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What kind of relief valve for an electric boiler?

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Mark_R
Mark_R Member Posts: 16
in Radiant Heating

I'm installing a monobloc heat pump.

On the glycol loop I'm installing a 9KW Thermolec backup electric boiler on a secondary loop via closely spaced tees from the main 1-1/2" glycol loop.

What is the proper relief valve to add to the output of the electric boiler? The manual only calls out a "safety valve". I think I only need pressure relief, not a T&P?

If it matters, the output of the boiler is on the side (horizontal), 1-1/2" FPT. I need to convert to 1" for the secondary loop piping.

Thanks.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,001

    What is the pressure rating of the LOWEST rated component? It may not be the boiler… you want a safety valve of that rating or lower, also rated for the output of the electric boiler.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 6,881

    it’s an electric boiler. Pressure and temperature

    Back flow preventer on inlet water if tied in.

  • hot_rod
    hot_rod Member Posts: 27,065

    Usually pressure and BTU capacity is how you would size a hydronic relief valve.

    This is the valve that shipped with my Viessmann Vitocal HP. Is has a 6 KW back up element built into the indoor module. The BTU capacity is probably a bit on the high side :)

    Screenshot 2026-01-07 at 2.55.37 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGrossIntplm.
  • Mark_R
    Mark_R Member Posts: 16

    No inlet water in the system, it's a glycol feeder.

    30psi is recommended by monobloc manual.

  • Jamie Hall
    Jamie Hall Member Posts: 27,001
    https://forum.heatinghelp.com/discussion/comment/1881209#Comment_1881209

    I don't think teperaturenis required on an electric boiler.. Water heater, yes. temo, I don't think so…

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    mattmia2kcoppSuperTechGGross
  • mattmia2
    mattmia2 Member Posts: 15,992

    The temp of a t&p valve is too close to the operating temp of an electric boiler, a hot spot could easily cause it to blow off so as far as I know boilers require a pressure only relief valve.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 19,881

    If it is an electric boiler it should be shipped with the valve or at the least the mfg should tell you what to use and how it should be installed.

    Seems to me a lot of liability for the boiler mfg not to provide a valve.

    Its been a very long time since boilers and water heater were shipped without relief valves.

    EdTheHeaterManAlan (California Radiant) ForbesSuperTechMad Dog_2
  • hot_rod
    hot_rod Member Posts: 27,065
    IMG_1759.jpeg
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mark_R
    Mark_R Member Posts: 16

    Well, I feel stupid now.

    Indeed there was a 30psi relief valve provided with the unit. It had fallen out of the box during unpacking and I missed it.

    Thanks for pointing that out.

    GGross
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 19,881

    It happens. Glad you found it.

    Some wouldn't have thought they needed a relief valve so your way ahead of the game. There are no stupid questions especially when it comes to a safety control

    GGross
  • hot_rod
    hot_rod Member Posts: 27,065

    The heat pump and boiler should both have a relief valve. Did the hp ship with one?
    Are you seperating the glycol loop from the heating loop with a plate hx? If so loop needs a relief valve, expansion tank and air separator

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GGross
  • Mark_R
    Mark_R Member Posts: 16
    https://forum.heatinghelp.com/discussion/comment/1881288#Comment_1881288

    No hx, glycol through the heating zones. In my case, the heating zones are toester fan forced coils.

    This is an Enertech system. The manual suggests a 30psi relief valve if required by local codes, but does not say its mandatory (page 25).

    The backup electric boiler is connected as shown:

    image.png

    The DHW is via a 80Gal. Turbomax reverse indirect hot water tank. It's not shown on the drawing below, but the tank did come with a 30psi pressure relief for the storage tank (glycol side). I'll be adding a T&P to the DHW side:

    image.png
  • hot_rod
    hot_rod Member Posts: 27,065

    One determining factor would be if the component can be isolated off from common piping that has the relief, it should have a dedicated pressure relief device.

    The Vitocal came with relief valves and fittings, installation manual shows a valve on the indoor and outdoor units. They seem pretty clear about installing relief valves.

    Relief valves protect your investment and they are fairly inexpensive. Plenty of choices out there.

    Also be sure to include the glycol % when sizing the expansion tank.

    Screenshot 2026-01-08 at 10.30.36 AM.png Screenshot 2026-01-08 at 10.23.20 AM.png Screenshot 2026-01-08 at 10.24.09 AM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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