Expansion tank on HWD system

Discussion in 'Water Heater Forum, Tanks' started by Young_apprentice, Apr 2, 2012.

  1. Young_apprentice

    Young_apprentice New Member

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    Hey everyone,

    just a quick question, im a plumbing apprentice so i have ALOT of them and i need a place to get answers!

    why does a expansion tank need to be installed on some hot water dist. systems but not others?

    FLEXCON- gave this answer in their Instruction manuel... Anyone care to translate for a poor apprentice?:)

    "thermal expansion tanks eliminate the problem of nuisance cycling of the safety
    relief valve when a backflow preventer or check valve is present in a domestic hot water system.
    Utilizing Flexcon’s patented double diaphragm system, they provide a reservoir for
    expanded water volume and help to prevent dangerous over pressurization. The diaphragm is
    permanently separated from the tank’s air charge. The tanks also feature a brass or stainless
    steel system connection for maximum corrosion resistance."

    Thanks guys!
  2. Hackney plumbing

    Hackney plumbing Homeowner

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  3. jimbo

    jimbo Plumber

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    Frankly, MOST systems call for an expansion tank. First, in theory, if the house has a pressure regulator, that may be a "one way" device. But many of them do have a bypass feature. But that simply prevents house-side pressure from rising above street-side. But if the street-side pressure is above 110 psi or so, then you may still get weeping from the water heater T/P valve
  4. Young_apprentice

    Young_apprentice New Member

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    Ok thanks hackney that made sense Volume control caused by the increase in temperature- thats the purpose of a expansion tank. I can see now how it would protect the t+p. As the heated water expands it has nowhere to go due to the check valve so it would normally expand and expand until the t+p senses the increases and blows out.
    So as the water expands and the pressure increases the expansion tank acts as a compartment for the pressurized water to go and the additional water presses against the diagphram expanding it allowing for more water to enter.

    Ok now im wondering why one would install a check valve at all--- to prevent heat loss by having hot water circulate back into the tank and up the cold loss? seems not really worth it.

    Also it said a pressure reducing valve could have the ability cause a system to become "Closed" how would it due that? because its a venturi valve small opening at the inlet??

    thanks guys!
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    A PRV is generally a one-way valve that opens only enough to enable the amount of pressure on the outlet it is set for. When there's no water flowing, it is closed, thus it acts like a checkvalve.

    As Terry mentioned, SOME of them have a bypass valve that can open, but it ONLY (should) open when the house pressure matches or tries to exceed the street pressure and that could be way higher than you want (and is the likely reason you installed a PRV in the first place).

    The ET acts like a buffer. If you have it sized properly, when the water expands, the pressure increase is barely noticeable as the air is compressed in the bladder tank. Only if the ET bladder precharge is incorrect, or it is improperly sized will the pressure rise much when the water expands. Think about blowing up a balloon...once you have initially blown it up part way (think precharge), it doesn't take much more pressure to continue to blow it up until you reach the limit and it aproaches breaking.

    Many utilities are adding checkvalves either in or near their meter to protect the rest of the system from potential contamination being pushed back into the neighborhood from a user if there's a crossover or contamination in the local system. So, many locales are now requiring an ET, even if there's no closed system there (yet) to save later problems when one is added.
    Last edited: Apr 2, 2012
  6. Runs with bison

    Runs with bison Member

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    They are necessary anywhere that the flow is not fully open back to the utility, because increasing water temperature results in expansion of an incompressible fluid (water.) My neighbor had a scary "event" the other day related to the water heater (backdraft--house fans will do that at times with naturally drafted systems when you close to many doors/windows) and while I was addressing the problem I noticed they had no thermal expansion tank and had a dripping T&P on a 90's era water heater. I stuck my peak indicator gauge on an outside faucet. It peaked at 175 psig during the burner cycle...and the gauge has tested true in the past. I suggested they call out a plumber and install a thermal expansion tank before the tank ruptures or some other weak link elsewhere in the house cuts loose.

    By the way, their PRV is the same model as mine and if memory serves both would theoretically allow backflow, but there is some sort of effective check in the city supply (and I've been taught to never rely on check valves which have failed spectacularly for me in the past...imagine 15,000 gpm at 150 psig reversing direction when a high head pump fails then blowing expensive catalyst solution out a low pressure stack and staining everything green), so I had a thermal expansion tank installed, and they need one as well. Theirs appears to have been this way for years (as was mine before I moved in) based on the etching of the concrete to the drain.
  7. Young_apprentice

    Young_apprentice New Member

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    Location:
    The CAPE
    Thanks guys this is helping me alot, just bare with me on some of question. Ok so back in the day before We put check valves, pressure relief valves, Backflow preventers, on everything to prevent contamination of our cities municipal supplies. any water that expanded ( i just read that water when heated expands up to 5%) could just flow right back into the municipal lines mix with the cold water and were all good thats when everything was "Open systems" Now due to cross contamination we have "Closed systems" as the backflow preventers have become in a sense barriers. I get that part but what i dont still understand is if Everything is pretty much a closed system now (I mean a meter acts as barrier right?) then why the heck do you need to even install a check valve before the expansion tank. What purpose does that check valve serve. I mean theirs already a Vacuum relief valve located on the Cold inlet. That should prevent a vacuum forming and back siphonage occuring whats the point of an check valve.


    Also i thought until now that the Check valve and the Expansion tank would be located on the hot lines, guess not but, why the Cold line? I mean i guess pressure is pressure and it is exerted equally
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
    New England
    There is no good reason to install a check valve when installing an ET. You install an ET when you have a closed system. Now, the utility may mandate a closed system, and if they do, they'd install it. You may create a closed system if you need to control the pressure by installing a PRV.

    Now, you generally need a checkvalve if you add a hot water recirculation system, but the location may or may not mandate use of an ET.

    Many localities require an ET, regardless, as part of their regulations.
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