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Rising water pressure, expansion tank practically new

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Phase III Smart 40 (36 gallon) IDWH with a pressure gauge at drain. Correctly sized thermal expansion tank correctly filled to house water pressure (60 lbs).
If one were to observe the pressure gauge after a deep draw, or for that matter any draw, what would the pressure gauge do? Would it stay at 60 lbs? Or fluctuate a few lbs?
I've spoken to at least one manufacturer and no on seems to be able to answer that question. My gauge is rising to at least 86 lbs. Maybe could've gone higher but the wife used some hot water and it dropped to 70 lbs. That can't be right. Either the tank is not doing its job or maybe I need a bigger tank, (I have a check valve at IDWH cold inlet right above expansion tank tee. The amount of 3/4" tubing to expansion tank is 52".


Comments

  • Ironman
    Ironman Member Posts: 7,379
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    If the incoming pressure is 60 psi, then the pressure in the tank WILL rise as the tank gets hotter. It has to. As the water is heated and expands, the pressure is gonna go up. It's no different than what happens in a boiler system.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,613
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    If I understand your post your water pressure is 60 ps which the air tank air side was pressureized to that figure. Did you set the air pressure to 60 with the tank disconnected from the system or installed on the system with no pressure on the water side?

    That being said if the above is correct the tank is not going to hold the system pressure at 60 when the water is heated. It will rise. I am assuming the water tank has a 150 psi relief valve.

    If the pressure rises to 86 or more I wouldn't be concerned unless it goes over 125

  • hot_rod
    hot_rod Member Posts: 22,251
    edited January 2021
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    The red pex with the check and an arrow pointing down? Is that a recirc line?
    86 psi is a bit high, most plumbing faucets, fixtures are rated to 80 psi, and plumbing codes state no less than 20 psi, no more than 80 psi. If the tank is sized and pre-charged properly 26 psi seems a bit too much increase. Here is the sizing math.

    https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • exheatingguy
    exheatingguy Member Posts: 9
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    Thank you Ironman for responding, but yes, as we all know as water is heated it expands/pressure rises, then what is the purpose for the expansion tank if the pressure is going to rise almost 50% above normal? Would it have risen a great deal more without an expansion tank? I thought maybe the cushion in the expansion tank would even out the pressure. Doesn't make sense to me.
  • exheatingguy
    exheatingguy Member Posts: 9
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    Thanks EBEBRATT-Ed for responding. Yes expansion tank installed correctly. I wonder if anyone ever valved off an expansion tank and drew down on their IDWH and monitored a pressure gauge just to see the difference an expansion tank makes, must be huge.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,613
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    @exheatingguy

    Most hot water tanks don't have expansion tanks. They don't have check valves either. If you have a check valve or a prv in the cold water line the hot water pressure can't back uo into the cold water which is what happens a t most jobs
  • hot_rod
    hot_rod Member Posts: 22,251
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    Does the home have a pressure reducing valve? What is the cold water pressure, does it change, maybe slowly creep up to city main pressure? Your gauge would show that. A common failure of pressure reducing valves is pressure creep.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    HomerJSmith
  • neilc
    neilc Member Posts: 2,707
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    draw down the domestic again and set the precharge to 65 and see if that makes a difference
    known to beat dead horses
  • exheatingguy
    exheatingguy Member Posts: 9
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    Wow, thanks everybody for responding.
    The check valves are for an issue I was having with "temperature creep" on the tempering valve, because of THAT fix I found out that I trapped the expansion tank on the wrong side of that check valve on the inlet of IDWH. I moved expansion tank to new location teed after the aforementioned check valve.
    Yes, PRV installed with probably overkill on gauges before and after. Don't believe pressure is creeping up, outlet pressure seems pretty steady but I'll keep an eye on it.
    I was thinking of changing pressure at the tank to match the highest pressure recorded, but then I was thinking shouldn't I lower it by the difference to make up for the expansion? Then I was thinking they're probably both wrong, there has to be another answer.
    The brass plug on the overhead horizontal copper line is where the tank used to be, I always thought about moving location due to hitting it with my forehead more than once.

  • Jamie Hall
    Jamie Hall Member Posts: 23,426
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    The pressure in the hot water side of your system cannot be less than the pressure in the rest of your water supply after the pressure reducing valve. It will go up as the water is heated, but should go back down as it cools. Depending on the size of the expansion tank and the water heater, it may go up actually quite a bit -- but not to worry, provided it does come back down when things cool off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,331
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    Hi @exheatingguy , That 87 psi you get, I see on the gauge under the pressure reducer. When it's reading high like that, what is the gauge at the drain valve, low on the side of the tank reading? I'm wondering if all those check valves are allowing pressure to build in some piping, but not the tank.

    Yours, Larry
  • exheatingguy
    exheatingguy Member Posts: 9
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    Hey Larry, that 87 psi is street pressure coming in which is typical, reading at gauge on IDWH is what is set at PRV, same as reading on upper gauge. Check valves not in play, expansion tank after the check valve.
    Jamie306A
  • Jamie306A
    Jamie306A Member Posts: 4
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    So, what you’re saying is street pressure is 87psi. And the problem you’re having is, the DHW is rising to 86psi?
  • exheatingguy
    exheatingguy Member Posts: 9
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    Yes Jamie, I realize now I should've said there is no problem with house water pressure and that its just related to IDWH being heated and that it raises water pressure at that drain gauge (which is protected by the expansion tank) to those aforementioned numbers.
    NEW UPDATE:
    I thought that the bladder might have gotten stuck somehow so I relieved water pressure, released air charge in expansion tank, filled IDWH, bled air from hot water faucets, disconnected recirculation pump, drew down on IDWH tank (shower), and checked pressure gauge at drain.
    Highest I got was 97 psi,
    so I guess expansion tank IS doing something.
    THEN I relieved water pressure again, filled expansion tank to 60 psi (house water pressure), turned water back on, bled hot water faucets, drew down on IDWH, monitored gauge at drain.
    Highest I got was 74 psi.

    I can't explain it, maybe it has something to with bladder beeng stuck and/or hot water recirc. being disconnected. Or neither.
    I'm going to call more Manufacturers of expansion tanks and see what variation of psi should you get, if any, on normal heat cool cycles of Hot water.

  • luckylindy39
    luckylindy39 Member Posts: 1
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    Did you get any response from companies? I have similar phenomenon on my system set at 65 psi
    , where gauge goes up to 80 psi before retreating to 65 . I am now video recording gauge to see the timing of the increase and decrease.
  • hot_rod
    hot_rod Member Posts: 22,251
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    I believe that brand and model of PRV is a bypass style that should allow pressure back into the main above 87 psi

    Now you may have it checked or a BFD at the meter?

    Looks like a # 15 thermal expansion tank? A bit small perhaps, which could explain the pressure increase
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • pedmec
    pedmec Member Posts: 987
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    Maybe i missed it but when you have the pressure increase at the tank do you check the pressure of the gauge downstream of the PRV?