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heat pump water heater with domestic coil backup

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WilliamGwiazdowski
WilliamGwiazdowski Member Posts: 100
edited November 26 in Domestic Hot Water

Next year I'm considering installing a 50 gallon hybrid electric heat pump water heater and abandoning the domestic coil in my nearly 30-year-old Burnham V7 steamer. However I would like to see if somehow I could still use the domestic coil for a backup should the HPWH have high-tech issues. How would I pipe it and how would I control the burner to choose whether the boiler is either warm start or cold start?

Unit I'm planning on installing :

Screenshot_20251124-200830.png

Current domestic coil piping:

KIMG8741.JPG

I don't know why the installer piped the hot out from bottom when it should be coming out of the top.

Planned installation spot :

KIMG8742.JPG

House used to have I believe it was an 80 gallon regular tank electric water heater so I already have a 30 amp circuit that is run to that spot and is dead ended inside of the panel and I've already run 3/4" pex to the sump pit for the condensate drain from the HPWH and I've run it along the steam system wet return. I have already done some checking and the wet return only ever gets warm to the touch during an extended call for heat.

Comments

  • mattmia2
    mattmia2 Member Posts: 15,067

    how low can the aquastat be set for the tankless coil? if you can set it below ambient then that will turn it off. otherwise you can put a toggle switch in series with it.

    you can pipe it in parallel and it will be one or the other or you can pipe it before the tank and it will het the water before it gets to the tank if the boiler s hot. you would need a bypass for the tank if you want to use it in place of the tank.

  • WilliamGwiazdowski
    WilliamGwiazdowski Member Posts: 100

    So the model that's installed appears to have two temperature ranges. 40 to 180° f or 100 to 240° f, so I think for simplicity if I do go the route of including the domestic coil in the new hot water loop I'll just do a switch in the aquastat wiring or I can even just pull it off of one of the T terminals on the primary control if I don't want to use it and have the boiler as a cold start.

  • WilliamGwiazdowski
    WilliamGwiazdowski Member Posts: 100

    Per the advice of @Intplm. , I'm going to run the HPWH as a standalone unit. Drew up a quick piping diagram today as well. Even though I am on well water with a 32 gallon pressure tank, the manufacturer still wants an expansion tank by the tank.

    KIMG8754.JPG
  • hot_rod
    hot_rod Member Posts: 26,254

    the expansion tank should be below the shut off valve to the hpwh

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    bburd
  • WilliamGwiazdowski
    WilliamGwiazdowski Member Posts: 100

    Thats just how I drew it. When I do it I'll make sure the expansion tank is below the shut off for the tank

    hot_rod
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,979

    Hi, Here's a quick sketch of how I like to valve things. HPs like cold water, so I'd put it first, then you have the coil as backup if needed. The three valve approach has been around a while for isolating and bypassing equipment.

    Yours, Larry

    diagram -1.jpg
  • hot_rod
    hot_rod Member Posts: 26,254

    I like Larrys thought process. It could be done with 4 valves also. Use 3 way manual ball valves to do two jobs with one valve. If you go that route make sure yo get the correct 3 way, they come on L or T versions.

    Screenshot 2025-11-28 at 1.42.23 PM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Larry Weingarten
  • WilliamGwiazdowski
    WilliamGwiazdowski Member Posts: 100

    Thanks gents however in the interest of simplicity I'm going to not even include the coil in the piping. Just straight into the HPWH with a tee for the boiler fill and a tee for the expansion tank

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