Can the check-valve in my hose bib cause high pressure readings?
March 29, 2019 9:48 PM   Subscribe

I had the main water shutoff and pressure reducing valve at my house replaced yesterday and I'm getting shockingly high pressure readings past the pressure reducing valve. I have a theory this is being caused by the check-valve in the back of my frost-proof hose bib. I can't find any google results or references to back this up. Did I fool myself the first time with the high readings or am I fooling myself the second time by explaining away the readings? Longer version below the fold.

Long story:

I had the main water shutoff and pressure reducing valve replaced yesterday. The main shutoff was a 25 year old gate valve so I had it replaced with a quarter turn ball valve. The pressure reducing valve was replaced at the same time due to replacement of the surrounding pipe. Today while in the hardware store I picked up a water pressure gauge meant to connect to a hose bib to double check the performance/setting of the new reducing valve. Conveniently (for whatever reason) my front hose bib is connected to the main water feed directly past the meter, before the pressure reducing valve and the back hose bib is connected past the reducing valve. This gives me a place to check the pressure on both sides of the valve. Both hose bibs are Woodford Model 17's.

I connected the pressure gauge and got the following results:
Front: 110 psi; seemed high for street pressure, but within possible.
Rear: 80 psi; way higher then I expected since the regulator was supposedly set to 50 psi and has a settable range of 25 to 75psi.

I went inside and ran a tap for a while thinking this must have been a fluke, headed back out and saw the same 80 psi. I then proceeded to try several iterations of removing and reconnecting the pressure gauge, going out front and out back, leaving it sitting for an hour, and turning on and off the water supply to the house. I observed the following:

When I opened the bib valve slowly the pressure would climb to 40 - 50psi, then as I passed the halfway open the pressure could climb rapidly, sometimes well into the 120 to 140 psi range. Once it hit 180psi. This freaked me the f out. I immediately texted the plumber who did the install, he responded he could not get to me tonight but would be able to swing by tomorrow. I was/am nervous about a catastrophic failure in the middle of the night so I did some google-ing and thinking. Google came up empty, suggesting the only reason pressure would rise would be due to thermal expansion. However I had a thermal expansion tank added at the same time for just such a reason, and the hot water was never used during my pressure testing so the tank should have been at a relatively consistent temperature and not in a cold to hot heating cycle.

My theory:

When I open the hose bib valve (slowly), the valve "tube" and gauge are filled with water and pressurized to the house pressure (hopefully 50psi). The end of the valve stem has a check valve on it, and as I further open the valve stem/check valve assembly moves towards the bib outlet, pushing the incompressible water into the gauge, causing high pressure readings. I could not find mention of this in any of my google searches. From my basic understanding of fluid dynamics once the valve is open at all; the position should not impact pressure at the gauge since there is no flow, however I could move the dial just by adjusting the bib valve position. I think I have confirmed my theory by loosening the gauge from the bib slightly once the bib is fully open to bleed off excess pressure, then tightening, at which time the gauge reads a level 50 psi as expected.

Can someone who knows something about plumbing weigh in? I don't want to waste the plumbers time on a weekend if this is operator error on my part.
posted by token-ring to Home & Garden (11 answers total)
Yes, I think your theory is correct. You can't accurately measure pressure unless the valve is fully open. There isn't a check valve. It is just a simple screw valve like any other simple valve, but with a very long stem, a foot or so, long, so that the washer and valve seat is far inside the wall. As you twist the handle, the valve stem screws outward toward the bib to raise the washer off the valve seat. Since the rubber washer is resilient, you have to screw the valve outward a certain amount before the water can flow. This reduces the space in the pipe between the washer and the gauge just slightly. But as you point out, water is incompressible, so even a tiny reduction in volume between the washer and the gauge before the washer lifts off the seat means the pressure goes very high. The only thing keeping it from going even higher is that the water is able to move into the bulb of the gauge and there is probably some air space in the pipe.

Once you have the valve open enough that water can flow past the rubber washer, you should have an accurate measurement. I wouldn't worry about the momentary fluctuation as you open the valve slowly. Just open it quickly all the way and be done with it. If the pressure reading is good when the valve is fully open, everything is okay.
posted by JackFlash at 10:53 PM on March 29, 2019

It's also possible that if you are doing multiple tests, loosening and tightening the valve, you may be making things worse because you are filling up the pipe with water between the valve seat and the bib opening with water, letting all the air escape. That means there is no air space to absorb the compression as you open the valve.

You actually would get a better result if you allow the bib to drain before you attach the gauge so there is air in the pipe to absorb the excess pressure as you slowly open the valve as described above. You should get the same reading with or without air in the pipe since the air will compress to match the water pressure, but with air in the pipe, you won't get the spike in pressure as you slowly open the valve.
posted by JackFlash at 11:04 PM on March 29, 2019

I think you're right token-ring — and there apparently is a check valve. It's #7 in the diagram and on the parts list.

You might be able to confirm your theory by finding a faucet in the house which matches the threading on your pressure tester. In my house, there's such a faucet on the water supplies to the clothes washer and the water heater drain.
posted by jamjam at 11:32 PM on March 29, 2019 [1 favorite]

Pressure reducing valves are not magic. The only way they can reduce pressure is by restricting flow: when the pressure downstream of the valve rises above the valve's set point, the valve responds by closing down. But once the valve is fully closed down, it cannot possibly reduce downstream pressure any further; and since water is both incompressible and massive, the pressure that the pipework downstream of a pressure reducing valve settles to under conditions of zero flow are in general unpredictable.

Consider conditions immediately downstream of the pressure reducing valve when there's an open tap somewhere in your house. This is a flow condition, so the pressure reducing valve can adjust its own aperture in such a way as to keep the pressure immediately downstream of it at the set point. But the lower the flow, the smaller the aperture the valve has to create in order to maintain itself, rather than the tap down at the end of the line, as the bottleneck.

As you close down that distant tap, the aperture in the pressure reducing valve has to track the aperture in the tap in order to keep your pipework at the set pressure. But the pressure reducing valve can't actually see the aperture in the distant tap, only the pressure in its own downstream side.

All the water flowing toward the distant tap has mass and therefore inertia. At the instant that the distant tap closes, that inertia will create a region of very high pressure immediately upstream of it, and that region will have a front that propagates backwards up the pipe at the speed of sound in water. When that front arrives at the pressure regulator it will continue to propagate through the regulator's aperture even as the regulator detects it and slams that aperture shut.

At this point you have pipework with a pressure higher than that set by the regulator, and a regulator with a fully closed aperture that simply has no way to get rid of that pressure. The aperture won't open again until the downstream pressure drops below the set point, but all the taps are closed and there's no way for this to happen.

If you want to guarantee that your pipework never gets stressed by internal pressures significantly higher than the nominal maximum set by your pressure reducing valve, you need to introduce compressible elements. This will give the shock fronts caused by turning off taps somewhere else to go, and you will end up with pipework pressure under no-flow conditions that's only very slightly above the set point of your regulator.
posted by flabdablet at 12:04 AM on March 30, 2019 [1 favorite]

There should be water hammer reducing air columns somewhere in the house. It’s just a one or two foot length of capped vertical pipe on a tee connector. Ask your plumber to install one where there’s exposed pipes, eg utility room.
posted by seanmpuckett at 4:07 AM on March 30, 2019 [1 favorite]

I left the valve on the bib overnight after bleeding the space beyond the check-valve (part #7 as jamjam pointed out). It read 50psi when I went to bed and again this morning, with the max indicator showing is reached 60psi at some point overnight (I'm guessing related to thermal cycling or a tap shutting off rapidly).

Thanks everyone for the responses.

jamjam and JackFlash: I just gave it a try at the washing machine connectors inside and it immediately rose to 40psi when the valve started to open and stayed steady as I opened it fully. I think this supports the idea of the high pressure as the hose bib opens being related to the check valve.

Since the washer connections are on the second floor and far away from the hose bib is a 10psi difference between the washer and hose bib typical?

flabdablet and seanmpuckett: There is an expansion tank prechared to 40psi of air in the crawlspace on the cold water line. I think that meets the compressible element/water hammer requirement.

I'm going to leave the gauge on the washer valve for the day and track the max pressure indicator, but I think this has it solved.
posted by token-ring at 6:06 AM on March 30, 2019

I think this supports the idea of the high pressure as the hose bib opens being related to the check valve.

On closer inspection of the exploded view provided on the Woodford site you linked to, I think you're right. The check valve appears to be mounted to the valve stem assembly in a way that would make it try to force water out of the bib as the valve is opened, and if the water can't flow that way because there's a closed pressure gauge attached to the bib, then the net effect is to couple the valve stem hydraulically to the pressure gauge's diaphragm and make the gauge reading depend far more on the valve stem's position than on the pressure of the water on the supply side of the check valve.

Even without being mounted to a valve stem shaft, a check valve between a pressure gauge and the water whose pressure you're trying to measure will make the gauge reading much closer to the maximum transient pressure ever seen in that water since the gauge was coupled up than to its current pressure.
posted by flabdablet at 6:41 AM on March 30, 2019

And yes, that expansion tank should work fine to allow your pipes to settle to an equilibrium pressure reasonably close to the set point of your pressure reducing valve. Whether or not it also prevents water hammer will depend on how close it is to any valve that tends to cause that hammer.

For example, if you've got a dishwasher or washing machine with a solenoid valve that slams shut quickly, as most of them do, the pipework supplying that appliance is still going to see high-pressure shock waves propagating from the valve to the expansion tank and you'll hear those as bangs. Adding a hammer arrestor (which is essentially just a tiny version of your expansion tank) right next to each such appliance will shut them up.
posted by flabdablet at 6:49 AM on March 30, 2019

The pressure spikes you're seeing (or were seeing) are basically water hammer. While your tester *can* hook up to the hose bibb connection, the hose bibb (due to its long chamber between the seat and the outlet) is probably the worst place to check pressure. The pressure reading at the washer connection (cold connection, right?) is much more reliable.
As others have said, the spikes you were seeing were transients.

Regarding static pressure differences vs elevation, the rule of thumb is half a psi per foot of elevation difference. A very generous fudge factor suggests the 10 psi is not unexpected.

Regarding water hammer arrestors, the old way was to put a foot-long vertical dead leg above each outlet. So instead of having an elbow out of the wall, you put a tee, like a capital T on its side, with the extra connection being the extra foot of pipe. Today, most codes call for hammer arrestors - a proper little device that is analogous to your expansion tank. The old way works for a while, then gunks up and is pretty useless.
posted by notsnot at 7:35 AM on March 30, 2019

I didn't see anything about whether the plumber checked the pressure after the pressure-reducing valve was replaced. I think all the discussion so far has settled the "measuring at the hose bib" question, but it's also possible that the PRV isn't set correctly. 40 psi when there's water flowing isn't a bad thing. Once you get your gauge in a place where the measurements are consistent watch it for a while especially after using a lot of hot water (assuming you don't have a on-demand system).
posted by achrise at 10:20 AM on March 30, 2019

If you've got a mains pressure storage tank hot water system, you will indeed record pipework pressures higher than those set by the PRV on the supply.

Consider the starting point where all your taps are closed, your expansion tank and PRV between them have set the standing pressure in your pipework at 50psi, and your hot water service is full of cold water.

As the hot water service heats up, the water inside it will expand. It can't get out through the hot water taps because those are all closed, so it's got nowhere to go except back into the cold water supply pipe.

This will force water into your expansion tank against the pressure of the air bag inside it, compressing the air in the bag and raising its pressure. That same raised pressure will be reflected everywhere inside your house plumbing.

If your expansion tank is too small, or so old that its bag has ruptured and allowed all the air inside to dissolve into the water, the pressure in the pipework will rise until it reaches the set point on the pressure/temperature relief valve that should be mounted right at your hot water service's hot water outlet. That valve will then open just enough to stop the pipework pressure exceeding its pressure set point.

If the supply-side PRV were not doing its job, and your house was being supplied with water at a pressure anywhere near high enough to risk injury to your pipework or hot water tank, you'd see the hot water service's PTR valve running continuously.

The PTR valve is quite an important piece of safety gear. They usually have a little valve easing lever that lets you open them manually, and it's worth complying with the manufacturer's instructions about how often to operate that, just to make sure that the mechanism will not be jammed shut in the unlikely event that you need to rely on it; mine says to do it every six months.
posted by flabdablet at 7:21 PM on March 30, 2019

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