HPWH for radiant heat
Would love some advice.
Here’s the situation. We have a 65g HPWH for our potable HW that probably needs replacing. It’s having issues. It is located in a 550 sq.ft. garage that never seems to go below 55 degrees F. The lower level of the house (1000 sq.ft), not counting the unheated garage, has radiant heating through a concrete slab. The house is well insulated (for the Pacific Northwest area) and the heating load is around 6600 Btu. for the afore mentioned lower level. This heat is currently provided by a 5kw electric boiler which ends up using almost 50% of our annual (all electric) energy. I would like to use a HPWH for both the potable and hydronic.
I’m aware of the Sanden equipment and have been told by the company rep. that we would be a perfect fit for their system based on the limited radiant system, our potable needs, and our temperate location. The downside when compared to the other option/s I’m considering is the cost of the equipment and installation, plus there is no back-up if the unit stops working unless of course I add something.
So here’s the big question. What if I use a new HPWH in the garage to do what I’m sure the Sanden could do? Would it be asking too much of a new Rheem Performance Platinum 80 gallon unit? The compressor on that unit is rated at 4200 Btu, but there is also a 4500w element for high demand. When the element comes on obviously our efficiency is back to where the boiler is, but we do not have too many very cold days here so can we heat the slab on most days without the element coming on, assuming the potable draw is not too high? The radiant is a closed system so I know I will need to use a heat exchanger, I’m thinking the Taco X block could work. The other option is possibly combining 2 40 or 50 gallon versions of the Rheem. I think my garage air volume is big enough for 2 units, and this way I would have 8400 total Btu of heat pump capacity in the hope that the elements would rarely if ever come on. The upsides (over the Sanden), in my mind at least, are savings on equipment and installation, and some redundancy as there are 2 HPHW heaters that also have elements should there be an issue with either heat pump. I know the Rheems are not made for this kind of use so I’m sure any warranty would be voided. As for efficiency, I’m not smart enough to figure that out. I believe the Sanden operates at somewhere between COP 1.5 to 6 or 7 at peak, although because the unit sits outside and we will be using the radiant and more hot potable when it’s cold, my guess is that a realistic COP over the year is probably around 3. The Rheems are rated at 3.5 to 4 COP depending on tank size. As they are not outside, I’m thinking they should still be quite efficient when the heat loads are high.
Sorry for the long description, but this is not a common situation so the solutions are probably limited, and I wanted to provide enough information in the hope that there is a specialist out there that can steer me in the right direction.
Thanks for any advice.
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