HPWH set point with mixing valve
Hello, I’m working through the planning for a PGH that I will construct in climate zone 6. I plan for an all-electric house with a heat pump water heater (HPWH) located in an insulated basement. I’d like to understand the energy tradeoffs in using a mixing (tempering) valve with a higher water heater set point.
A higher set point, say 140 vs 120 F, could provide several benefits:
– less use of resistance heating
– potential use of a smaller tank, e.g., 60 vs 80 gal
– reduction of Legionnaires disease risk
But comes with a few possible drawbacks:
– greater standby losses
– lower heat pump efficiency
– potential increased wear and tear on equipment, e.g., compressor, sacrificial anode
– requirement for a mixing valve to prevent scalding
There was a nice DoE paper (https://www.nrel.gov/docs/fy16osti/64904.pdf) on the field performance of heat pump water heaters that concluded the highest COPs were observed in 80 gal Stiebel Eltron units which were set to 140 F. However, it didn’t really isolate the set point well vs. tank size and had a limited number of data points.
So, if you have a HPWH or an opinion, I’d like to know: did you include a mixing valve (may be code in some places)? What are you using as a set point temperature? Did you see a benefit? What decrease was observed in resistance heat use? If water usage was not sufficient to trigger resistance heating, what was the increased energy usage due to the higher set point?
If anyone is aware of good analysis on this, please point me to it. I’m sure there is some rule of thumb along the lines of increase setpoint until electric resistance usage occurs on <5% of water draws. I just haven’t seen it or the analysis to support it.
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