it only reads 40 psi. Based on the literature I found online regarding this PRV, I think I should only be seeing a 20 psi drop if the flow was 20gpm which I'm pretty sure this single faucet is nowhere near flowing. Again this is with a 3/4" PRV and 3/4" PEX tubing leading to and from it. I don't know the diameter of the water main in the basement but it looks larger than the 3/4" PEX leading from it so I'd guess it was closer to 1". Unfortunately, I also don't know what the pressure is before the PRV. This is a new subdivision so I'd think the exterior water lines would be in good condition.

Is it likely that this PRV is defective or otherwise needs maintenance (clogged intake screen)? I thought I'd try asking here before trying to clean the screen or replacing it.

Thanks in advance.

 

[Reach4]

Does your outside faucet in front connect before the PRV? If so, measure the pressure there. If no, try cranking the PRV up to 80, and see if that happens.

Your idea of a clogged intake screen could certainly cause a pressure drop.

 

[Sponsor]

 

[Slicker]

Does your outside faucet in front connect before the PRV? If so, measure the pressure there. If no, try cranking the PRV up to 80, and see if that happens.

Your idea of a clogged intake screen could certainly cause a pressure drop.

All the outside faucets are connected after the PRV. I turned the adjustment screw on the PRV all the way in and the pressure gauge read 70 psi. When I turned on the faucet it read 50 psi so it looks like I'm still getting a 20 psi drop. Thanks.

 

[wwhitney]

I adjusted the PRV (a Zurn NR3XL 3/4") at the water main so that we had 60 PSI static pressure measured at the tank-less water heater cold intake service port a couple of feet from the PRV.
[ . . . ]
I went ahead and read the pressure (again at the cold water service port) with a single faucet open (dynamic pressure?) and it only reads 40 psi.

What was the flow rate from that faucet during the second pressure measurement?

If it was an indoor faucet flowing 1.5 gpm, then you have a severe restriction between the water main and the measuring point (or rather, the least upstream point that is common to your measuring point and your open faucet).

Cheers, Wayne

 

[Jadnashua]

Short answer ...no you should not have a 20# drop with flow. It could be a restriction (well that’s true regardless) or a defective PRV..

 

[Slicker]

What was the flow rate from that faucet during the second pressure measurement?

If it was an indoor faucet flowing 1.5 gpm, then you have a severe restriction between the water main and the measuring point (or rather, the least upstream point that is common to your measuring point and your open faucet).

Cheers, Wayne

It was an indoor faucet and just the cold handle. As an experiment, I turned on the faucet (cold handle again) and flushed the toilet. When I did that the pressure went from 70 to 50 again. However, after that I only got a 10 PSI drop with just the faucet and a 20 PSI drop with just the toilet.

So it looks like the next step here is to open up the PRV and check the screen, or go ahead and replace it. If I do end up replacing it, would there be any advantage to using a different manufacturer (Watts?) or different model in the Zurn range - e.g. XL70?

Thanks again for all the replies.

 

[Reach4]

So it looks like the next step here is to open up the PRV and check the screen, or go ahead and replace it. If I do end up replacing it, would there be any advantage to using a different manufacturer (Watts?) or different model in the Zurn range - e.g. XL70?

I have great hopes for cleaning the screen. Next I would try the RK-35 repair kit https://www.zurn.com/products/water-control/pressure-reducing-valves/rk34-nr3xl

 

[Slicker]

I have great hopes for cleaning the screen. Next I would try the RK-35 repair kit https://www.zurn.com/products/water-control/pressure-reducing-valves/rk34-nr3xl

So no luck with the screen. It was clear of debris when I opened the valve up. Rinsed it out anyhow and getting same 20psi drop when a fixture is turned on. Next up is to either rebuild or replace the PRV.

Quick question. I figured out that the supply line from the street is 1" which transitions immediately to 3/4" pex and the PRV. Would there be any advantage to upgrading the pipes to 1" from the supply line to where the 3/4" pex branches off to go to the water heater and rest of house (its only about 2 ft from PRV to where the lines start branching off)? Similarly any advantage to leaving the 3/4" pex and installing a 1" PRV? Or would the increase in flow (assuming there is any) not really be worth the effort for either option?

 

[Jadnashua]

It depends on how long the section is from where it comes in at 1" to where it goes to 3/4". When it is short enough, it doesn't affect the dynamic pressure all that much...it's when it is longer. There are calculators that can tell you the pressure drop through various pipes. It also depends on your peak volume, as the friction (therefore pressure drop) goes up the faster you try to get the water (or any fluid) to flow.

One way they gauge what size supply line you need is by counting up the fixtures in your home.

 

[wwhitney]

Can you temporarily bypass the PRV to check that the pressure drop goes away? That would be definitive.

I didn't quite follow your piping layout (a diagram would help), but here's a relevant comparison you can use:

1" type K copper has an ID of 0.995"
3/4" PEX has an ID of (7/9)*(7/8) = 0.681"
pressure loss due to friction is modeled by the Hazen Williams formula as d^(-4.8655)
The roughness coefficient for the two materials is about the same.

The upshot is that for the pipe only, ignoring fittings, 1' of 3/4" PEX is equivalent to (.995/.681)^4.8655 = 6.3' of 1" copper. PEX fittings may be comparatively more constrictive than copper fittings, but I haven't compared the published equivalent length numbers (and that would depend on which type of fittings you use).

So you can use that to figure out the relative impact of the the PEX.

Cheers, Wayne

 

[Reach4]

I turned the adjustment screw on the PRV all the way in and the pressure gauge read 70 psi.

I am not sure you need a PRV. If the outside pressure is less than 80, I would not want a PRV.

If you make plumbing changes and keep the PRV, I would want a pressure gauge before the PRV.

 

[Bannerman]

Your home's plumbing was designed with a 1" supply line. You did not specify which material the 1" supply is made from, but I will assume copper.

If the incoming pressure is consistently 70 psi, I agree that the PRV could be removed. Some municipal systems experience considerably higher pressures during the night when little water is normally consumed and the water storage tower water height becomes maximized.

If a PRV is required, I'm not understanding why a model designed for a 3/4" water supply would be installed.

The Zurn spec sheet graph shows that specific 3/4" PRV, will impart a 6 psi pressure loss @ 10 gpm, 10 psi @ 15 gpm, 15 psi @ 20 gpm, and 20 psi @ 25 gpm.

The 1" version of the same model is 4.5 psi @ 10 gpm, <8 psi @ 15 gpm, 12 psi @ 20 gpm, 16 psi @ 25 gpm and 20 psi @ 30 gpm. These rates are based on a 50 psi pressure differential through the PRV.

As PEX has a smaller ID than an equivalent diameter copper pipe, the additional bottleneck through the 2 - 3/4" PEX sections is further restricting flow to your 1" main line and branch runs downstream of the PRV.

There may be alternate 1" Zurn models or other brand PRVs with lower pressure loss characteristics. A full 1" PRV will also allow the 3/4" PEX bottlenecks to be eliminated which combined, should result in higher flow capacity downstream including when multiple fixtures are being utilized at the same time.

 

[Slicker]

I thought it might be easier if I just post some photos of the current set up. Seems like there's a lot of potential issues that might explain what's going on. The supply line into the house doesn't look like copper. Some sort of plastic. There's a brass adapter on it which appears to be labeled 1" to 3/4". The rest of the piping from that point on appears to be Zurn 3/4" PEX with brass or plastic fittings.

Again, I appreciate all the replies I've gotten on this thread.