Smart thermostats garnered a lot of energy industry and media attention in 2012 and will likely continue to do so as the market continues to grow (for example, a recent GigaOM article claims Nest is shipping 40,000 to 50,000 thermostats each month).  While the energy industry tries to figure out a) what smart thermostats are capable of and b) if consumers will pay to swap out their “dumb” thermostats, it’s clear these devices have the potential to help create more efficient homes by enabling consumers to adjust their energy use.  Still, some consumers continue to debate whether smart thermostats can actually save energy.  Thus, it’s a good time to review that ways in which thermostat setbacks can save energy.

Thermostat setbacks are defined as setting a thermostat at a lower – or higher, depending on the season – temperature than normal so the HVAC system will run less often.  Typically, setbacks are deployed when less heating or cooling is needed, i.e., during the day when occupants are at work, or at night when occupants are sleeping.  The common misconception around setbacks is that the extra energy needed to recover the original temperature nullifies the energy saved by using the setback, and can even raise energy bills.  The fact is, that’s not how setback savings works.

How It Works

The savings from temperature setbacks are directly related to the amount of time spent at the lower temperature setpoint (or, in summer, a higher setpoint).  The energy savings accrued while the indoor temperature falls (or rises) is approximately equal to the additional energy needed to bring the indoor temperature back to the original setpoint.  Since those two conditions cancel out, the measurable savings amass during the time spent at the lower setpoint.

Let’s look at a specific example in the winter.  Consider a thermostat using a comfort setting of 70°F while the home is occupied and a setback setting of 62°F while the home is unoccupied.  At 9 a.m., the thermostat lowers the indoor temperature from 70°F to 62°F.  At 5 p.m., the thermostat brings the home back to 70°F.

The savings accumulate during the 8-hour span while the home is at 62°F; again, the assumption is that the energy saved while the house dropped from 70°F to 62°F equals the energy required to bring the house back to 70°F.

Some argue about setback savings because they don’t agree with that key assumption.  Variables like a home’s physical characteristics (envelope, insulation, solar heat gain, etc.) as well as the HVAC system’s efficiency all help determine the effectiveness of setback savings.  In general, the older a home and/or its HVAC equipment, the more likely efficiency losses are present, especially while the HVAC system recovers from a setback.

However, it’s worth pointing out that the HVAC system doesn’t work harder per se to recover the original temperature; the system just cycles longer.  As North Dakota State University’s (NDSU) “Thermostat Setbacks Do Pay Off” article puts it, “It is not like the throttle on your favorite automobile, where the harder you push, the harder the motor works.  Heating systems are simply on or off.”

Still don’t believe the savings setbacks are selling?  Feel free to comment below to provide your arguments.