What to Set the Thermostat to in Summer and Winter (and Why It's Different)

Energy Efficiency The Leonard Splaine Team December 1, 2020

Illustration of Sonny setting a thermostat between a summer and winter wall

Every June and every December, the same question shows up on our service calls: what should the thermostat actually be set to? One number gets quoted for summer, a different one for winter, and plenty of households run a quiet tug-of-war over both. The difference isn’t arbitrary. It comes from how your body sheds heat, how humidity behaves, and how the energy math changes with the seasons.

Here’s where to set it, and why the two numbers aren’t the same.

Quick answer

Set the thermostat to 78°F in summer while you’re home, and let the house drift into the mid 80s when you’ll be out for four hours or more. In winter, 68°F while you’re home and awake, then 7 – 10 degrees cooler overnight and while the house is empty. Each degree you move toward the outdoor temperature saves real money: roughly 6 – 8 percent of cooling cost per degree above 78 in summer, and about 1 percent of heating cost per degree of setback held for eight hours in winter. The one exception is a heat pump, which needs gentler setbacks. We’ll get to that.

The summer setting: 78°F when home, higher when away

78°F sounds warm to a lot of people, and on the first day it usually feels warm. Give it a week. Your body acclimates faster than you’d expect, and a ceiling fan buys another 3 – 4 degrees of perceived comfort for pennies, because moving air pulls heat off your skin even when the room temperature hasn’t changed.

The bigger lever is what happens while you’re gone. If the house will be empty for four hours or more, there’s no reason to cool it for the furniture. Raise the setting to 85°F or so and let a schedule bring it back down before you return. Cooling an empty house to 72°F all day is the most common energy leak we see in Northern Virginia summer bills.

The winter setting: 68°F by day, cooler at night

In heating season, 68°F while you’re home and awake is the sweet spot between comfort and cost. A sweater is cheaper than a degree.

Overnight is where winter gives you free savings. Most people sleep better in a cooler room, somewhere in the 60 – 67°F range, so dropping the thermostat 7 – 10 degrees at bedtime costs nothing in comfort and shows up directly on the bill. The Department of Energy’s long-standing estimate: a 10-degree setback held eight hours a day trims 5 – 15 percent off annual heating costs. Blankets do the rest.

Why the two numbers aren’t the same

If 78°F is comfortable in July, why does the same house want 68°F in January? Three reasons.

Humidity does half the work in summer. Your air conditioner isn’t just cooling the air, it’s wringing moisture out of it, and dry air at 78°F feels noticeably cooler than muggy air at 75°F, because sweat evaporates instead of sitting on your skin. Northern Virginia summers are humid enough that once indoor humidity sits in the 40 – 50 percent range, 78°F is genuinely comfortable. Winter flips the equation: cold air holds less moisture, indoor air dries out, and dry air pulls heat off your body faster, so you need a warmer thermostat reading to feel the same.

Your body is dressed for the season. In summer you’re in shorts and your body is primed to shed heat. In winter you’re layered up and your metabolism has shifted toward holding heat in. The comfortable air temperature moves with you.

The energy math rewards a smaller gap. Your system’s workload is driven by the difference between indoors and outdoors. Holding 72°F against a 95°F afternoon is a 23-degree fight; holding 78°F is a 17-degree fight, and the smaller gap also slows how fast heat leaks back in through walls and windows. The same logic runs in reverse in winter, which is why 68°F beats 72°F on the gas bill.

The setback strategy, degree by degree

The pattern behind both seasons is the same: hold the comfortable number only when someone is there to enjoy it.

Summer, per degree above 78°F: roughly 6 – 8 percent off cooling costs.
Winter, per degree of setback: about 1 percent off heating costs for every eight hours the setback holds.
The four-hour rule: if the house will be empty that long, a setback pays for itself. Shorter than that, the recovery run mostly cancels the savings.

The old myth says re-cooling or re-heating the house costs more than you saved. It doesn’t, at least not with a furnace or conventional AC. The recovery run is real, but hours of reduced runtime beat it comfortably.

The heat pump exception

If a heat pump heats your home, keep winter setbacks small: 2 – 3 degrees, not 10. A big morning jump from 60°F to 68°F asks for more heat than the heat pump can deliver quickly, so the thermostat calls in the electric auxiliary strips, which make heat at three to four times the cost. One aggressive setback can hand back a whole night’s savings in one morning. We cover the full story in why aux heat keeps coming on. Better yet, use a thermostat with an adaptive heat pump recovery mode, which warms the house back up gradually without waking the strips.

Let the schedule do the remembering

None of this works if it depends on someone touching the thermostat four times a day. A programmable thermostat runs the whole strategy on autopilot: 78°F when you’re home in summer, 85°F while you’re at work, 68°F on winter evenings, 60°F overnight. A smart thermostat goes further, learning your patterns, noticing when the house is empty, and letting you adjust from your phone. Either one typically pays for itself within a year or two of following the settings above, which is more than we can say for the manual dial that’s been holding 72°F since it was installed.

Frequently asked questions

Is 78°F really comfortable in summer? With humidity under control and a ceiling fan running, yes, for most people within about a week. If 78°F still feels sticky, the problem is usually humidity, not temperature, and worth having checked.

What temperature should the house be at night in winter? Somewhere in the 60 – 67°F range suits most sleepers. Start with a 7-degree setback and adjust from there, unless you have a heat pump, in which case stay within 2 – 3 degrees.

Does turning the AC way down cool the house faster? No. Your AC removes heat at one fixed rate, so setting it to 65°F doesn’t cool faster than setting it to 78°F. It just overshoots the target and runs up the bill.

What should indoor humidity be? Aim for 30 – 45 percent in summer and below 40 percent in winter to avoid window condensation. If your system can’t hold those ranges, a whole-home dehumidifier or humidifier fills the gap.

Want the thermostat schedule set up right, or wondering why 78°F still feels muggy? Leonard Splaine has tuned Northern Virginia heating and cooling for three generations. Call 703-494-2855 or book a seasonal tune-up.