It may be interesting to note that many heat pumps become quite inefficient whenever the outside temperatures fall much below 35°F. It’s at that point that many of these units switch over to resistive heating in order to warm a home. Please read the excerpt below:
“Heat pumps are only effective to about 35 degrees. Anything lower than that, and you have to use your emergency heat. The two reasons for this are that when the outdoor air temperature gets too cold, the unit cannot extract as much heat from the outdoor air to keep up with heating demand.
Second, condensation forms on the outdoor unit when it is running. In temps above 32 deg (freezing point of water), the condensation simply drains away from the unit. Below 32 deg the condensation freezes to the coil and blocks air flow. When this happens, the unit must go into defrost mode to clear the ice. During this time the heat pump is not heating your house. It’s heating the outdoor unit to clear the ice.”
So, at about 35°F, your heat pump also switches over to a set of resistive heating coils located in the inside furnace. This form of heating is much less efficient and power consumption can soar. My heat pump is a two stage heat pump which can even be less efficient as will be explained below:
“A single stage heat pump uses the same rate of transfer at all times during operation. A dual stage (also called a 2-stage), uses 2 different rates of transfer. Put simply, these 2 stages are kind of like a “high” and a “low”. When the desired temperature and measured temperature are fairly close in number, the unit will not need to work as hard, thus using the “low” setting. In this case, a low rate of transfer is all that is necessary for getting to the desired temperature, so why push it and use more energy than is necessary for the job? Conversely, when the difference in temperature is high, and the unit needs to change the temperature several degrees quickly, the dual stage heat pump will switch over to the high rate of transfer and more energy will be used to get the job done.”
This ability to go from a low setting to a high setting simply means that at the high setting, my house will get warmed more quickly. Useful if it’s really getting cold outside and the unit is struggling to keep up. However, not so good from an energy consumption standpoint!
Let’s see this from the standpoint of actual data. Below is a graph of what happened to the outside temperature during the night of November 17-18, 2012. As you can see, at about midnight the temperature dropped into the freezing range (yellow area of graph).
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Correspondingly, the heat pump began to kick on into Stage 1 (note it never made it to Stage 2 due to both the low setting for the house proper and the fact that it never got seriously cold outside).Let's now look at the corresponding power graph for that period of time as measured by my TED energy monitor:
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All those little spikes represent my small space heater kicking on and off all night long. In spite of how it looks, this little unit did not consume much power as it was heating just a 1600 square foot area (my bedroom). The stage 1 spikes were indicative of the switch over by the heat pump sensor to relatively frequent bouts of resistive heating (5 from 12 midnight to 8AM). Once again, my low thermostat setting for the rest of the house combined with the fact that the lowest low was 27°F. is really what saved the overall situation. Had I been trying to heat the entire home using just the heat pump, the situation would have been much different. Now, take a look at the actual power consumed both before and after the mercury dipped below 35°F.:
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Wow! There was roughly a 50% increase in the amount of energy consumed (all things being equal) after the drop below 35F. I also noticed a residual effect as the house, once cooled, was reluctant to come back up to a 'normal' reading of 60°F.
Disclaimer: There is no way that the information present here could be construed as scientific. It was presented honestly but be forewarned that the author has absolutely no credentials in the areas of heating and cooling.