Thermostat setbacks: how much they save, and when they don't

Why a setback saves energy

A house loses heat in proportion to the difference between the indoor and outdoor temperature. Lower the indoor temperature and the house loses heat more slowly, so the heating system has less to replace. The common belief that the furnace has to work so hard to warm the house back up that the saving disappears is wrong: the energy needed to reheat the house is less than the energy saved while it was cooler, because the cooler house was losing heat more slowly the whole time. The same logic applies to cooling in summer, with the thermostat set higher while the house is empty.

The rule of thumb

The US Department of Energy puts the saving at about 1% of the annual heating bill for each degree Fahrenheit of setback held for eight hours a day, and says a household can save up to 10% a year by turning the thermostat back 7 to 10 °F for eight hours a day. The thermostat setback savings calculator applies that rule:

The calculator caps the saving at 20%, because the rule is a straight-line approximation and does not hold for very deep or very long setbacks.

Adding a daytime setback while the house is empty for another eight hours roughly doubles the saving, to about $180. The saving is a share of the bill, so it is worth most where heating is expensive: the guide to comparing fuels per million BTU shows that propane and oil cost about twice as much per BTU as gas, which doubles the dollar value of the same setback.

Where the setback is worth more and less

The percentage saving is larger in mild climates than in cold ones, because a 7 °F setback is a bigger share of a smaller indoor-outdoor difference. The dollar saving is larger in cold climates, because the bill is larger. The guide to degree days explains why: a season’s heating energy is proportional to the degree days, and a setback removes a share of them.

Heat pumps: the strip trap

A heat pump recovers from a setback slowly, because it has modest capacity, especially in cold weather. Many older thermostats respond to a large gap between the setting and the room temperature by bringing on the electric backup strips, which cost about three times as much per BTU as the compressor. A morning recovery on strips can cancel a large part of the night’s saving.

The calculator subtracts a penalty for heat pumps with backup strips: 0.6% for each degree of setback beyond 3 °F.

With a thermostat that has adaptive recovery and locks out the strips during recovery, the full 12% is available; with an old thermostat that brings on the strips every morning, the saving can fall below half of that. For heat pumps, smaller setbacks of 2 to 4 °F, or none, are often the better choice. The guide to heat pumps in cold climates explains why the strips are so costly, and the heat pump size calculator shows how much capacity a unit has in reserve at a given temperature.

Radiant floors and high-mass systems

Radiant floor heating and large cast-iron radiator systems store a lot of heat and respond slowly, often taking several hours to recover. A deep setback leaves the house cold well into the morning. Small setbacks, or none, suit them better; the saving from a setback is real but the comfort cost is large.

Summer setups

For cooling, raising the thermostat while the house is empty saves in the same way. Raising it from 75 to 82 °F for 8 hours during the workday saves several percent of the cooling bill. In humid climates, a long setup can let the indoor humidity climb, so the air conditioner has to remove moisture as well as heat when it starts again. Moderate setups of 4 to 5 °F avoid that. The HVAC running cost calculator shows what an hour of cooling costs, which is what a setup saves.

Smart and programmable thermostats

A programmable thermostat saves only if it is programmed, and studies of real households have found that many are left on hold or in a manual mode. Smart thermostats help by learning schedules, detecting when the house is empty, and managing heat pump recovery. ENERGY STAR certifies smart thermostats that show savings in field data. The saving still comes from the setback, not from the device: a household that already turns the heat down by hand gains little from a new thermostat.

Checking your result

The calculator works from the season’s heating cost. If you do not know it, the degree-day fuel estimate builds one from the house’s design load, climate and fuel. Compare the saving with the bill a year later, adjusted for the winter’s degree days, to see whether the setback is doing what the arithmetic says.