A Family Adventure


Tyson, Emilie & Isaac

Heat Pump performance during the cold snap

January 11, 2022
Emilie Phillips

This summer we installed a heat pump in our hangar. It is a Chiltrix CX34 air to water heat pump. Yesterday we had our first chance to stress test it’s performance. Outside, the temperature dropped to single digits. Heat pumps have troubles heating when the outside temperature is significantly lower than the indoor temperature. So I was curious how ours did.

Our heat pump install is rather atypical. If you look online for suggested heat pump size per square foot, you get ~20-30 BTU per square foot for residential construction. So our 50 foot by 60 foot hangar should need ~60,000 BTU (maybe more since our square feet are two stories tall). The single Chiltrix that we installed is 34,000 BTU.

  • Our hangar is super insulated. The winter before we installed the heat pump, Tyson heated the hangar with the baseboard and two space heaters. Based on increased electricity usage, that was about 2,500 BTU
  • Unlike the residential rules of thumb, most of the time we want our hangar around 50F so the airplanes don’t fog up when we take them out to fly.
  • The radiant floor is intended as a slow to change base thermal mass. So we didn’t need large capacity to quickly heat up the hangar air.
  • To get better efficiency, I have the heat pump programmed to primarily come on during the day. When the PV panels are producing, and the outside air is warming.

Combining all that, we decided not to over size the heat pump: less cost up front, and hopefully less cost over time.

The heat pump heats the radiant floor in the hangar. We have two zones. The main hangar thermostat program goes up to 50F during the day and down 40F at night. The shop target is 65 during the day and 45F at night.

A lot of heat pumps have a back up resistive heater. The Chiltrix doesn’t appear to have a resistive heater built in. Instead, they offer a backup heater in their water buffer tank. We already had a baseboard heater in the shop, so we didn’t add any additional backup. Looking at the Sense power data from yesterday, the heat pump did run overnight to keep the floors warm and it continued running during the day. At 9AM or so, Tyson went out to the shop to work. He turned the baseboard heat on to bring the air temperature up to 68F.

Hangar Electricity

The heat pump is the lumpy shaped consumption. The baseboard first turned on solid, then it toggles rapidly on and off. Next, here is a plot where you can see hourly energy consumption as compared to indoor and outdoor temperature.

Temperatures vs Energy Consumption

We were able to keep the shop temperature at 68F. (Not shown here, the rest of the hangar stayed around 48F.) When we bought the heat pump, people were concerned it wasn’t big enough despite our calculations. Much of the success is due, not to the heat pump, but to the energy efficient construction of the hangar.

How efficient were we? For the whole day we used 29.2 kWh and produced 17.1 kWh of solar. According to some averages for the US in 2016, a typical US house uses 32 kWh to heat on a typical winter day. We aren’t keeping the hangar as warm as a typical house, but it also wasn’t a typical day.

The next question, is could the heat pump have heated the hangar temperature up to 68F on it’s own. Tyson tried setting the buffer tank target temperature to 90F. The heat pump could only heat the water+glycol mix to 63F. And the floor stayed at 50F. Possibly if we had run the heat pump all day and all night it could have slowly moved the floor up.

sketch of system temperatures

In conclusion: the system works as we wanted it to. I would have to do more detailed studies to determine how efficient it is.

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Comments (2)

  • I appreciate your post because I haven’t found much user information on A2W HPs. A good thread on Greenbuilding.com. More comments than comprehensive reporting (like yours) but residential and northern New England so matching my situation in southeastern Ontario.
    It must have been an interesting challenge to air seal & insulate a hanger door!
    Details on your data logger would be very useful.
    Thanks!

    • The electricity tracker is a unit from https://sense.com/

      For temperature, we are using a zwave temperature sensor (https://aeotec.com) connected to home assistant (https://www.home-assistant.io/)

      We used fairly standard super-insulation techniques on the hangar door:
      * reduce bridging losses by using a layer of rigid foam between the exterior sheeting and the metal structure.
      * spray foam the bays of the metal frame for more R value at the lowest weight.
      * put thick strips of flexible foam to seal each of the joints.

      The one place that isn’t well sealed is the two bottom corners. Air and mice come in. We’ve been stuffing a foam block there.