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Thread: Troubleshooting - Expansion tank or PRV?

  1. #1
    DIY Junior Member talltree's Avatar
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    Default Troubleshooting - Expansion tank or PRV?

    1st symptom - about a week ago noticed slightly higher pressure when turning on faucets at intermittent times, pressure drops back to normal in seconds.
    2nd symptom - a few days ago the PVR started an intermittent trickle of a leak.

    If I open a cold water faucet the PVR leak stops immediately.

    We have an expansion tank on the inlet side of the hot water heater. Do expansion tanks go bad and cause this type of problem?

  2. #2
    In the Trades Gary Swart's Avatar
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    Problem is the PRV. Expansion tanks, like every made made device, can and do fail, but that is not this problem. Expansion tanks provide temporary storage for the water expansion created by the water heater. A failed expansion tank would cause the T/P on the water heater to trip briefly to reduce the excess pressure.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, not sure everyone is getting the terms correct...if the valve on the WH is intermittently leaking, yes, the expansion tank is probably bad. PRV's don't tend to leak (except internally an thus fail to regulate pressure well).

    So, if you are talking about the T&P safety valve on the WH that is releasing water, then yes, the expansion tank is shot and needs to be replaced.

    If you tap on it and it is a dull thud, rather than ring like a bell (well, a muted bell), or if you check the pressure on the Schrader valve (like a tire valve) on the bottom of the tank, and water comes out, it needs to be replaced.

    A PRV (pressure reduction valve, and sometimes called a pressure relief valve - those are NOT the same things!) can let the pressure rise if it is leaking. But, pressure from expansion goes down almost immediately when you open a tap.

    The T&P opens at around 150psi, if you have a prv, it is usually set to maybe 60. The amount of water it takes to drop isn't much, but you could notice a spurt of higher pressure when opening a tap.

    If you actually have a pressure relief valve AND an expansion tank, either or both could be bad, or you may have a bad pressure reduction valve.
    Last edited by jadnashua; 12-24-2013 at 09:01 PM.
    Jim DeBruycker
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    In the Trades Gary Swart's Avatar
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    As I often do, I read the OP too quickly and didn't see the misuse of terms. I concur with Jim's comments. Good catch, Jim.

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    Moderator & Master Plumber hj's Avatar
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    1. The expansion tank only absorbs the intermittent pressure increase due to thermal expansion. IF a PRV, (pressure reducing valve), is defective, the pressure will increase, regardless of the existence of an expansion tank, and the PRV, (pressure relief valve), will discharge.
    2. Relieving the excess pressure caused by thermal expansion and a bad expansion tank will occur almost instantaneously so the OP would probably not notice it.
    3. Excess pressure due to a malfunctioning PRV and a "good" expansion tank will take longer as the tank pushes the excess water out as it returns to its design pressure.
    Licensed residential and commercial plumber

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by jadnashua View Post
    Well, not sure everyone is getting the terms correct....
    You mean he's not talking about his Personal Video Recorder?

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    DIY Junior Member talltree's Avatar
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    Quote Originally Posted by jadnashua View Post
    So, if you are talking about the T&P safety valve on the WH that is releasing water, then yes, the expansion tank is shot and needs to be replaced.
    Sorry, yes, T&P safety valve trickles when water heater cranks up.

  8. #8
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by talltree View Post
    ...turning on faucets at intermittent times, pressure drops back to normal in seconds.
    How many seconds? A closed system with no place for expansion would drop the pressure back to normal within a second. More than a second suggests that there is air in the expansion tank that sustains the pressure for longer.

    T&P valves can and do go bad. I always hate to pull the lever on them to test them as is called for since there is a good chance it will start to leak.

  9. #9
    DIY Junior Member talltree's Avatar
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    The faucet release of pressure is quick. Maybe a second or two.

    Quote Originally Posted by LLigetfa View Post
    T&P valves can and do go bad. I always hate to pull the lever on them to test them as is called for since there is a good chance it will start to leak.
    The T&P valve was replaced a few years ago. I suppose it could bad but in the past when I've had them go they have dumped more water. Pulling the test lever releases water as it should and then closes immediately with no leak.

    The rap test produces a muted ring. What is the proper pressure for the tank?
    Last edited by talltree; 12-25-2013 at 12:18 PM.

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by talltree View Post
    Sorry, yes, T&P safety valve trickles when water heater cranks up.

    Maybe your tank needs more air added. Sounds like it may be waterlogged.

    I am no pro. I just lay pipe, when she lets me.


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  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    One decisive test for a failed expansion tank...if it leaks water when you check the air pressure. But, if it is undercharged (still a fault, but maybe in the air valve) it could be nearly full of water. Often, you can tell if it is waterlogged by how it sounds, or its weight (primarily full of air, it is light, full of water, it is MUCH heavier).

    To test definitively, shut off the water, open a valve to release any pressure, check the air pressure of the ET, and if required, set it to your static water pressure. If it holds, it is working. If it leaks, it has failed. Once you've decided it is good, you can then shut the valve you opened, and then turn the water back on.

    Unless you have a monster WH, the quantity of expansion from reheating a cold WH is in the few cup range, and is easily absorbed by the expansion tank with little noticeable pressure change. The quantity of water the expansion tank needs to hold varies by the size of the WH tank, and the amount of temperature change to the water being heated. For the majority of home WH, from full cold to full hot, the expansion of say a 50g tank is in the order of 0.3g at typical in/out water temps, less for a partial draw.
    Jim DeBruycker
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    DIY Junior Member talltree's Avatar
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    Quote Originally Posted by jadnashua View Post
    Unless you have a monster WH, the quantity of expansion from reheating a cold WH is in the few cup range, and is easily absorbed by the expansion tank with little noticeable pressure change.
    Thanks for that info. That is just about what trickles out during the heating phase.

  13. #13
    DIY Senior Member kcodyjr's Avatar
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    Quote Originally Posted by talltree View Post
    What is the proper pressure for the tank?
    Same as the static system pressure, that is, what the gauge reads when nothing is flowing, is what I've been reading.

    Since street pressure varies through the 24hr period, it seems to me a pressure reducing valve (regulator) is necessary for a meaningful calibration. Without one, I'm going to ask the experts to clarify whether the expansion tank air bladder should match the nominal or peak pressure.

    Practically, as more water flows into the tank, the air in the bladder will compress, raising its pressure to match the water pushing against it. The benefit of calibrating it carefully is to minimize the amount of water entering, and thus minimize the amount of time it takes for pressure to return to normal once a valve is opened. Laboratory-grade precision isn't really necessary. If one were to try, my gut instinct is that the air pressure should be *just* a hair higher than the regulator's normal highest drift - they aren't perfect either.

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    Jack of all trades DonL's Avatar
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    Quote Originally Posted by jadnashua View Post
    One decisive test for a failed expansion tank...if it leaks water when you check the air pressure. But, if it is undercharged (still a fault, but maybe in the air valve) it could be nearly full of water. Often, you can tell if it is waterlogged by how it sounds, or its weight (primarily full of air, it is light, full of water, it is MUCH heavier).

    To test definitively, shut off the water, open a valve to release any pressure, check the air pressure of the ET, and if required, set it to your static water pressure. If it holds, it is working. If it leaks, it has failed. Once you've decided it is good, you can then shut the valve you opened, and then turn the water back on.

    Unless you have a monster WH, the quantity of expansion from reheating a cold WH is in the few cup range, and is easily absorbed by the expansion tank with little noticeable pressure change. The quantity of water the expansion tank needs to hold varies by the size of the WH tank, and the amount of temperature change to the water being heated. For the majority of home WH, from full cold to full hot, the expansion of say a 50g tank is in the order of 0.3g at typical in/out water temps, less for a partial draw.

    Nice post Jim. You have a way with words.


    I think one of the problems with Schrader Valves, is that people loose the cap.


    The Cap is needed to insure no leak.
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    Moderator & Master Plumber hj's Avatar
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    The expansion tank will ALWAYS be at system pressure. The question is about the ratio of water to air. Low air pressure means more water in the tank, high pressure lets less water in. ZERO pressure means the tank is full of water, MAXIMUM air pressure means there is NO water in the tank. Optimum starting air pressure is somewhere between the two points. If you START with pressure equal to the incoming water pressure, water does not start to fill the tank until the pressure starts to rise, giving the maximum capacity, even though the actual volume involved may be in the "tablespoon" range. If you took a sealed container full of water, and could screw a solid rod into the water, without leakage, you would see "massive amounts" of increase even when it was just entering the water.
    Last edited by hj; 12-26-2013 at 06:53 AM.
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