Why Setbacks Save Money
A house loses heat in proportion to the gap between indoors and outdoors. Let the inside cool by 8°F overnight and, for those hours, the gap and the heat loss both shrink. The furnace does have to make that heat back up in the morning, but that recovery is smaller than what the cooler house stopped losing all night, so you come out ahead. That's why "keeping it at one temperature all day is cheaper" is a myth for furnaces and boilers.
The size of the saving follows a simple rule: about 1% of your heating bill for every degree set back for 8 hours (NYSERDA's thermostat worksheet; DOE puts it as up to 10% a year for a 7–10°F setback 8 hours a day). The calculator adds up every period: a cooler normal setting, a night setback and an away setback.
Why Climate Changes the Percentage
DOE notes that the percentage saved is greater in milder climates. A setback trims the same few degrees off the indoor-outdoor gap everywhere, but where winters are mild that gap is small to begin with, so the same setback is a larger share of the bill. Using NOAA's 1991–2020 normals for Chicago (5,974 heating degree days, with a heating season of roughly October through April), a degree for 8 hours works out to about 1.2% there. In Minnesota or New England it's close to the 1% rule; in the South it's higher, though on a much smaller bill.
Heat Pumps Are the Exception
DOE warns that setting back a heat pump in heating mode can cancel the savings. When a heat pump has to recover several degrees on a cold morning, many thermostats call in the backup electric strips, which cost roughly three times as much per hour of heat. Keep heat pump setbacks to 2–3°F, use a thermostat with adaptive or heat-pump recovery, and check what the strips cost with the aux heat calculator. A dual-fuel system recovers on the furnace when it's cold, so larger setbacks work there; set the changeover with the dual-fuel switchover calculator.
In-floor radiant heat has the opposite problem: the slab takes many hours to cool and reheat, so short setbacks barely change the temperature it holds and save little.
Lower Limits Worth Keeping
- 55°F for an empty house in winter. Pipes in exterior walls, crawlspaces and unheated garages can freeze before the thermostat reading looks dangerous.
- Comfort for sleepers. If a 62°F bedroom keeps someone awake, a smaller setback plus a warmer blanket keeps most of the saving; the personal heating calculator compares an electric blanket against turning the heat up.
- Older or ill household members. Health beats savings; the settings here are a starting point, not a target.