An outdoor thermostat is an optional device for heat pump systems that can be used to reduce energy costs during the heating cycle. Auxiliary electric heat strips assist the heat pump on cold days during this cycle but draw excessive amounts of power. This device is used to tell the heating system the outside air temperature at which the auxiliary heat strips should be turned on.
The outdoor thermostat connects to the heat pump's control circuitry. When actual outdoor temperature is available, it tells the system how cold it must be before the heat strips can energize. This temperature can be adjusted between 45 degrees F. and zero degrees F. Manufacturers usually set the default temperature to 40 degrees F. if the thermostat device is not used.
Here's where a little experimentation with your home energy monitor system can pay off. First, establish a baseline load in kilowatt-hours for your heat pump using load-profiling, net-metering or sub-metering techniques with the outdoor thermostat set at 40 degrees F. or left disconnected.
Log kilowatt-hour usage between the hours of 10:00 PM and 6:00 AM and record the lowest overnight temperature using a simple outdoor digital thermometer with a min-max function. Leave your indoor thermostat set at the same temperature throughout the entire process. Gather enough data points to graph kilowatt-hours against overnight lows from the mid-teens to the high thirties in degrees F.
We recommend using the overnight time frame to isolate the heating load from other loads that will normally be off during this time. You might consider shutting off your electric hot water heater manually, or use a timer, for cleaner data during this period. Since outdoor temperatures are generally lowest at this time of day the system will be working at its hardest.
When you have enough data points to define a line, lower the thermostat setting by five degrees. Continue to log kilowatt-hours and overnight lows plugging the data into your graph until you can see a pattern that can be represented with a new line.
Is this pattern above or below your the 40 degree line? If the kilowatt hours read higher, reset the thermostat to 37 degrees F. You are very close to the optimum setting. If the kilowatt-hours read lower, reset the thermostat to 30 degrees and gather a new data set.
Continue stepping down the outdoor thermostat in five degree increments until you see the overall power consumption begin to rise. Increase the thermostat setting by two or three degrees and test the new setting again. It should be very close to the optimal setting for your home where heat pump run-time and heat strip usage are balanced to minimize energy consumption.
After you have established the optimal setting let it run for a few weeks. Check your next electric bill. Compare the kilowatt-hours consumed with what you used during the same month a year ago. The difference in dollars will be your savings during the winter months assuming other factors remain equal.
For additional energy saving ideas for HVAC equipment, please visit the following: