I was recently asked on a Google+ discussion if you should turn your air conditioner OFF when gone. Plenty of room for disagreement even among the professionals. When taking a purely energy conservation viewpoint, you’ll definitely save when using setback periods. However, we buy these HVAC systems for comfort. How comfortable is an 80*F home after climbing from your 65*F car and the 50*F vents blowing on you during your commute? Not really comfortable.

There’s a simple solution to help – a programmable thermostat with dehumidification capabilities. Research shows use that humans are most comfortable when the humidity is 40-60% and the indoor temperature is 68-78*F. A home at 78*F and 50% humidity has the same comfort feel to most people as a home at 70*F and 60% humidity.

What about dehumidification? Let’s consider the function of an HVAC system in cooling mode. It’s purpose is to cool and remove moisture. If you’re in a deep setup, then the AC won’t be running. If it’s not running, then it’s not removing moisture. So, I prefer the programmable thermostats that can also dehumidify. They do this by cycling the system for 10 minute periods when there is no active call for cooling.

With that, I recommend a setback similar to the EPA (Energy Star):http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=TH and http://www.energystar.gov/index.cfm?c=thermostats.pr_thermostats_guidelines.

Keys to the savings: setback period should be at least ten hours and 7*F (heating) and 4*F (cooling). Read the footnote on the second link.

What about my own home? Cooling 78*F. Heating 68*F. Set to dehumidify to 50%. Today it’s 78*F in the home with a humidity of 44%. Very comfortable. Actually a bit of a chill as I entered from work. Set it and forget it because we are gone less than ten hours at a time.