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DHW Heatpump with modcon combi

I'm considering upgrading my heat to a modcon (Laars MFTCW199NA2XN combi). I also have a heat pump water heater for my DHW (AO Smith 50gal) that I love, but would like to have additional hot water capacity at times. If I pass the outlet of the HPWH to the DHW side of the combi before it goes to the distribution, will the combi short-fire when hot water is demanded? The thinking is that when the HPWH is empty and can't keep up, the combi starts burning and gives unlimited hot water. I would attempt to keep the 3/4" pipe between the HPWH and the combi as short and insulated as possible, but 5-10 feet could be nearly a 1/4 gal of room-temp water flowing into the combi before the heated HPWH reaches it.

Do these generally fire on flow? Flow + temp? Temp only? Instantly vs 10-second delay on flow? Will this work?

I could do a tiny recirc post-combi or custom electronics to delay the flow sensor being read by the combi controller, but curious if someone has tried this before.


  • gsk3
    gsk3 Member Posts: 21
    Usually with disparate sized tanks in series you’d set the second tank to a slightly lower temp. If you put in a mixing valve you could do 130 on the heat pump and 120 on the boiler or something.

    If you plumb it right you can isolate one tank or the other for redundancy.
  • hot_rod
    hot_rod Member Posts: 21,874
    i ran a few Lochinvar combis with solar pre-heat and they worked fine, they would fire, then ramp down to low fire. Look into how that brand modulates. Some or the tankless water heaters did not get along well with solar pre heat. Eventually a few brands offered a solar version of their tankless

    In several states mix valves are required on all water heaters, regardless of the type.

    caleffi 520 are quick responding mix valves that were designed for tankless and combi type water heaters.

    There is also a method to select between the two sources with 2 thermostatic valves
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream