Normal pressure drop?

My pressure guage at the entrance of my house reads ~60 psi. WHen i open the farthest Lav cold water, the pressure drops to about 45 psi. Is that normal? I am in the country on city water. There is a pressure regulating valve at the meter by the road, and it enters my house about 365' later. The lav is roughly 75' of developed length of pipe away from where it enters the house. The toilet next to the lav also causes the pressure to drop about 12 psi.

I don't know exactly how big the pipe is from the meter, but the previous owner told me he thought it might be 1-1/4". Coming into my house, though, is 3/4" pvc. The rest of my house is plumbed with CPVC, with 3/4" main trunk and 1/2" branches to fixtures.

One more thing, the water pressure out of the lav faucet is high for a few seconds, then drops off.

SOunds like the PRV may be failing.

jadnashua said:

SOunds like the PRV may be failing.

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Is there an easy way to fix it? Can I adjust the screw up and down to see if something is froze up? Or will it require taking it apart and cleaning something?

Prv

If the PRV were at the house, I would say it was failing. Since it is so far away, that could be normal friction loss, especially since we do not know the pipe size. A pressure reading at the PRV, some have a plugged opening for a gauge, is the only way to tell if it is not working properly.

Also, is there any significant elevation change from the street to the house? If the pipe is too small, or it has a kink in it from some action (drive a truck over it, or a big tree or something), your static pressure would be fine, but as soon as you try to run any volume, it just can't provide the volume and keep the pressure up.

I finally got around to finding out what size of pipe i have from the PRV to the house. Coming out of the PRV meter box is 3/4" copper for about 12". Then there is a transition to...3/4" PVC. Yeah, that is a little upsetting. I thought that maybe it transitioned up to a larger size because a 3/4" copper to 3/4" PVC fitting was easier to find, but I dug about 5 foot away from the meter box and it was still 3/4". I stopped there. I guess its possible that there is a full 10' stick of 3/4" then a transition to something bigger, but I doubt it. I am going to find that out tonight.

As of right now I am planning on running 1-1/2" from the PRV to teh house and abandoning my current line. I will be able to save about 100' of developed length or so by taking a different route.

What fitting should i use to go from 3/4" copper to the 1-1/2" PVC underground?

Thanks

tbdub said:

My pressure guage at the entrance of my house reads ~60 psi. WHen i open the farthest Lav cold water, the pressure drops to about 45 psi. Is that normal? I am in the country on city water.

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At what flow rate? 2 GPM?
I know they keep pipe velocities below 2 to 5 fps to reduce friction.

I'm trying to get my water co. to tell me the "pump curve" that I would see at my house inlet.

This website
http://www.engineeringtoolbox.com/
lets you figure your whole plumbing system as if it were an electrical circuit, but you need to know what your WaCo is giving you.

The most you can get out of a 3/4" meter is 14 gpm. Of course, that's fine if your house is within fifty feet, but when you're talking ~400' you've got a whole new problem.

1-1/2" PVC should be pretty good. It's sufficient for about twice the fixtures in a 1-bath home according to UPC if the street pressure range is 45 - 60 lbs and the distance is about 400'. There is also some friction loss in elbows but I'm guessing it's a pretty straight run.

http://www.engineeringtoolbox.com/pe-pipe-pressure-loss-d_619.html

1-1/4" plastic pipe at 13 GPM is 2.7 FPS and @ 400' is about a 4 PSI drop.

Thatguy said:

I'm trying to get my water co. to tell me the "pump curve" that I would see at my house inlet.

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You won't get a pump curve, or it will be meaningless, as you don't have the entire network (the municipal piping grid and pumps to work from.) I used to model and troubleshoot firewater & cooling water systems for refineries, especially expansions. We would have multiple pumps of varying capacity and quite a few branches with large elevation changes, many exchangers, and critical scenarios.

The pump curve only really matters if you are a signficant user of the pump capacity. At a few gpm max, you are not. At a few hundred or a thousand gpm you could become imporant to the pump and impact its discharge pressure.

Better to put in a test gauge and read the pressures yourself from the supply side of the PRV. That way you can determine a range of supply pressures that apply to your application at various times of day/season. Once you have the supply you can use the PRV curves to determine the nameplate DP for various demands.

As for the OP, the PRV sounds suspect. Try putting a gauge near the PRV discharge and the house to determine the pipe DP when there is even a small flow. (Be prepared to swap gauges as a cross check.) I've got a weak PRV in my home and it has about 10 psi drop from the static set point with an insignificant flow.

My suggestion is to be ready to replace the PRV (with a larger size if needed) but to hold off on the new piping until it is clear that the PRV is not the culprit. Afterall, that is a lot of line!

Herk said:

The most you can get out of a 3/4" meter is 14 gpm.

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Can you explain that to me? According to this meter, badger meter, it says typical operating range is 3/4 -35 GPM and max continuous is 25gpm.

Runs with bison said:

The pump curve only really matters if you are a signficant user of the pump capacity. At a few gpm max, you are not. At a few hundred or a thousand gpm you could become imporant to the pump and impact its discharge pressure.

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So the pump curve at my house entrance is essentially flat at 30 to 80 PSI or so.

Thatguy said:

So the pump curve at my house entrance is essentially flat at 30 to 80 PSI or so.

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On the upstream supply side of the PRV it should be essentially flat...at least until the flowrate becomes great enough that you begin losing some pressure through the lines and meter. (This would be part of the system curve for the supply side.)

Really the best thing to do would be to attach a gauge on the supply side and then so some measured water draws. That will let you create a supply side system curve.

Then of course you have the home piping network and the PRV. The PRV takes increasing DP as the flowrate rises. There are curves for this.

Runs with bison said:

On the upstream supply side of the PRV it should be essentially flat...at least until the flowrate becomes great enough that you begin losing some pressure through the lines and meter. (This would be part of the system curve for the supply side.)

Really the best thing to do would be to attach a gauge on the supply side and then so some measured water draws. That will let you create a supply side system curve.

Then of course you have the home piping network and the PRV. The PRV takes increasing DP as the flowrate rises. There are curves for this.

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You have a link to a typical PRV? I doubt that I have one in my house.

Thatguy said:

You have a link to a typical PRV? I doubt that I have one in my house.

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Watts Pressure Reducing Valve

tbdub said:

Watts Pressure Reducing Valve

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Thanks; if PSI = volts, this is a voltage regulator.
http://www.national.com/mpf/LM/LM317.html

Tbdub said:

Can you explain that to me?

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Yes, I can.

I used to install sprinkler systems and for any practical purposes 14 gallons per minute is maximum because if you try to get more your pressure drops. Hook a hose and a sprinkler to your front and back hydrants on the house and just see the trickle you get out of the faucet with a 3/4" water meter. You're using 14 gpm but your pressure has become too low to be useful. It's also based upon turbulence and friction loss and other factors.

I have a 3/4" water meter and am able to run four 40' sprinklers at one time, but the bath sure takes a long time to fill up.

So, when I said that you can only get 14 gallons per minute, I meant that practically.

Friction loss goes up as the water speed (feet per second) goes up. I wouldn't design a sprinkler system without knowing how many gallons per minute I could practically expect while keeping a usable pressure in the line. And I wouldn't supply a house without knowing how many fixture units were in it and what the distances and type(s) of pipe is/are.

You don't want the water flowing too fast in the pipe - it can lead to erosion. So, while, if the pressure is sufficient, you can get more water through the pipe, you don't want to do it.

tbdub said:

Can you explain that to me? According to this meter, badger meter, it says typical operating range is 3/4 -35 GPM and max continuous is 25gpm.

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It's advertising exaggeration. Our Elster C700 (formerly ABB, formerly Kent) meters are rated basically the same but we see 18 gpm as a max. Our test facility is 100 meters from the primary pump for our city and water pressure at the outlet of the water plant is steady at 100 psi.