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Is it better to cool your house all day, or adjust the A/C setting on your way out the door? Westend61 via Getty Images

Hot summer days can mean high electricity bills. People want to stay comfortable without wasting energy and money. Maybe your household has fought over the best strategy for cooling your space. Which is more efficient: running the air conditioning all summer long without break, or turning it off during the day when you’re not there to enjoy it?

We are a team of architectural and building systems engineers who used energy models that simulate heat transfer and A/C system performance to tackle this perennial question: Will you need to remove more heat from your home by continuously removing heat throughout the day or removing excess heat only at the end of the day?

The answer boils down to how energy intensive it is to remove heat from your home. It’s influenced by many factors such as how well your house is insulated, the size and type of your air conditioner and outdoor temperature and humidity.

According to our unpublished calculations, letting your home heat up while you’re out at work and cooling it when you get home can use less energy than keeping it consistently cool – but it depends.

Blast A/C all day, even when you’re away?

First, think about how heat accumulates in the first place. It flows into your home when the building has less stored heat than outside. If the amount of heat flowing into your home is given by a rate of “1 unit per hour,” your A/C will always have 1 unit of heat to remove every hour. If you turn off your A/C and let the heat accumulate, you could have up to eight hours’ worth of heat at the end of the day.

It’s often less than that, though – homes have a limit to how much heat they can store. And the amount of heat that enters your home depends on how hot the building was to begin with. For example, if your home can only store 5 units of thermal energy before coming to an equilibrium with the outdoor air temperature, then at the end of the day you will only ever have to remove 5 units of heat at most.

Additionally, as your home heats up, the process of heat transfer slows down; eventually it reaches zero heat transfer at equilibrium, when the temperature inside is the same as the temperature outside. Your A/C also cools less effectively in extreme heat, so keeping it off during the hottest parts of the day can increase overall efficiency of the system. These effects mean there’s no one straightforward answer to whether you should blast the A/C all day or wait until you get back home in the evening.

Energy used by different A/C strategies

Consider a test case of a small home with typical insulation in two warm climates: dry (Arizona) and humid (Georgia). Using energy modeling software created by the U.S. National Renewable Energy Laboratory for analyzing energy use in residential buildings, we looked at multiple test cases for energy use in this hypothetical 1,200 square-foot (110 square-meter) home.

We considered three temperature strategy scenarios. One has the indoor temperature set to a constant 76 degrees Fahrenheit (24.4 degrees Celsius). A second lets the temperature float up to 89 F (31.6 C) during an eight-hour workday – a “setback.” The last uses a temperature setback to 89 F (31.6 C) for a short four-hour workday.

Within these three scenarios, we looked at three different A/C technologies: a single stage central A/C, a central air source heat pump (ASHP) and minisplit heat pump units. Central A/C units are typical of current residential buildings, while heat pumps are gaining popularity due to their improved efficiency. Central ASHPs are easily used in one-to-one replacements of central A/C units; minisplits are more efficient than central A/C but costly to set up.

We wanted to see how energy use from A/C varied across these cases. We knew that regardless of the HVAC technology used, the A/C system would surge when the thermostat setpoint returned to 76 F (24.4 C) and also for all three cases in the late afternoon when outdoor air temperatures are usually the highest. In the setback cases, we programmed the A/C to start cooling the space before the resident is back, ensuring thermal comfort by the time they get home.

Energy models can show how much energy a house will use under particular conditions – like Phoenix’s hot, dry summer weather. The researchers ran the numbers on three different HVAC technologies and three different temperature-setting strategies. Pigott/Scheib/Baker/CU Boulder , CC BY-ND The researchers used the same three different HVAC technologies and three temperature-setting strategies, but this time for a house in hot and humid Atlanta. Pigott/Scheib/Baker/CU Boulder , CC BY-ND

What we found was that even when the A/C temporarily spikes to recover from the higher indoor temperatures, the overall energy consumption in the setback cases is still less than when maintaining a constant temperature throughout the day. On an annual scale with a conventional central A/C, this could result in energy savings of up to 11%.

However, the energy savings may decrease if the home is better insulated, the A/C is more efficient or the climate has less dramatic temperature swings.

The central air source heat pump and minisplit heat pump are more efficient overall but yield less savings from temperature setbacks. An eight-hour setback on weekdays provides savings regardless of the system type, while the benefits gleaned from a four-hour setback are less straightforward.

Aisling Pigott receives funding from the Department of Energy.

Jennifer Scheib receives funding from the Department of Energy.

Kyri Baker receives funding from the Department of Energy, the Alfred P. Sloan Foundation, and the National Science Foundation.

15 ways to stay cool this summer without AC

2021 is already shaping up to be a hot year, with warmer than average spring temperatures and hot streaks across much of the continental U.S into June. In May 2021,  there were 947 record warm daily high temperatures recorded across the U.S., according to the National Oceanic and Atmospheric Administration (NOAA). Stacker scoured scientific studies, DIY websites, and even American history to come up with 15 ways to stay cool on warm days without AC.

For anyone not living along the northern edges or mountains of the U.S., the dog days of summer feature weeks—if not months—on end of intolerable heat. Sizzling temperatures have been shown to negatively affect memory, reaction time, productivity, moods, and even suicide rates. One study by Harvard University in 2016 tracked students before and after a heatwave and determined that higher temperatures correlated with decreased cognitive function; another by Stanford University in 2018 studied the links between suicide rates and temperature and ultimately theorized that bumps in temperature from now through the year 2050 may correlate with an added 21,000 deaths from suicide.

For the times we can’t be in air conditioning 24/7, there are thankfully many low-cost, energy-efficient ways to cool off during summer heat that work and don’t involve sitting in a bath of ice water all day. These tricks can make a difference when your attention is required in a classroom or office setting, or if you’re working outside and need relief from the hot sun.

Keep reading to see a simple hack for using skin care products to bring down your core temperature, how hot beverages may actually cool you off, and how to up your use of fans to beat the heat this summer.

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Despite one’s best efforts to reapply sunscreen as directed, being out in the sun all day can still lead to sunburn and skin damage. The medicinal use of aloe vera to help treat skin ailments can be tracked all the way back to the 4th century B.C. The antiseptic aloe vera is ultimately a wonder plant: In skin care applications it acts as a cleanser, astringent, and moisturizer, and it can stimulate the growth of new skin tissue, while also calming the nervous system. For an extra cooling effect, mix water and aloe vera in ice cube trays, and pop them into the freezer for use when needed.

Whether your skincare routine involves multiple products or just one, keeping them in the refrigerator can make for a refreshing reprieve from the heat. Many lotions, serums, sprays, and masks will even keep their potency for longer when kept cold.

There is no better way to heat up a home in the summer than by switching on the oven, burners, or toaster oven. Avoid that entirely by grilling outside, eating out, or switching things up with meals that don’t require the stove on the hottest days. Salads, cold sandwiches, cold soups like gazpacho, and nori rolls are all great options for hot days.

A well-placed bandana that’s been soaked in cold water will work wonders to cool you off. Be sure to tie it around a major pressure point (ankles, wrists, neck, or elbows) and it can provide an instant chill.

Strategic placement of fans throughout the home can completely change an indoor climate. Window fans pulling in cooler outside air should be positioned on the first floor of the shadiest, coolest side of the home (most likely in a north-facing window). A fan blowing out the window, which will suck warmer air from inside, should be placed on the sunniest, hottest side of the second floor (or same floor if it’s a one-story home or apartment). Ceiling fans should be rotating counter-clockwise (from your vantage point below it)—don’t forget to reverse this direction during cooler months!

Hot water bottles are built to keep people warm on cold nights, but they can also serve the opposite purpose. Fill your hot water bottle (or a regular water bottle, if that’s what you have) with tap water, put it in the freezer for a few hours, and then sleep with the bottle at your feet. Your feet are one of the most temperature-sensitive parts of your body, so keeping them cool will help the rest of your body stay cool.

While a west-facing home may be perfect in winter, it makes cooling the house down in the summer a bit more challenging. Stock up on reusable UV window film to keep the sun’s harmful rays out and the cool air in (it will also work to keep your home warm in the winter). Tinfoil and space blankets work great for blocking heat from windows, too, and may save you a few bucks.

Cotton is a great fabric for the summer whether you live in the dry heat of Arizona or humid Florida; this light material allows air to circulate without getting trapped in one place, cooling your body naturally. Other fabrics that can help you fight off the sweat are chambray, rayon, linen, and synthetic blends, and lighter colors are generally cooler than darker colors.

Drinking a lot of water is one way to stay hydrated during the summer, but it can also be done through eating foods that have high water content. This includes fruits and vegetables such as melons, cucumbers, lemons, grapes, leafy greens, and berries. Spicy foods prepared with chili peppers are great coolers as well: the chilis increase circulation and cause perspiration. And while ice cream might seem refreshing in the moment, it will only make you feel warmer in the long run—flavored ice is a good alternative.

It seems counterintuitive, but drinking something hot on a warm, dry day can help to cool a body off. A Thermal Ergonomics Lab study found that drinking a hot beverage increases sweat production, which in turn cools the body off. There’s just one caveat—the sweat needs to be able to evaporate in short order. This trick will not work if you’re wearing a lot of clothing that can trap sweat or if the scorching day is paired with high humidity.

You may also like: 50 activities to keep children engaged this summer

Strategically opening windows at night when the air is coolest and keeping them closed (with curtains drawn!) during the day is one of the most straightforward ways to keep a home cooler in summer months. This method is most effective if you have adequate insulation in the walls to trap the cooler night air inside. Another window trick is to take advantage of convection: When double-hung windows are open, keep them as such at the top and the bottom. Having the top window cracked allows hot air to escape, while the bottom window being cracked will suck in cooler air from outside.

Plan ahead for hot nights: fold up a top sheet, put it in a bag, and set said bag in the freezer a few hours before bed. When it’s time to hit the hay, take the sheet out and lay it on top of you. For a last-minute (slightly messier) iteration of the same technique, before going to bed wet down a top sheet in cold water, wring the sheet out well, and put it over yourself.

In addition to keeping windows closed during the day and open at night, use blackout curtains to trap cool air inside and keep the heat from coming in. They’ve been said to reduce heat transfer through windows by as much as 24%—that’s a lot of degrees when it’s sweltering outside.

Pranayama is the practice of breath control—and utilizing it can make you feel noticeably cooler. Two breaths, in particular, that can help are the sitali (“cooling breath”) and sitkari (“sipping” or “hissing”). In the sitali, the mouth is opened into an “O,” and the tongue folded lengthwise into a tube to suck air through. After sticking the curled tongue out almost an inch from the lips, a big breath is drawn in, using the tongue like a drinking straw. This will pull cool air into the diaphragm. After breathing in, the mouth is closed and air slowly exhaled through the nose. The sitkari variation of this breath involves closing the top and bottom teeth together with lips open in a smile. Long, slow breaths are drawn in this position through the teeth and mouth, then the mouth is closed and the breath exhaled slowly through the nose.

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