Most people believe turning off their air conditioners when they're not at home saves them money. Well, researchers from the University of Colorado Boulder put that theory to the test.
The team of researchers used energy models that simulate heat transfer and AC system performance to tackle the question everyone wants the answer to: Does turning the air conditioning off when you’re not home actually save energy?
They found that it depends on the type of cooling system in your home, the climate of the area in which you live and the type of construction of your home.
The researchers found that 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.
According to the study, your AC cools less effectively in extreme heat, so keeping it off during the hottest parts of the day can increase the overall efficiency of the system. These effects mean there’s not a single straightforward answer to whether you should blast the AC all day or wait until you get home from work.
Using energy modeling software created by the U.S. National Renewable Energy Laboratory for analyzing energy use in residential buildings, the University of Colorado Boulder team looked at multiple test cases for energy use in a hypothetical 1,200-square-foot home.
During their study, the researchers considered three temperature strategy scenarios in two warm climates: Arizona and Georgia.
One had the indoor temperature set to a constant 76 degrees Fahrenheit (24.4 degrees Celsius). A second let the temperature get to 89 degrees Fahrenheit (31.6 degrees Celsius) during an eight-hour workday – a "setback." The last used a temperature setback to 89 degrees Fahrenheit (31.6 degrees Celsius) for a shorter four-hour workday.
They looked at three different AC technologies: a single-stage central AC, a central air source heat pump (ASHP) and mini-split heat pump units. Central AC units are typical of what most people use today.
They found that when the AC 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.
The researchers stated that on an annual scale with a conventional central AC, this could result in energy savings of up to 11%.
They also found that the central air source heat pump and mini-split heat pump were more efficient overall but yielded a lower savings from temperature setbacks.
An eight-hour setback on weekdays provided savings regardless of the system type, while the benefits from a four-hour setback were less straightforward.