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Indirect HWH requires its own expansion tank?

Al Letellier_9
Al Letellier_9 Member Posts: 929
1- An expansion tank on the cold water side of the indirect is common to prevent popping the safety. A lot of water districts are using backflow preventers and thus you have a closed system that can't absorb the expansion in the tank
2- A properly sized thermal mixing valve will have only minimal flow restiction and should always be install on an indirect (my opinion, not the law). It allows you to run the tank at a higher temp to take care of any extreme demands while controlling the supply temp
3- AS to a bypass, it depends on what temp your system is running at. A normal hydronic system, demand start shouldn't require a bypass, but if you have some low temp zones,that could pull the boiler down under 130*, you should consider protection for the boiler.

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  • D107
    D107 Member Posts: 1,849
    Contractor gave me estimate for indirect SuperStor

    piped to my gas-fired Weil Mclain which included an ST-12 expansion tank in addition to the one for the heating. Is this usual?

    2. Also said adding a mixing valve as a scald protect (in addition to the aquastat) would constrict the flow.

    3. And that a bypass on a residential unit with 3/4" supply and return lines (3 zones) would not be necessary as such small piping couldn't produce enough water at a low enough temp to shock the (cast iron) Weil McLain.


  • Big Ed
    Big Ed Member Posts: 1,117

    If there is a check valve or pressure reducing valve on the main you will need the ST tank.

    I believe the manufacturer does not sell the mix valve as a anti scald protection.

    You need to contact Weil McLain and ask them .It will depend on the type of radiation Baseboard my guess they may not recommend a bypass.

    Indirect should be piped larger then 3/4"..Would be 1" or larger on a SuperStore .I know the 30 gallon uses 1" boiler supply..
  • jim lockard
    jim lockard Member Posts: 1,059

    in the State of Virginia requires an expand. tank for domestic hot water tanks on public water.
    Mixing valves I use them with indirect tanks off the boiler- great safty if anything ever failed.
    As for low temp. water shocking the boiler what did he say about flue gas condensation rotting out the flue. The bypass piping will not hurt anything just as easy to do it.
    Best Wishes J.Lockard
  • Weezbo
    Weezbo Member Posts: 6,232


    i consider a st-12 s.o.p. a anti scald the right way to roll and because ihappen to like partial differential reset i see no problem with a by pass.

    would all these things work without these items ? Yah sure would,...until....then what happens someone else will comealong and say "well,...when the sensor,control whatever allowed the water to get that hot it should have had a anti scald devise on it." or "well, the problem originally stemed from there not being a place for the water to expand and thats why you need a ST-12 " or "Well, it is my considered opinion that ....because the indirect did not have domestic priority the 3/4" take off allowed the full flow of the water straight back into the boiler off the indirect,this in turn allowed the field to cool off substantially and that is what caused the boiler to experience the shock..." so even if something is yet to be a code matters not one jot. Each contractor has thier own set of standards yet we may all have various codes we follow as a basic minimum....
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