It’s a question most HVAC engineers have been asked at least once – do HVAC setbacks really save energy? Does it make sense to let the space heat up, and then run the system hard to bring the temperature back down? A group of HVAC engineers using models that include building thermal mass and cooling system efficiencies finally have an answer – well, it depends….
Of course that answer works for any engineering question. So, the researchers created a “typical small US home” – about 1,200 square feet, typical level of wall and roof insulation, and looked at a hot/humid climate (Georgia) and a hot/dry climate (Arizona). The modeled cooling loads using a package from NREL that includes thermal mass, solar radiation, and other factors. Considered were three situations – a constant 76°F through the unoccupied day, an 89°F setback for an 8 hour unoccupied period, and an 89°F setback for a shorter 4 hour unoccupied period. The unit was tasked with bringing space conditions back to 76°F at the end of the unoccupied period.
The results showed that the 8 hour setback saves around 10% of the cooling energy for a typical central HVAC system. Savings for heat pumps and mini-split systems were slightly less. Increased insulation in the home and higher efficiency HVAC units tended to reduce the savings. The bottom line is that there is the potential save energy in the summer with setbacks – for most homeowners.
