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Thread: Expansion Tank Sizing

  1. #1
    DIY Junior Member Schieftain's Avatar
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    Default Expansion Tank Sizing

    The T&P valve on our HW heater starting leaking, so I replaced it, and new one still leaks. Getting about 6-8 oz of water every other day, sometimes every day.

    Checked my system pressure and am getting around 74psi when HW heater is not running, but 100-115 psi when it has been running for awhile. (And I assume, close to 150psi, since that's the max pressure on the T&P valve?)

    Temp on HW heater is set at 120. Heater is 50 gallons. 14 years old, A.O. Smith.

    We do have a backflow preventer valve, so from what I've been reading, it sounds like I need an expansion tank, but am confused as to how to calculate what size tank I need. For sizing, do I use the lower system pressure reading of 72 (when the heater isn't causing a spike?), or the maximum pressure I'm seeing when the heater IS causing a spike? Does my question make sense? The calculators ask for a "supply pressure or maximum system pressure."

    Also, should I install a pressure reducing valve? Or is the expansion tank itself all I need?

    Thanks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Technically, you don't 'need' a PRV...code requires one if your static or normal pressure exceeds 80psi. If you install a properly sized expansion tank, the pressure should stay pretty constant. BUT, note that if your 'normal' pressure is 74, it COULD rise significantly higher at night when few people are using water in town and the water company is pumping more water to fill the water towers for the next day. In that case, to keep it below the 80psi, a PRV would technically be required. I'd use your static pressure as the starting point. Once you get the expansion tank in, if your pressure gauge doesn't have a tattle tale hand to record peak pressure, pick one of those up. Leave it for say 24-hours, and see if it then gets above 80psi...if it does, add a PRV.
    Jim DeBruycker
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    In the Trades Tom Sawyer's Avatar
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    A Watts ST5 is the norm for your installation.
    74 is getting close, I would put a prv in.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  4. #4
    DIY Junior Member Schieftain's Avatar
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    My gauge does have a tattle-tale and it's showing 115.

    The regular needle is now at 100 (we ran some things to make the HW heater come on, and it must have been running for a while.)

    I will leave it on for 24 hours and see what happens overnight.

  5. #5
    DIY Junior Member Schieftain's Avatar
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    Default

    Now back to 62.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Unless something is leaking, once it gets to a peak, it should stay there. The checkvalve may not be perfect, a toilet fill valve may leak, maybe a hose bib, etc...the T&P shouldn't open until it gets to 150psi. Sounds like a PRV may be a good addition. That, in combination with an expansion tank, and things should remain stable putting less stress on things like hoses to faucets, washing machine, ice maker, toilets, etc.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    In the Trades Gary Swart's Avatar
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    The reason your TP is tripping is because the backflow preventer creates a closed system. You get the same results with a PRV because a PRV has a backflow preventer built in to it. If you want to prove this point, attach a gauge to a hot water faucet and turn the faucet on. Next, run another hot water faucet until the water heater kicks on and start to heat. Watch the gauge. You will see the pressure rise very rapidly until it hits 150 PSI and the TP opens. I concur with those who have advised a PRV. Your pressure is higher than necessary at best and likely too high during periods of low water use on your main. 60 PSI is plenty of pressure for a home. A tad more or less is OK, mines is set at 50 PSI. I don't know why you have a BF preventer, but unless it is part of a sprinkler system, I think you could replace it with a PRV. The sizing of an expansion tank is on the box the tank comes in, but the ST5 is probably correct. Not sure sizing is too critical. Balance the air pressure in the tank with the pressure you set the PRV at, but use care when adding air to the tank as it takes very little volume of air to raise the pressure considerably. Too much pressure will rupture the bladder in the tank.

  8. #8
    In the Trades Tom Sawyer's Avatar
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    Unless there is a commercial water heater or very large mains an st5 is all you need here. Don't overcomplicate things
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  9. #9
    DIY Junior Member Schieftain's Avatar
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    Could I try just the tank first, and watch my pressure? Not worry about a prv unless it goes over... X?

    Also, just want to confirm, you guys are saying for calculating tank size, I use my static pressure, not what it spikes at, right?

    Our Home Depot has the Watts tanks (PLT-5, PLT-12...).

  10. #10
    In the Trades Tom Sawyer's Avatar
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    Sure you can. You want. The plt5
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    IF your supply doesn't vary much (that depends a lot on your location and special circumstances), the expansion tank could be all you need and it certainly doesn't hurt to install one and monitor things (unless you are going to pay a plumber, then the cost to call him back would make it much more expensive).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    In the Trades Gary Swart's Avatar
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    I'm still curious about the back flow preventer. Why is there one? I'd still recommend a PRV even if 72 psi is as high as it gets, that's more pressure than needed and very hard on fixtures with valves like DW, Clothes washer, ice makers, toilets, and the like. If you need a BF, then let the PRV provide that as well as keeping the pressure down.

  13. #13
    In the Trades Tom Sawyer's Avatar
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    The bfp is a code requirement
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  14. #14
    DIY Junior Member Schieftain's Avatar
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    BFP came with the house. I didn't even know we had one until I started reading about this stuff. It's not a big one with turn valves, just the kind that looks like a big fat coupling.

    Our static pressure seems to have settled at around 64. Will check again in the morning.

  15. #15
    In the Trades Gary Swart's Avatar
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    OK, if it's a code requirement in Pennsylvania, wouldn't the PRV meet that requirement? I'm just curious.

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