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Thread: Air temperature affecting expansion tank air psi

  1. #1
    DIY Junior Member DRG32's Avatar
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    Default Air temperature affecting expansion tank air psi

    I've got an expansion tank in my attic near the hot water heater and the air psi in the expansion tank can vary 5 or 6 psi depending on the air temperature in the attic. I live in Texas where it gets very hot, and freezes from time to time. I added air when the temperature was about 70* or so thinking that is a good median temp. Any other tips or ideas? Is that much of a difference a big deal? Should I wrap insulation around the tank? Thanks in advance.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    The air tank pressure will ALWAYS be at the same prsssure as your water system, once you turn the water on. IF it is varying, it is because your water pressure is changing, NOT air temperature. However, the air pressure is fairly immaterial as long as it is NOT zero, (but ideally at lest 20 psi), and not above about 100 psi. Between those two values the VOLUME of air in the tank will change, but it will still be at the water system's pressure, and do its job.
    Licensed residential and commercial plumber

  3. #3
    DIY Junior Member DRG32's Avatar
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    I've got a pressure reducing valve and the pressure is set at 59 psi (check it often) The pressure will slightly increase while the hot water heater is running (after someone takes a shower). I always cut the water off and drain the lines prior to checking the air pressure in the expansion tank. I've just noticed that when the attic is warmer the pressure is slightly higher, and the pressure is slightly less when the attic is cool. I won't worry much if the difference is not that big of deal.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Pv=nRT. If you look it up, it will tell you that pressure-volume is directly related to the temperature. In your case, you could consider volume a constant (not exactly true, but close enough) so both temp and pressure will track linearly. Your tire pressure on your car changes with ambient temperatures, too. Just set it for your nominal water pressure when the temp's nominal, and that's the best you can do.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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