PRV: Should I get constant static pressure?

[buckyswider]

Hi all, I just had my defective PRV replaced. I was getting 95-105PSI from the street (measured on my "Flo" device) and when I tried to adjust the PRV it started to drip- so I engaged a plumber to replace it.

So now that is all done, and I am seeing a much-needed pressure drop (I got tired of changing toilet fill valves every year!). But I'm seeing something that may or may not be odd depending on the expected behavior of the PRV.

After my initial adjustment, the reading usually hangs out around 65PSI. But a some times the static pressure (i.e. no water flowing, I learned enough in my fire service pump classes to know the difference between static & residual pressures!) can vary- anywhere from 50 to 90PSI.

So I guess the question is: How is this thing *supposed* to work? It it just supposed to reduce the incoming pressure by a set amount? Given that the instructions say that the "factory preset" is 50PSI, I wouldn't think so- I mean how can they preset for 50PSI when they have no clue how much incoming pressure it's going to get? OR, is it as I suspect-it's supposed to regulate the pressure to a set amount (give or take a few PSI- it's not a swiss watch!)

I did contact Watts last week with this question and they have not responded

Thanks!

 

[Sylvan]

"So I guess the question is: How is this thing *supposed* to work? It it just supposed to reduce the incoming pressure by a set amount?"

YES

Cold water when heated to boiling point will increase in pressure about 4% Fingering your PRV is SET for 65 PSI

There would be no reason to have an expansion tank other on a hot water boiler where the "SET" pressure is 12 - 15 PSI ad the relief valve is set for 30 PSI

More pressure is not always a good thing

 

[Sponsor]

 

[Reach4]

After my initial adjustment, the reading usually hangs out around 65PSI. But a some times the static pressure (i.e. no water flowing, I learned enough in my fire service pump classes to know the difference between static & residual pressures!) can vary- anywhere from 50 to 90PSI.

It is supposed to reduce the pressure for incoming water to a fairly constant value. However if the water inside expands due to thermal expansion, it will not be able to do that. The PRV should have thermal expansion tank that can accept the increased water volume from when cool water comes into the water heater, and then expands. The tank should have the air precharge pressure to set to the same as the PRV water pressure, but the air precharge needs to be set when the water pressure is near zero.

The expansion tank should be big enough, and some factors come into play. The capacity of the water heater, and the temperature rise that the water heater must heat to are important to get big enough.

If you slightly dribble water from a faucet, then any then any thermal expansion pressure gets released. The PRV pressure will be what you measure then.

There is another factor in the pressure you can see. As more gpm passes thru the PRV, there is an increased pressure drop. The amount of drop you see from that depends on the PRV including its size.

Impressive graph. What gathered the data? Looking at your graph, I could imagine if you had a long hot shower and turned off all water use maybe 11:30, water expanded as it heated, and then tiny water leaks gave the slow fall? A toilet flush at 12:30 would have brought the pressure right down.

It could also be that your PRV has a problem.

 

[hj]

COrrection to the above reply. It does NOT reduce the pressure by a set amount. It reduces it to the pressure setting regardless of the incoming pressure, unless that pressure is greater than the valve is designed for, but then you need a different valve. IF it were a defective regulating valve, the pressure would have remained high until a faucet was opened, then it would have dropped to the set pressure, but would then have returned to the high pressure. Since it did not do that, the assumption is that it was thermal expansion.

 

[Dj2]

Another thing is your water pressure gauge: how reliable is it?

 

[buckyswider]

Thanks for the replies! Sorry I took so long- my notifications aren't coming through for some reason. I just happened to go back and hit F5

First off, the pressure graph is from my "Flo" device. https://meetflo.com/ It's an automatic leak detection/shut off valve and also does telemtry. So I can get pressure, flow, and usage data. It's a little primitive right now- the only time 15 minute interval data is available if for the current day only. Good question about the calibration/reliability of the device. I'm assuming it's OK. Ha.

Now onto the thermal expansion...in reach's scenario, where a hot shower is taken at 11:30PM, wouldn't the pressure DECREASE after the shower and then slowly INCREASE after the water heater starts getting the 'replacement' water hot? I will experiment- next time I see a high pressure real-time, I will give the john a flush and see if the pressure drops back down to ~65.

Interesting theory about the slow drip- the Flo device does a daily 'health check' where it shuts the valve down* (in the wee hours of the morning) for up to 9 minutes. It measures the pressure on the house side of the valve and then monitors it for those 9 minutes to see if it decreases. If it does (no idea of their granularity or allowed thresholds) it tells you there's a drip detected. Well, everything was fine up until the day the plumber came over and replaced the PRV and installed a new toilet. Every since that exact day the health check have all come back "Small drip detected". Could just be a coincidence. But I've hunted high and low and found no drips anywhere. I wonder if it's possible that the fill valve in the new toilet (American Standard dual-flush H2Option) could have a problem? Next debugging step (for the drip) is to shut off various fixtures/lines to see if I can successfully run a health check, and that would help narrow it down.

* - If during a health check the Flo detects a sudden large demand- i.e. a flush or a tap opening- it immediately opens up and cancels the health check so you're not without water when necessary.

Anyhow, I'll watch the pressure and report back with my flush results when I observe a high pressure...

Thanks all!!

 

[Reach4]

Now onto the thermal expansion...in reach's scenario, where a hot shower is taken at 11:30PM, wouldn't the pressure DECREASE after the shower and then slowly INCREASE after the water heater starts getting the 'replacement' water hot?

Pressure will go to the regulation pressure of the PRV when significant water is being used.

The pressure rises when heating occurs after water use has stopped.

 

[Jadnashua]

Toilet fill valves tend to be a way for high water pressure to be bled off (i.e., when older, they sometimes leak). When yours was replaced, that stopped happening. You need to check your expansion tank's precharge and to verify that the diaphragm, isn't shot and it's leaking (and thus not doing its job), or, it's slowly lost its precharge.

If the ET is sized and pressurized properly, the static pressure will only rise slightly after the WH runs.

 

[buckyswider]

Thanks again guys!

Reach, I think I understand your line of thought now- it went up after the heating, but then started decreasing with a small drip over time.

Jad, I had replaced my fill valve a week before getting the toilet replaced- it was while replacing the valve that I decided that it was time to get a new loo. And unfortunately I have no expansion tank. In a bit I will shut of the toilet line valve and run a health test to see if the new toilet is causing the alert.

I didn't see a pike in the last 12 hours, so no opportunity to test the theorem. However, I had downloaded some pressure data a few days ago and happened to notice something that may be relevant.

At 2AM I was cruisin' along nicely with 66.71 PSI. Then at 02:15 a sudden spike to 91.68 before a rather quick drop off to normal over the next 90 minutes.

I'm not really sure if this tells us anything, but maybe. There was definitely no hot water usage for several hours prior. But maybe the DWH cooled off enough and decided it needed some warmth? And if that did happen, would be expect (even in the absence of a ET) this high of an increase? I'm not sure of the range of temp rise that the DWH thermostat would effect- couldn't be more than a few degrees, could it? (Remember, *if* the DWH had kicked on it wasn't the result of water usage).

 

[Reach4]

1. Water is nearly in-compressible. Some plumbing is more expandable than others. With no thermal expansion tank, your 25 psi rise could conceivably happen due to a cycling on during no-water-use.

2. Your PRV may be limiting your pressure from expansion by releasing water back into the supply line. This feature is called an integral bypass, or some such. https://www.zurn.com/media-library/web_documents/pdfs/catalogs/zmktg480-45-pdf.aspx says

The pressure downstream of a PRV can build from thermal expansion or the use of pumps. When downstream pressure builds and exceeds inlet pressure, an integral bypass bleeds off excess pressure by allowing water to flow upstream through the bypass, limiting pressure build in a system to that of the inlet pressure. With an integral bypass, pressure in a system will never exceed that of the inlet pressure if there is no backflow prevention device installed.​

If the water meter has a check valve, this bypass feature will not work.

3. Thermal expansion to 100 or even 120 is not a big deal. Even with a good thermal expansion tank there can be enough pressure that some nervous Nellie will predict doom. The first opening of a faucet will bring the pressure back down.

 

[buckyswider]

DING DING DING we have a WINNAH!

I just got out of the shower and glanced at the FLO dashboard. 100.3PSI! Hit the "low flush" button on the new john- watched the pressure drop to the residual ~45PSI while the toilet tank refilled (3 quarts only- love it!). Watched the pressure slowly (over a minute or so) work its way back up and stabilize around 64! So yep, it seems likely that the DWH is causing thermal expansion which is driving up the static pressure in the system!

I checked out a yootoob video about DWH expansion tanks, and the first one I grabbed blamed several DWH failures on thermal expansion- that's why his *new* plumber recommended an expansion tank. Thankfully that hasn't happened to me (yet)- I have an indirect DHW tank that was installed in 2005 when I replaced my boiler and re-piped everything into a primary/secondary setup to add some in-floor radiant.

So now I'm wondering if I should install an expansion tank or not. Real Estate is my big problem with this- it's an old house (~225 years) with a basement with an 80 inch ceiling clearance. With the aforementioned new boiler piping I'm not sure where I could tuck in an expansion tank. But I'll take a closer look and then go from there.

As far as backflow preventer device- I don't think the PRV does it (it does however have a thermal expansion bypass). I don't think my water meter has it either- but it is semi-new, as the utility installed a new one probably 10 years ago to facilitate AMR. Why I don't *think* it has one is because prior to the PRV being installed I was seeing phantom flow on the FLO dashboard. Their support team attributed this to fluctuation on the supply line- water flowing back and forth thru their device- and they can't distinguish which direction the water is flowing. They recommended installation of a check valve. Which is a very long way of saying that if the meter had a check valve, I don't think I would have seen this! Interestingly the replacement of the PRV seems to have cure the phantom flow, even though it doesn't have a check valve function.

And now going reading through the leaflet that came with the PRV they have a little troubleshooting blurb titled "High System Pressure" where they list thermal expansion as a possible cause. I should have RTFM!!

 

[buckyswider]

Oh, and the "small drip detected" problem seems to be gone. When the toilet was replaced, I asked the plumber to fix/replace the shut off for it, as it required superhuman strength to operate. I shut it off, rean a health check, and it ran clean. I opened it back up, ran another health check and...clean! And it ran clean overnight too. I guess it had something to do with that valve (although I could detect zero moisture around it) but I guess I should just be happy it's fixed...

 

[Jadnashua]

A bypass valve on a PRV can only open once the internal pressure reaches and usually exceeds the supply pressure. If you're goal is to maintain a fairly static pressure, that sort of defeats the purpose!

Things like the supply hoses to faucets, toilets, washing machines, etc. generally are a tubing with a slight amount of flex. When the pressure rises, it causes them to slightly balloon. This puts wear on them. Worst case, long-term, they could burst prematurely. In a really tight system, it's not at all unusual for the T&P valve on the water heater to weep some each time the WH is heating water (once the use stops). Depending on how much hot water needs to be replenished, the expansion can be up to a few cups of water or so...the bigger the use, the bigger the tank, and the bigger the temperature rise, the more expansion you'll get. So, it will be more in the winter with cold water coming in versus the warmer water of the summer.

Put in an ET. Things will last longer, and your pressure will stay much more constant.

 

[buckyswider]

Thanks! Will figure out where to fit an ET. Is there an effective limit as to how close it needs to be to the water heater inlet?

 

[Reach4]

Thanks! Will figure out where to fit an ET. Is there an effective limit as to how close it needs to be to the water heater inlet?

No, tho some will claim that the tank needs to be between the stop valve and the WH. If you close the stop valve, turn off the WH. People with wells have big tanks at the input of the water or even at the well in the yard, and they don't need a separate expansion tank.

 

[Jadnashua]

Ideally, the ET should be where it will always do its job. Putting a valve between it and the WH can defeat that. Functionally, as long as there's no valve between it and the WH, it will work, regardless of how far away it is. So, you could T it in close, and run the line wherever it needs to be to work and fit the tank.

 

[buckyswider]

Excellent thanks guys! It's on the to-do list!!