Programmable thermostats reduce heating energy use

Thermostats are inexpensive temperature sensitive switches that can be connected to a heating system to regulate the temperature. A programmable thermostat permits different temperatures to be preset, usually together with the time of day or week. For more information visit: www.espenergy.com/programmable_thermostats.htm.

The following chart, generated in a home with a programmable thermostat controlled heating system, shows temperatures and natural gas use over a 48 hour period.

The green line shows the temperature in the room having the programmable thermostat, i.e. the kitchen.

Temperatures Feb 24 - 26

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Programmable thermostat energy savings

During the first 24 hour period the programmable thermostat was set as follows:

09:00 to 09:45 - 72°F,
09:45 to 17:00 - 65°F,
17:00 to 21:00 - 70°F,
21:00 to 07:00 - 55°F,
07:00 to 09:00 - 67°F

During the second 24 period the programmable thermostat was set as follows:

09:00 to 10:40 - 67°F,
10:40 to 09:00 - 70°F

Thermostat operation is indicated by the 'sawtooth' portions of the green line where, in this case, the temperature frequently moves between 68 and 70°F.

Natural gas use more than doubled in the second 24 hour period when the programmable portion of the thermostat was effectively not used. During 'sawtooth' period the heating system is continually being turned on and off or 'short cycling'. In the first 24 hours the heating system switched on 7 times only, in the second 24 period 24 times.

Short cycling heating systems are considerably less efficient in terms of energy use than heating systems that are programmed to operate at significantly different temperatures during the day. This in part accounts for the reduction of energy used during the first 24 hour period.

The chart conclusively disproves a common misconception that it costs less to maintain a constant temperature in a home.

Test notes

Average daily outside temperatures over the 48 hour period were almost constant.

Temperatures were logged every 5 minutes in 2 locations by a La Crosse wireless data logging weather station WS-8610U.

Natural Gas use was monitored by making daily meter readings.

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