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Relief dripping when recirc is on

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metrosilo
metrosilo Member Posts: 33
edited December 2022 in Domestic Hot Water
I tried looking this up but couldn’t find anyone who has asked this question before. 
One of my buildings has a regular 40gal domestic hot water heater. There’s a recirc line with a three speed grunfos pump. 
When the pump is on, the relief starts dripping. When it’s off, the relief stops. Anyone have thoughts on why this is happening.  

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  • hot_rod
    hot_rod Member Posts: 22,157
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    What pressure is the system running at?
    Is there a n expansion tank on the water heater?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,302
    edited December 2022
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    Hi @metrosilo , I agree with Hot Rod that this sounds like thermal expansion. Can you give us a bit more info on the bigger system? Is there a pressure regulator or any sort of backflow device or check valve on the cold supply? You might get a 0-200 psi gauge and put it on the heater's drain valve to see what's happening. If you can find the gauge with the red pointer that records maximum pressure, that would help. Look for something like this: https://www.walmart.com/ip/CARBO-Instruments-2-1-2-Pressure-Gauge-Water-Pressure-Test-Gauge-3-4-Female-Hose-Thread-0-200-PSI-with-Red-Pointer/267890424

    Yours, Larry
  • metrosilo
    metrosilo Member Posts: 33
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    Pressures are at about 60 psi. I was thinking of thermal expansion but to reach 125psi is a big leap from 60. 
    There is no expansion tank on this system. It’s basically a regular hot water tank with recirc. Would the pump create pressure on incoming cold water line. 
    It’s a long shot but maybe the water at the bottom of the tank, Where the cold water is coming in, has thermal expansion and increasing temp/pressure causing relief to drip.  
  • bburd
    bburd Member Posts: 917
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    Nowadays, water meters frequently contain backflow preventers. So do pressure reducing valves. It doesn’t take much heat in a water heater to spike the pressure throughout the system if this is the case.
    —
    Bburd
    Larry Weingarten
  • hot_rod
    hot_rod Member Posts: 22,157
    edited December 2022
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    A gauge like this screwed onto a hose connection, even the drain of the tank will answer your pressure question. The lazy hand, indicator hand will record the highest pressure for you.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,534
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    @metrosilo

    Understand that water can't be compressed. I'll give you an example.

    We had a job with a 500-gallon chilled water tank. This was a new job. The customer complained that the relief valve was dripping, and they would drain water from the system.

    There was an expansion tank piped to the 500-gallon tank and it had a valve between the expansion tank and the 500-gallon tank which was open.

    I drained literally 1COFFEE CUP of water from the 500-gallon tan and the pressure dropped too normal.

    I figured the expansion tank was bad, But I climbed up on 500-gallon tank to look around and found another shut off valve going to the expansion tank that was out of site and had never been opened.

    If this HW tank is not used (no one drawing water every so often) then expansion could be the problem

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