Momentarily high water pressure when first opening a faucet; is my expansion tank operating as designed?

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Temp945

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Hi all,

Sometimes (but not all the time), when I first open a faucet, the water pressure is momentarily higher than normal for about 3-5 seconds. This short burst of higher pressure is not extreme but is definitely noticeable.

Here are a few relevant facts about my system:

1. I have a pressure reducing valve after the water meter set to 50 PSI.
2. I have a water pressure gauge installed on the cold line immediately above the water heater that shows a constant ~50 PSI.
3. I have an expansion tank installed on the cold line next to the water heater with the air bladder pressurized to 50 PSI.
4. I replaced all of the old galvanized pipe with Uponor ProPex (PEX-A).
5. The water heater's TPR valve is not opening or leaking.

I believe the momentarily high water pressure occurs when the water heater has caused thermal expansion and a faucet has not yet been opened to release the pressure. I also believe that the purpose of an expansion tank is to mitigate this issue, and the ~50 PSI I observe on the pressure gauge seems to confirm that the expansion tank is operating as designed. So why am I still observing the momentary burst of water pressure when first opening a faucet?

I have three guesses:

1. The expansion tank is doing its job and filling with a small volume of water. When I open a faucet, the initial burst of water occurs because the air bladder in the expansion tank has become pressurized to more than 50 PSI by the expanded water. The pressurized air bladder therefore pushes the water out of the expansion tank in a relatively short burst as the air bladder returns to 50 PSI.
2. The expansion tank is doing its job and filling with a small volume of water, but the flexible PEX-A lines are also absorbing some of the thermal expansion similar to the expansion tank. When I open a faucet, the initial burst of water occurs because the PEX-A lines are releasing the thermal expansion.
3. Both of the above are occurring.

Is it normal to observe momentarily high water pressure when opening a faucet, even with a thermal expansion tank installed? If so, are my explanations for why this occurs correct?

Thanks for reading!
 

LLigetfa

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Yes, it is normal to have a rise in pressure. The amount of rise depends on the air volume in the expansion tank and the water volume plus degrees of temperature rise in the water heater. Also, as you mentioned, stretch in the PEX lines also stores some pressure.

Even if you are not seeing a static pressure rise at the gauge, you are seeing momentarily higher flow due to the extra stored pressure/volume as you open the tap.
 

Temp945

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Thank you, LLigetfa!

This issue is a bit confusing because short bursts of high water pressure when opening a faucet are evidence that a thermal expansion tank is needed in the first place! So some people may be confused because short bursts of high water pressure will continue even after installing a thermal expansion tank.
 

LLigetfa

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The reason for the short burst is a change in pressure/volume and friction loss.

It is the rise in pressure that the expansion tank mitigates. So long as you monitor the gauge for a rise in pressure and the pressure does not rise to levels that stress the plumbing and pop off the TPR, then the short burst can be ignored.
 

Temp945

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I have been closely monitoring the pressure gauge for increased pressure but it is always within 2 PSI of 50. This has confused me because I can also clearly observe the short bursts of water pressure when first opening the faucet sometimes. Now it makes sense!
 

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Is that burst typically in the morning after no water use? If so, I expect a slightly leaky PRV. Have you tried checking the pressure gauge after not having used water for many hours?
 

Temp945

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Yes, the burst is only after no water use for a while. I assumed this was due to the water heater coming on and causing thermal expansion. In my many dozens (hundreds?) of observations of the pressure gauge, it never exceeds about 50 PSI.

I last replaced the PRV in 2016.

Reach, are you saying that the short burst of pressure should NOT be caused by an expansion tank releasing pressure after thermal expansion?
 

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Yes. If the burst was bigger several minutes after a hot shower, expect thermal expansion. But if it is bigger after a longish non-use of water, expect PRV leak. Check the water pressure before your first flush, or other water use, of the morning.

There are rebuild kits for some PRVs.

I have never used a PRV, but if I were putting one in, I would consider one that used this cartridge: https://www.supplyhouse.com/Cash-Acme-23148-EB-25-Cartridge-Kit-1-2-to-1-Models

Sure looks easy. I am not a pro.
 
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John Gayewski

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Your gauge should show any pressure increase. Unless the gauge is broken your pressure being at 50 should cause the same amount of water to be dispelled. If your system never climbs according to your gauge, but your seeing evidence of high pressure, then I would suspect the gauge.
 

Temp945

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The gauge does go down when a faucet is opened and then climbs back up to 50 when the faucet is closed, so it does appear to be working. It just never exceeds about 50.

Given the variety of answers, I think should try three separate troubleshooting steps:

1. Verify the pressure gauge is working:
a. Increase the PRV setting and see if it increases the pressure shown on the gauge.

2. Check if the water heater is causing the thermal expansion:
a. Verify system pressure is at 50 PSI by observing the gauge
b. Verify the expansion tank air bladder is inflated to 50 PSI with a separate gauge
c. Temporarily turn the temperature on the water heater from 120F to 130F to force it to heat up substantially
d. As the water heats, check the pressures on the cold line and on the expansion tank air bladder
e. When the water is finished heating, again check the pressures as above
f. Open a faucet and observe whether there is a short burst of water pressure

3. Check if the PRV is leaking and causing a build up of pressure:
a. Before bed, turn off the water heater
b. Verify system pressure is at 50 PSI by observing the pressure gauge
c. In the morning, check if the pressure gauge reads higher than 50 PSI
d. Open a faucet and observe whether there is a short burst of water pressure
 

LLigetfa

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The gauge does go down when a faucet is opened and then climbs back up to 50 when the faucet is closed
That is a clear indication that you have more friction loss between the gauge and the upstream water source than the gauge and the downstream faucet.
 

LLigetfa

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Do note that water consuming devices such as RO filters, ice makers, humidifiers, trap primers, and even some toilet fill valves can bleed off expansion volume.
 

John Gayewski

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Your water heater is definitely causing thermal expansion and in turn will carouse the pressure to rise. An expansion tank doesn't maintain a steady pressure on the system it just keeps the system below the max 80psi. Your pressure should be changing pretty regularly, slowly and consistently depending on what's happening at the moment.
 

Reach4

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While many agree that the thermal expansion should not increase the PSI to over 80, I disagree.

My argument is that the thermal expansion tank makers recommendations result in pressures over 100. Your argument is that the codes say that the water pressure should not be above 80 psi, and do not state an exception.. And I argue that if we consider 80 psi to be a hard limit, then the great majority of thermal expansion tanks are undersized.

https://terrylove.com/forums/index....g-regulator-question.94022/page-2#post-677156 has my thoughts about there not being a problem for a tank WH if the thermal expansion results in a 100 or even 120 psi for a bit. If you designed

If you had incoming water at 70 psi, and if you were determined to not have the pressure rise over 80 after a hot shower in Minnesota, the thermal expansion tank would have to be much bigger than normal.

Examine that Watts calculator at https://tools.watts.com/ETP/
Look at https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf

150 PSI in the WH tank will not damage the tank, but you don't want the T&P valve to be discharging regularly.
 
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John Gayewski

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While many agree that the thermal expansion should not increase the PSI to over 80, I disagree.

My argument is that the thermal expansion tank makers recommendations result in pressures over 100. Your argument is that the codes say that the water pressure should not be above 80 psi, and do not state an exception.. And I argue that if we consider 80 psi to be a hard limit, then the great majority of thermal expansion tanks are undersized.

https://terrylove.com/forums/index....g-regulator-question.94022/page-2#post-677156 has my thoughts about there not being a problem for a tank WH if the thermal expansion results in a 100 or even 120 psi for a bit. If you designed

If you had incoming water at 70 psi, and if you were determined to not have the pressure rise over 80 after a hot shower in Minnesota, the thermal expansion tank would have to be much bigger than normal.

Examine that Watts calculator at https://tools.watts.com/ETP/
Look at https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf

150 PSI in the WH tank will not damage the tank, but you don't want the T&P valve to be discharging regularly.
Your opinion vs every manufacturer of plumbing fixtures, the code, and experience. Just becuse someone tests something at 150psi or something is rated to take 150 psi doesn't mean the system can take 150. I regularly open and read plumbing fixture instructions, they pretty much all (with few exceptions) say not to exceed 80. There's really no reason to suggest to people who may not know that going over 80 is a good idea as it can only hurt. Reality is 50 is usually plenty and 60 is more than enough.
 

Reach4

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I was not suggesting that the PRV be set above 60. I was suggesting that thermal expansion could raise the pressure to 100+ with no worry. My appeal to authority is from the tank makers including Amtrol and Watts.

But if we were to want to not ever exceed 80, then thermal expansion tanks would need to be much bigger than they are now.
 

John Gayewski

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I was not suggesting that the PRV be set above 60. I was suggesting that thermal expansion could raise the pressure to 100+ with no worry. My appeal to authority is from the tank makers including Amtrol and Watts.

But if we were to want to not ever exceed 80, then thermal expansion tanks would need to be much bigger than they are now.
I appreciate your thoughts. There are several sizes of expansion tanks available.

The expansion tanks commonly available say they can withstand 150, should not operate above 80.

A few weeks ago I went to a house that had flooded their basement. Two hoses blew off that had been there for ten years (since the house was built). I redid the hoses and noticed they had an expansion tank which isn't normal here. I checked the expansion tank it was water logged. Suspecting they had a private water service I checked their yard and found they had a prv. Sure enough the prv was not shutting and letting the static pressure rise to 95.

These two hoses flooded their basement and the cause was high pressure... along with improper install of these hoses that happened to go to a roman tub. The hoses lasted ten years installed improperly the high pressure set them off.
 
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