What to Set Your Thermostat To, Season by Season

The commonly quoted starting points are 68 degrees in winter and 78 in summer. Treat those as a reference, not a rule handed down. Comfort depends on humidity, air movement, what you are wearing and who is in the house. A rowhouse in Takoma Park with original single pane windows feels colder at 68 than a tight 1990s build does at 66, because your body is radiating heat toward cold glass and drafty walls even when the air temperature reads exactly the same on both thermostats.

A workable range for this area is 76 to 78 in summer and 66 to 70 in winter. Pick the warmest summer number and the coolest winter number you are genuinely comfortable at, then leave it alone. Nudging the thermostat two degrees each way all day costs more than settling on one setting and living with it. If you have an infant, someone elderly, or someone with a medical condition in the house, comfort and safety come first and the utility bill comes second. That is not a close call.

Each degree you move the setting changes the size of the temperature gap between inside and outside, and heat always flows across that gap. A wider gap means more heat pushing in during August or leaking out in January, so the system runs more hours to hold the line. The figure usually quoted is somewhere around 1 to 3 percent of the heating or cooling portion of the bill per degree, over a full season. It is a rough number that varies with insulation and how leaky the house is.

The exact percentage is not the point. The point is that the single biggest lever you personally control on a Pepco summer bill or a Washington Gas winter bill is the setpoint, followed closely by how many hours the house sits at that setpoint with nobody in it. Insulation and air sealing change the same equation permanently rather than degree by degree, which is why attic insulation and duct sealing usually pay back better than chasing one more degree of discomfort.

A setback means letting the house drift while you are asleep or at work, then bringing it back. In a gas heated house it works well. Set back 7 to 10 degrees for an 8 hour stretch overnight or during the workday and you save real money, because the furnace simply runs fewer hours and recovery is quick once it fires. A house does not burn more energy reheating than it saved while coasting. That is a persistent myth and the physics does not support it.

Setbacks work in summer too, though the payoff is smaller, partly because the outdoor temperature swing is smaller and partly because a hot humid house takes time to dry back out. Letting the house drift to 82 while you are at work and pulling it down to 76 an hour before you get home is reasonable. Letting it climb to 88 through a humid August week is not, because you then spend the evening pulling moisture back out of the drywall, furniture and carpet.

A heat pump moves heat rather than creating it, and it does that efficiently as long as it runs steadily. It is not built for a fast recovery. Ask it to close a gap of several degrees at once and the thermostat sees a deficit it cannot make up quickly, so it brings on auxiliary heat, which in most homes around here is a bank of electric resistance strips in the air handler. Those strips are straight electric heat at roughly three times the running cost of the heat pump.

So on a heat pump, keep setbacks small, two or three degrees, or skip them entirely and hold one temperature. If the thermostat has adaptive or smart recovery, turn it on so it begins the recovery early and gently instead of demanding it all at once. And do not reach for the emergency heat setting when the house feels a little chilly. Emergency heat locks out the compressor and runs the strips alone. It is for a failed outdoor unit, not for an impatient evening.

AUTO runs the indoor blower only while the system is actively heating or cooling. ON runs the blower continuously. ON gives you more even temperatures room to room and keeps air moving through the filter, which some people prefer for dust control. In a Mid-Atlantic summer it carries a real penalty. When the compressor cycles off, the indoor coil is still wet with the moisture it just condensed out of your air, and a blower running across that wet coil evaporates the water right back into the house.

The result is a house that reaches its temperature and still feels clammy at 76 degrees. For most homes here, AUTO is the better setting from May through September. In winter the humidity concern disappears, so ON is fine if you like the evenness, at the cost of some extra blower electricity. A variable speed blower changes the math, because it can run slowly and continuously at low power without dragging as much moisture back off the coil.

A programmable thermostat is worth exactly what you program into it. Build four periods for a weekday, wake, away, home and sleep, plus a simpler weekend schedule. Then leave it alone for two weeks and see how it actually feels before you start adjusting. Most people set an aggressive schedule, get uncomfortable in week one, press hold, and never take it off hold again. A modest schedule you can live with beats an ambitious one you override by February.

Smart thermostats add geofencing, phone control, usage reports, and in some cases enrollment in utility programs available to Pepco and BGE customers. Some models also have a humidity setting. If yours does, target 45 to 50 percent relative humidity in summer. On equipment that supports it, that setting lets the thermostat overcool slightly or slow the blower to wring out more moisture on a muggy day. In winter with a whole house humidifier, 30 to 40 percent is the usual target, and lower during a hard cold snap so windows do not run with condensation.