Does turning off the air conditioner when you’re not home save energy?

Hot summer days can mean big electric bills. People want to feel comfortable without spending energy and money. Maybe your family has been struggling with the best strategy for cooling a room. Which is more efficient: running the air conditioner all summer without a break, or turning it off during the day when you are away to enjoy it?

We a collective arch and building systems engineers who used energy models that simulate heat transfer and air conditioning system performance to address the perennial question: Do 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 comes down to how energy intensive it is to remove heat from your home. This is affected by many factors, such as how well insulated your home is, the size and type of your air conditioner, and the temperature and humidity outside.

According to our unpublished calculations, letting your home heat up when you’re at work and cool it down when you get home may use less energy than keeping it cool all the time, but it depends.

Turn on the air conditioner all day, even when you’re gone?

First, consider how heat is stored. It enters your home when less heat is stored inside the building than outside. If the amount of heat entering your home is defined as “1 unit per hour”, your air conditioner will always have 1 unit of heat to remove every hour. If you turn off the air conditioning and let the heat build up, you can have up to eight hours of heat at the end of the day.

However, it is often less – houses 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 house can only store five units of thermal energy before coming into equilibrium with the outside air temperature, then at the end of the day you will need to remove no more than five units of heat.

Also, when your home heats up, the heat transfer process slows down; eventually it reaches zero heat transfer in equilibrium when the temperature inside is the same as the temperature outside. Your air conditioner also cools less efficiently in extreme heat, so turning it off during the hottest part of the day can increase the overall efficiency of the system. These effects mean that there is no one-size-fits-all answer to the question of whether you should turn off the air conditioner all day or wait until you get home in the evening.

Energy used by different conditioning strategies

Consider a test case of a small house with conventional insulation in two warm climates: dry (Arizona) and humid (Georgia). Using Program for simulation of energy created by the US National Renewable Energy Laboratory to analyze energy use in residential buildings, we looked at several tests of energy use in this hypothetical 1,200 square foot (110 square meter) home.

We considered three temperature strategy scenarios. One has a constant room temperature of 76° Fahrenheit (24.4° Celsius). A second allows the temperature to rise to 89°F (31.6°C) during an eight-hour workday – this is a “rollback”. The latter uses a temperature drop of 89°F (31.6°C) for a short four-hour workday.

Within these three scenarios, we considered three different conditioning technologies: one stage central air conditioninga air central heat pump (ASHP)and minisplit heat pumps. Central air conditioning units are typical of modern homes, while heat pumps are gaining popularity due to their increased efficiency. Central ASHPs are easily used in individual replacement of central air conditioning units; minisplit is more efficient than central air conditioning but expensive to set up.

We wanted to see how air conditioning energy use varied in these cases. We knew that regardless of the HVAC technology used, the A/C system would kick in when the thermostat setpoint returned to 76°F (24.4°C), and in all three cases late at night, when the outside air temperature is typically at its highest. In cases of failure, we have programmed the air conditioner to start cooling the room before the occupant returns, ensuring thermal comfort by the time they return home.

We found that even when the air conditioner is temporarily ramped up to recover from higher room temperatures, the total energy consumption during blackouts is still less than when the temperature is maintained throughout the day. On an annual scale with a conventional central air conditioner, this can result in energy savings of up to 11 percent.

However, energy savings can be reduced if the home is better insulated, the air conditioner is more efficient, or the climate has less extreme temperature swings.

A central air source heat pump and a minisplit heat pump are more efficient overall, but offer less savings from temperature fluctuations. An eight-hour weekday break provides savings regardless of system type, while the benefits derived from a four-hour break are less straightforward.

Aisling Pigott is a graduate student in the field of architectural construction, University of Colorado Boulder; Jennifer Scheib is an associate professor of building systems engineering, University of Colorado Boulder; and Kyrie Baker is an associate professor of building systems engineering, University of Colorado Boulder

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