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Old 01-07-2016, 09:44 AM   #1
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Water Pressure Spikes


I could do with ideas for addressing a water pressure spike problem. About 5 years ago, shortly after buying the house, I discovered the water pressure was ~80 psi, with occasional spikes to ~120; and we lacked a PRV. A plumber installed a PRV, set it at 40psi and installed an expansion tank on the electric water heater, also set to 40 psi. We've been dissatisfied with the low shower pressure for years (it's upstairs) so I finally decided to address it, but checked the water pressure as the first step. Static pressure is 40psi, but we were getting spikes up to 120psi during the evening/night. My first thought was thermal expansion due to a ruptured diaphragm on the expansion tank, but depressing the schrader valve on the tank only let out air. I then tested the pressure in the expansion tank, taking the following steps:

1. Shut off power to the water heater
2. Closed the cold water flow to the water heater
3. Opened a hot water faucet (ran dry very quickly)
4. Let some water out of the water heater to take pressure off the diaphragm in the expansion tank
5. Tested the air pressure in the expansion tank (~36psi)
6. Put the air in the expansion tank back to ~40 psi.

In short, the expansion tank seems fine.

That suggests the PRV is faulty, but we know the mains pressure was 80psi 5 years ago when we set the PRV to 40psi. It doesn't seem reasonable to think that in that time the mains pressure in the neighborhood has coincidentally been dropped to 40 psi, the PRV has failed and the 120 spikes we're getting are coming from the mains. That said, I can think of no other way we could be getting these spikes (I haven't directly tested the PRV yet because it's in the crawl space on the other side of the house to the crawl space entrance and take 5+ mins crawling to get to it).

Lastly, my hunch is that it's related to the water heater because the spikes seem to be at night (that's when my wife and daughter drain the heater with showers and baths). Right now I'm trying to cause spikes by running the hot water for a while, shutting it off and watching the pressure gauge on an outside faucet. But, I've only seen small pressure changes which I can't be sure are related.

Any thoughts would be much appreciated because I'm out of ideas.
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Old 01-07-2016, 11:11 AM   #2
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What size is your water heater and what size is your expansion tank?
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Old 01-07-2016, 11:21 AM   #3
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Water Pressure Spikes


jmon thanks for replying. I'll have to get the sizes for both when I get home from work (~5 hours). I do remember checking them both when the plumber installed the expansion tank (I wanted to get informed) and I think the tank's the right size for the heater, but I'll get them both later today.
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Old 01-07-2016, 11:34 AM   #4
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Thanks. More than likely the problem is with your prv as you mentioned. They do go bad, or they can get clogged with sediment, etc., which may affect the way they work.

When you get home and have some time, post a picture of the prv and water heater set-up. Thanks.
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Old 01-07-2016, 03:59 PM   #5
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Adjust it so you have 60 Lbs pressure. That is what fixtures are designed for 40 Lbs is to low.
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Old 01-07-2016, 04:56 PM   #6
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Water Pressure Spikes


Thanks for replying Ghostmaker. Is there some way the low pressure can be causing the spikes? I'll happily increase the static pressure, but have been anxious about doing anything until I understand what causing these spikes. Thanks again.
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Old 01-07-2016, 05:02 PM   #7
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Water Pressure Spikes


jmon the water heater is 47 gallons and the expansion tank is 2 gallons. Here's a pic of the water heater specs and the attachment of the expansion tank to the cold water line above the water heater. I won't have a pic of the PRV until I can get under the house at the weekend, but will get one then. Thanks for taking the time to give it a thought.
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Old 01-07-2016, 08:14 PM   #8
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You said in your write up you had pressure spikes at night. That may be when there is low use of the water system. If your spiking to 120 lbs you might just add a second PRV after the primary. Then adjust them for a 60 lb output. A single PRV can only do so much.
Two-Stage Serial Reduction Configuration


The two-stage serial reduction approach uses two valves in series to reduce or eliminate extreme variations between the water main's inflow pressure and the desired, final reduced pressure. Two stage reduction is recommended when initial pressures are 200psi or greater, or when the desired pressure reduction ratio is greater than 4:1, e.g., from 200psi to 50psi, or where the inflow pressure fluctuates greatly. The advantage of two-stage serial reduction is that neither valve is subjected to extreme pressure differentials, thus prolonging valve life and delivering more precise pressure regulation.





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Old 01-08-2016, 08:00 AM   #9
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Water Pressure Spikes


Thanks Ghostmaker, I had no idea two PRVs on a line were even possible. I'll test the PRV this weekend - adjust it up and down to see what happens to the pressure and open it fully to see what mains pressure I'm getting. Is there any other test of the PRV you would recommend?

Last night I put the gauge on the outside faucet (40 psi), checked it again before shower/bath time, it was still 40psi, and again within a few minutes of showers/baths finishing (and before any faucets were turned on or toilets flushed or water run in any way), and it was 40 psi, but at some point in between the red needle on the gauge had gone up to 120psi. I left the gauge on overnight but when I came out this morning again it read 40 psi and there had been no movement of the red needle, that also still read 40psi. This suggests thermal expansion, but the diaphragm in the expansion tank hasn't ruptured, the pressure is set to the right level and the tank is the right size for the water heater.....
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Old 01-08-2016, 09:24 AM   #10
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If your water system is gravity fed, look at the elevation difference between your house and the nearest gravity tank. Gravity tanks are usually refilled during low use periods by increasing system pressure.

If you are at the "bottom of the line" and the elevation difference is great, when the pumps start the surge could overwhelm your PRV. Another indicator for series PRVs. You could also need a model that handles higher pressure "first in line".
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Old 01-08-2016, 09:47 AM   #11
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Thanks GrayHair. I've no idea what system is used. I've just taken a look at the city water distribution system specs (see below). Does it suggest a gravity fed system (it means little to me)?

Over 246 miles of water distribution and transmission mains ranging in size from 2-inch to 30-inch diameter.
Water system storage includes:

o Two Overlook Reservoirs (combined 12 million gallons)

o 150,000 gallon Overlook standpipe

o 300,000 gallon Lakemont #1 standpipe

o 1,000,000 gallon Lakemont #2 standpipe

o 1,500,000 gallon Wildwood elevated tank
Three booster pump stations:

o Overlook

o Wildwood

o Regal Drive

Thanks again
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Old 01-08-2016, 01:29 PM   #12
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Assessing water distribution systems is not exactly my forté, but I'd say it's a mixed system. The reason I brought it up is past experience testing fire sprinkler electronic supervisory systems by flowing water through the system.


On a wet fire sprinkler system (water already in the pipes throughout the building), there is a device to stop the non-potable water in the building pipes from getting back into the potable water supply. When gravity storage tanks are refilled by increasing water system pressure, system pressure increases and gets trapped. This device also traps increased pressure, so system pressure is often a lot higher than supply pressure.


My reason for mentioning a different model PRV in front of another is that, in my experience, "one size fits all" actually fits none very well.



And you don't flow-test fire sprinkler systems on a Friday unless you want to respond during the weekend (the surge from increased supply pressure occasionally causes false alarms).
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Old 01-08-2016, 05:53 PM   #13
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What you should do is get the pressure before the pr valve. Remove it and see what your incoming pressure truly is.
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Old 01-11-2016, 09:33 AM   #14
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Thanks Ghostmaker. Removing the PRV and checking the incoming pressure is a step I may need to take, and adding a second PRV sounds like a good possibility as well, though both are beyond my competence (any real plumbing work is beyond me). Because the spikes are seemingly irregular I want to do what I can to figure out what's going on before bringing in a plumber.

Over the weekend I adjusted the PRV, increasing pressure to ~56-60 psi and adjusted the expansion tank accordingly. Since then I've only experienced spikes up to ~70-80 psi but it's not been 48 hours.

jmon here's a pic of the PRV (it's not easily read but it's a Watts).

I'll keep monitoring the pressure but I appreciate everyone's input (I had fixated on thermal expansion but everyone else's focus on the PRV seems warranted as I'm having no luck getting 120 psi spikes running the hot water heater dry - I can only get spikes up to ~70-80).

Does anyone have any experience with the Watts Governor 80M-1 (the ballcock model)? If I can't figure out where the spikes are coming from I'm tempted to add that to a toilet for extra security.
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