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Rheem heat pump water heater ducting

fourforhome | Posted in Energy Efficiency and Durability on

I’m in climate zone 4C, 22°F design temp, 4500 HDD, high radon potential (conditioned crawl not allowed). My heating season is basically mid-Nov through end of April. Cooling from mid-June to mid-September.
Energy Star is rewarding the use of an HPWH and the local utility is offering a $500 rebate for such (free money).
Does it make more sense to
1) Install the WH in the garage, which gives up the cooling in the summer and the COP in the winter (it will use more resistance heat in the winter);
2) Duct it to the attic or crawl space all the time;
3) Put it in the laundry room and add a register from the ducted multi-split to make up the stolen BTUs (35 gallons/day * 8.33 lbs/gal * ∆70°F = 20.4k btu)
4) Put it in the laundry room and duct the air discharge to a larger room to damp the effect of the cooled air in small room.

The Rheem Performance Platinum Hybrid Electric 50 Gal. water heater can be ducted (340′ rigid/125′ flex equivalent using 8″ ducts). (

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  1. fourforhome | | #1

    Let's see. Hypothetically, the HPWH uses 1kW of power to produce 3kW worth of heated water. The multi-split uses 1 kW of power to produce the 3kW that the HPWH takes. Therefore, the system uses 2kW of power to produce 3kW worth of heated water. System COP is 1.5 when the HPWH and multi-split COPs are both 3.0.
    During the cooling season, the HPWH offsets the use of the multi-split so it uses 1kw to produce 3kW which saves the multi-split 1kW to NOT cool 3kW worth. A 1:1 exchange for a true COP of 3.0.
    If I'm doing this right: 1.5 COP x .5 year + 3.0 COP x .25 year + 2.25 COP (shoulder season) x .25 year = 2.0625 COP for the whole year. That saves about $185/year.
    But I think I'm underestimating the cooling season benefit. The heated water is free because the electricity would have been used anyway to cool the air.
    For .5 years it uses 2kW to get 3kW; .25 years it uses 1.33kW to get 3kW; for .25 years it uses zero to get 3kW. Weighting the average I get 2+2+1.33+0=5.33kW to get 3kWx4=12kW worth of heated water for a whole year COP of 12/5.33 = 2.25 for a savings of $205/year.
    The Rheem COP/EF is stated as 3.74, not 3.0. The multi-split ducted COP is stated around 3.4, not 3.0. Overall, putting the HPWH in the house is a net benefit over using a ERWH.

  2. propeller | | #2

    If I may share my experience with a ducted hpwh located inside the house. The de-humidifying and cooling effect is welcome most of the year as long as you have some internal heat gain to compensate. On sunny days our windows heat gains suffice otherwise the masonry heater does the job. A good setup for empty nester, might be an issue with a family. In our case the hpwh typically run 2 to 3 hours per day. Look for one that you can schedule to run overnight to avoid the cold air discomfort and the noise.

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