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Combi Boiler water heater side... need a T&P

nate379
nate379 Member Posts: 78
in Domestic Hot Water

The install of my combi boiler had the domestic water side just done up with Pex off the boiler, a pair of ball valves and boiler style spigot valves for flushing. No "combo" valve. I'm not sure why, as the heat piping has the standard combo valves.

The domestic water combo valve kits come with a T&P on the hot side, which makes sense for a tankless water heater.

Does a combi boiler need that as well or does the PRV on the boiler side function take care of that?

I spent some time looking this up and I found varying info.

Thanks for the help.

Comments

  • mattmia2
    mattmia2 Member Posts: 13,943

    any time you are heating water you need a relief valve. i believe it needs to be a t&p valve but some application of boilers heating dhw can exceed the temp rating of t&p vavles and require pressure only valves.

    the dhw is isolated from the boiler water so it needs a relief valve before any valves on the outlet of the domestic side of the hx.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,426
    edited September 1

    The only time you need a T & P is if you have hot water storage.

    1. A tankless heater in an oil or gas boiler doesn't need a T & P
    2. Indirect and gas or oil tank type heaters have them as do HP water heaters
    3. Tankless gas fired WH I don't know but doubt it.

    I don't know what the minimum storage requirements that are would require a T & P

    Any water heating device needs a T & P or a straight pressure relief.

    A combi is basically just a HX and I don't think it needs a T & P.

    kcopp
  • Jamie Hall
    Jamie Hall Member Posts: 26,299

    Um… it's very rare, but I have to disagree, @EBEBRATT-Ed . If there is any sort of check valve on the feed line to the water heating device — check valve, PRV, backflow preventer, whatever, you need at least a pressure relief valve on the hot side.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 25,543

    I think they get just a pressure relief, often includes in the isolation valve set. I don't know there is a place where a probe would go in, as the T&P have temperature probe.

    You can also get away with a small "softball" expansion tank.

    Screenshot 2025-09-01 at 8.37.39 AM.png Screenshot 2025-09-01 at 8.39.15 AM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • nate379
    nate379 Member Posts: 78

    What I'm calling the combo valve is the insolation valves in the above pics.

    It's on a well, there's nothing stopping backflow from the combi to the well pressure tank. There's just a check valve at the well pump. Well has

    How this all came about is I descale flushed the hot water side and was wondering why the heat side had a nice valve set but the hot water was kind of a janky setup with a ton of pex fittings. After finding the correct isolation valves, I noticed they all seem to come with provisions for a T&P.

  • kcopp
    kcopp Member Posts: 4,583

    Just a relief valve…. nothing more.

    I have never seen a temp and pressure relief valve on the isolation valve set ups.

  • GGross
    GGross Member Posts: 1,598
    edited September 5

    Not sure what code says where you are but I have always sold and installed a relief valve with tankless water heaters. As others said its not a T&P though, just a P lol.

    I believe this cash acme valve is the one included with the caleffi tankless water heater service valve kits

    https://www.cashacme.com/us/en/temperature-pressure-relief-valves/fwl-2-pressure-only-safety-relief-valve

    nate379

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