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Three engineers have run the numbers on whether or not turning the air conditioning off when you’re not home actually saves energy

People want to use energy that minimizes the amount of money they need to be able to purchase.

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Aisling Pigott, a Ph.D. student in architectural engineering at the University of Colorado Boulder, Kyri Baker, an assistant professor of building systems engineering, and Jennifer Scheib, an assistant teaching professor of building systems engineering, all at the University of Colorado Boulder

Aisling Pigott, a Ph.D. student in architectural engineering at the University of Colorado Boulder, Kyri Baker, an assistant professor of building systems engineering, and Jennifer Scheib, an assistant teaching professor of building systems engineering, all at the University of Colorado Boulder

1 day ago Aisling Pigott, a Ph.D. student in architectural engineering at the University of Colorado Boulder, Kyri Baker, an assistant professor of building systems engineering, and Jennifer Scheib, an assistant teaching professor of building systems engineering, all at the University of Colorado Boulder

High electricity costs can result from warm summer days. People strive to maintain their comfort level without squandering their resources. Perhaps there have been arguments in your home about the most effective cooling plan. Which is more efficient, turning off the air conditioning during the day when no one is there to enjoy it or running it nonstop throughout the summer?

We are a group of architects and engineers who studied the performance of air conditioning systems using energy models that replicate heat transport. If you continuously remove heat from your house throughout the day, will you need to do so or will you only need to do it at night?

The amount of energy required to remove heat from your home determines the response. It is affected by a variety of elements, including how well-insulated your home is, the size and kind of your air conditioner, and the temperature and humidity outside.

In some cases, according to our unpublished calculations, keeping your home constantly chilly can use more energy than letting it warm up while you’re away at work then cooling it when you get home.

A/C on full blast all day, even when you’re not there?

First, consider the first mechanisms that cause heat to build up. When the structure has less heat stored than the outside, it enters your home. Your A/C will always have 1 unit of heat to remove each hour if the pace at which heat enters your home is “1 unit per hour.” You can have up to eight hours’ worth of heat at the end of the day if you turn off your air conditioning and let the heat build up.

However, because homes have a limit to how much heat they can store, it’s frequently less than that. Additionally, how hot the structure was when it was first constructed determines how much heat enters your home. You will only ever need to remove a maximum of 5 units of heat from your home, for instance, if it can only hold 5 thermal units before reaching thermal equilibrium with the outside air temperature.

Additionally, as your house warms up, the heat transfer mechanism slows down until it eventually reaches equilibrium, which occurs when the inside and outside temperatures are equal. Additionally, your air conditioner cools less efficiently in high temperatures, so turning it off during the hottest hours of the day might improve the system’s overall effectiveness. Because of these consequences, there is no simple solution to the question of whether you should run the air conditioning all day or wait until you go back home in the evening.

energy usage of various air conditioning methods

Think of a test scenario where a tiny house with conventional insulation is located in two warm climates: dry (Arizona) and humid (Florida) (Georgia). We examined several test cases for the energy use in this hypothetical 1,200 square foot (110 square meter) home using energy modeling software developed by the U.S. National Renewable Energy Laboratory for studying energy use in residential buildings.

We took three temperature strategy situations into account. One has the interior temperature set to 76 degrees Fahrenheit all the time (24.4 degrees Celsius). A “setback” occurs when the temperature reaches 89 F (31.6 C) for a second during an eight-hour shift. The final one uses a short, four-hour workday with a temperature reduction to 89 F (31.6 C).

We were interested in how these situations’ A/C energy usage varied. We knew that in all three circumstances, in the late afternoon when external air temperatures are typically the highest, the A/C system would surge regardless of the HVAC technology employed when the thermostat setpoint returned to 76 F (24.4 C). To ensure thermal comfort by the time the resident returns home in the setback scenarios, we programmed the A/C to begin cooling the area before they do.

As a result of the higher indoor temperatures, we discovered that even when the A/C temporarily rises to compensate, the overall energy consumption in the setback situations is still lower than when maintaining a constant temperature throughout the day. With a traditional central air conditioner, this might lead to yearly energy savings of up to 11%.

If the house is more insulated, the air conditioner is more effective, or the climate has less extreme temperature swings, the energy savings could, however, be reduced.

This story was first posted on The Conversation, a non-profit news outlet that shares viewpoints from academic authorities.

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Department of Energy funding is given to Aisling Pigott.

The Department of Energy provides funding for Jennifer Scheib.

The National Science Foundation, the Alfred P. Sloan Foundation, and the Department of Energy all provide support to Kyri Baker.