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We have a concrete slab radiant floor that is currently heating by propane tankless boiler (Lochinvar NKC199N), but as you know LPG is expensive and not quite efficient. Now, I am thinking about adding a Hybrid Water Heater tank dedicates only to my concrete slab radiant, to send 120F water to the slab using heat pump mode only. I am looking at Rheem Performance Platinum 50 Gal.

So question are:

  • Could the hybrid water heater keep up with concrete slab in Winter? We are in Massachusetts, cold days could drop to single digit Fahrenheit.
  • If we send 120F in the slab and comes out 100F, how long does it take to heat 100F water? or start at the beginning, how long does it take to heat 50F water from the well.
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  • I have radiant floor heating using a boiler and I've watched the temperature in the supply and return on my system a lot. I suspect that once you raise the slab temperature a hybrid tank will be able to maintain temp, but I think it will take forever to get to temp
    – Matthew
    Feb 16 at 4:38
  • Based on @Matthew's comment, it seems that if you keep the existing boiler running to keep temp up while you're installing the heat pump, then shut everything down and do a reasonably quick switch over, you're probably going to be fine. You don't say how big the slab is, but even on a 0°F day, it's not likely going to lose heat so quickly that being unheated for an hour is going to create a significant issue. There's a lot of thermal mass in a concrete floor and the rest of the house is insulated...
    – FreeMan
    Feb 16 at 13:26

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LP is expensive, but you can get very efficient (95+%) propane water heaters (e.g. Polaris.) You can also mitigate the cost somewhat by having enough tank to supply a whole year and buying only when it's cheapest (typically in the summer.)

The problem with using a hybrid heat pump to heat your floor slab that heats your house is that the hybrid heat pump pulls heat from the air inside your house.

It's like cooling your house by opening the refrigerator door. Which, incidentally, does not work.

In mild climates where the outside air does not drop below the minimum intake air temperature for the heater, this can work with an outside vent kit. In cold climates, you need an actual source of heat to make it work (a cold climate air-to-air air heat pump is one such option.) A ground source (or "water to water") heat pump is another option.

ThreePhaseEel will be along in a minute to mention "Eco-Cute" outside air-to-water heat pumps using CO2 refrigerant, but the US distributor shortens &/or voids the warranty for a variety of things, including using them for space heating, so that's a very expensive leap of faith if you choose it, since it's hugely expensive and they also void the warranty if you don't have a very expensive plumber install it. Hopefully some competition will come along and improve that situation, but for now, it does not look practical to me, and I'd love to have it be practical.

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