We recently moved into a 5 bedroom home in central Massachusetts with the following system that was installed in 2006:

  • 3 zone hydro-air heat with Veissman Vitorond 100 boiler and Riello 40-F5 burner. Last test showed 85.5% efficiency
  • 80 gallon Superstore ULTRA indirect hot water tank
  • 3x First Co. 36HBXB-HW air handler with Heat Pump R-22 TXV installed, some zones are slightly smaller but they all have a heat pump according to the "installed with" label
  • Nest E thermostats w/o any wiring for heat pump
  • Electricity costs me $0.38/KWH - my total cost after delivery and fees

Heat pumps are all the rage, so I'm wondering if I should have the Nest thermostats wired with the heat pump for alternative heat, or if it wouldn't be a savings because we pay a lot for electricity?

System photo

  • Are you asking if the 'stat should be configured to use the heat pump as aux/backup heating in case the hydro can't keep up, or are you asking something else?
    – FreeMan
    Nov 17, 2023 at 14:04
  • Typically a HP would be your primary heat source, and your "something else" your auxiliary heat. Does your air handler have an electric heat kit installed?
    – Huesmann
    Nov 17, 2023 at 14:17

1 Answer 1


Take the price of 1kWh of fuel, divide by 0.85 which is your efficiency: that's the price of 1kWh of heating with fuel. Since it is usually billed by volume, you'll need the energy density which is about 10.3 kWh/litre for heating oil.

Take the price of the electric kWh, divide by the heat pump COP: that's the price of 1kWh of heating with the heat pump. Note COP will depend on outside temperature and indoor air air temperature, so make sure to check the heat pump documentation.

The valve should be closed, otherwise the hot water will go around the house heating installation and straight back to the boiler:

enter image description here

All these pipes are probably wasting ~500W of heat into your basement, so insulating foam sleeves would probably be a good investment.

  • Good feedback, thanks Bob! Nov 29, 2023 at 1:03
  • For follow-up: 1 liter is 0.264 gallons, and we pay $3.50 per gallon, so 10.3kwh from oil costs $0.92 USD. Our electricity costs $0.38/kwh, so 10.3kwh from electric heat costs roughly $3.91. So electricity is 4.25x the cost of oil for us. Nov 29, 2023 at 1:13
  • OK! In this case you'd need to turn on the heat pump when the outside temperature is not too low so it can have COP>4.25. That's a pretty high threshold. You should read the manual for yours, it should mention COP versus outside air temperature. Would be a good idea to run it once in a while, just in case oil prices go through the roof, but for now it looks like you won't save much...
    – bobflux
    Nov 29, 2023 at 9:22
  • One more note, the advice to close that valve added a lot of air to the system. I heard it immediately after, and when I asked the guy who installed it about it he said to keep it open for pressure reasons. Dec 6, 2023 at 13:38
  • Well a valve on a pipe can't put air in the system... Most likely it caused water to flow in pipes that were full of air because there was no flow in them so they didn't purge.
    – bobflux
    Dec 6, 2023 at 14:22

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