1

See a similar question I posted a few years ago: Heat pump minimum compressor temprature

I have a custom built new construction home, but I was not involved with the build process, so I don't know the thought behind some of the equipment decisions.

My home is located where we have very cold winters (Nights at -10F are not uncommon).

I have a multi zone air handler that is connected to a 2 stage ground source geo-thermal heat pump, along with 2 stage natural gas auxiliary heat. In addition, the entire home is equipped with zoned hydronic floor heating backed by a natural gas fired boiler.

The zone controller for the air handler has settings to configure to when to "Stage up", with combinations of Temperature setpoint to actual temperature delta, number of zones calling, outside air temperature, and time spent running in current stage. These settings were never setup by the installer, and due to my rural location, getting any type of an HVAC person out (Especially one that understands the system) comes with a very hefty service fee.

From the zone controller point of view:
Stage 1 - Ground Geo Stage 1
Stage 2 - Ground Geo Stage 2
Stage 3 - Aux heat (Natural gas) stage 1
Stage 4 - Aux heat (Natural gas) stage 2

The floor hydronic heating is serviced by a bunch of "dumb" honeywell thermostats and not tied into the air handler at all.

Specific questions:

  1. Since I have a ground source geo-thermal unit, is there ever a time when I should run the Auxiliary heat? (Leaving number of zones calling, Outside Air Temperature, and time spent running in current stage disabled)?

  2. What is the most economical way to balance the Hydronic floor heat with the heat provided by the air handler?

Specific Equipment (If needed):

  • Air Handler / Furnace: Carrier Infinity 59TN6A (96.7% Efficiency Rating)
  • Hydronic Boiler: Navien NHB-080 (95% Efficiency Rating)
  • Ground Source Heatpump: Hydron Module BT060 (Stage 1 CoP - 4.1, Stage 2 CoP 3.7)

Energy Costs (If needed):

  • Electrical: $0.1057 / kWh
  • Natural Gas: $0.1414 / Therm

Ground Source Heat Pump Floor Heat Floor Boiler

2
  • Is that ground-source heatpump tied into the hydronic system? What's providing domestic hot water in this system, for that matter? Sep 22 '20 at 0:02
  • The ground source heatpump is not tied into the hydronic floor heating system. Domestic hot water is provided by two tanks in series. The first tank heat is supplied by the de-superheater on the ground source heatpump, the second tank is just a standard Natural Gas water heater.
    – cyclops
    Sep 22 '20 at 12:01
2

If your numbers are accurate (that seems like a shockingly low price per therm for 2020):

10.57 cents buys you 13,989.2 BTU at a COP of 4.1 in the heat pump.

14.14 cents buys you 96,700 btu in the furnace, or 95,000 in the boiler.

A dollar buys you 132,348 BTU at the heat pump, or 671,852 at the boiler, or 683,875 at the furnace ("heating fuel cost", ignoring the fact that all of them use additional electricity to move the heat around)

At those prices, you want to use the heat pump for cooling only, unless you have excess electricity to burn (Solar panels and no buy-back, just offset) or the two gas sources can't keep up on a cold day.

2
  • Good catch on the prices (I mis-typed on my natural gas charge), and have updated the post accordingly.
    – cyclops
    Sep 21 '20 at 21:27
  • Even at the updated prices: Does make one wonder who specified a ground-source heat pump for this install. Those have a rather high installed cost to be (basically) a really good air conditioner, unless your gas price skyrockets, or you have solar power to burn.
    – Ecnerwal
    Sep 21 '20 at 23:43

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