Turn a furnace or boiler thermostat down 7 to 10°F for 8 hours a day

Turn the thermostat on a furnace or boiler down 7 to 10°F for 8 hours a day in fall and winter, then back up. DOE's Energy Saver guide (2022) says that routine can save as much as 10% per year on heating and cooling, with the thermostat turned up in spring and summer. The one condition sits in the same passage: if you have a heat pump, don't do this without a smart or programmable thermostat designed for use with heat pumps, which is why a heat pump owner should hold a heat pump steady instead. The setback is free, and renters and owners can both do it: whoever controls the thermostat.

Drop 7 to 10°F for 8 hours, asleep or away

The rule has three numbers: 7 to 10°F, 8 hours a day, fall and winter. DOE's 2022 guide puts the saving for that routine, with the thermostat turned up in spring and summer, at as much as 10% per year on heating and cooling, and its smart thermostat section describes schedules that move to energy-saving temperatures when you are asleep or away. Night is the easy window. If the house sits empty all day, the 8 hours can go there instead.

The rule fits a furnace or a boiler. ENERGY STAR's heat pump page draws the line in one sentence: unlike a furnace or boiler, heat pumps do not save energy by turning it down when you're away or asleep. A furnace paired with a heat pump still means you have a heat pump, so DOE's thermostat condition applies to it.

A colder house at night is the whole point. Running a plug-in heater in the bedroom during the setback is a separate sum, because a space heater saves only if the rest of the house gets colder.

What as much as 10% comes to in a gas-heated home

Example, with published figures and one assumption. EIA's RECS 2020, Table CE3.6 lists what homes heated mainly with natural gas spent in 2020: an average of $519 on space heating and $299 on air conditioning, each averaged over the households that use that end use. The example's one assumption is a gas-heated household that spent both averages: $519 + $299 = $818 on heating and cooling in 2020. As much as 10% of $818 is as much as $81.80 a year, at 2020 prices.

The household's schedule follows DOE's 2022 guide, with times the example chose: 8°F below the daytime setting from 10 pm to 6 am on weekdays and from 11 pm to 7 am on weekends, which is 8 hours every night, and the thermostat turned up for cooling in spring and summer. The 8°F sits inside DOE's 7 to 10°F range.

Prices have moved since 2020. Run the same sum on your own year: add what heating and cooling cost you, then read DOE's as much as 10% from the 2022 guide as the ceiling, not a promise.

Set the schedule once on a programmable or smart thermostat

A manual thermostat works if you turn it down at bedtime and up when you get up. That costs nothing. A programmable thermostat does it for you: DOE's 2022 guide says it can store multiple daily settings that you can manually override without affecting the rest of the daily or weekly program, and that programmable models are cheaper than smart thermostats. A smart thermostat can learn the temperatures you like and build the schedule itself. Either one is a small buy.

ENERGY STAR's smart thermostat page adds one buying check: make sure the model is compatible with your heating and cooling system. The same page describes geofencing, which sets the system back when you leave for the day and adjusts the temperature again while you are on the way home.

Renters program the thermostat they have and ask the landlord before replacing it. An owner who would rather not wire a new one hands the job to an HVAC contractor.

Turn it up in spring and summer, and set back a heat pump only with its own thermostat

The same DOE passage reverses in warm months: turn the thermostat up in spring and summer. In summer, the cooling tips in the 2022 guide say to set it as high as is comfortable when you're home and awake, to raise it when you're sleeping or away, and to mind the comfort and safety of any pets. A ceiling fan saves only when you raise the thermostat while it runs.

On a heat pump, DOE allows the setback only with a smart or programmable thermostat designed for use with heat pumps; otherwise hold a heat pump steady. Not sure whether the system includes one? Ask an HVAC contractor before you program a setback.