Modern central air systems both heat the house in summer and cool it in winter, and a single thermostat typically controls both of these functions. The thermostat can be programmed to begin heating when the temperature falls past a certain preset value and to begin cooling when it rises past another preset value. What happens on a spring day, however, when the temperature fluctuates between these values?
Cycling on and off wastes energy, and it's extremely hard on the HVAC system, so to prevent it, modern thermostats have a deadband, which is a temperature range in which the system neither heats nor cools. The temperature range is factory set, and it's usually between 5 and 10 degrees. You can change it, but it isn't recommended.
The deadband on a thermostat is the temperature range within which neither the heating nor cooling system functions. It's there to prevent the system from cycling on and off too frequently and to prevent the heating and cooling systems from operating simultaneously.
Deadband Operates in Auto Mode
You won't find a deadband on a manual thermostat that controls only a furnace or an air conditioner, and you definitely won't find it on an older electromechanical thermostat or any thermostat that you have to set manually. It's only present on modern digital thermostats that can be placed in "auto" mode, and it only works when the "auto" mode is selected. It won't work when the thermostat is in "heat," "fan" or "emergency heat" mode.
It works like this: Suppose the deadband range is 4 degrees, and you set the temperature at 70 degrees. In the summer, the air conditioner will switch off at that temperature, and it won't start again until the temperature reaches 72 degrees. In the winter, the furnace will shut off at 70 degrees, and it won't switch back on until the temperature falls to 68 degrees. This guarantees that the heating and cooling functions cannot operate simultaneously.
A Longer Deadband Saves Energy
The deadband range can be set as long as 10 degrees, which keeps the system off for a longer period and saves energy. The tradeoff is the comfort level in the home, which may be unacceptable for elderly or infirm people living in the home. In a nutshell, a longer deadband is more economical, while a shorter deadband is more effective at maintaining the desired temperature.
The most energy savings result from setting the temperature higher in the summer and lower in the winter, but adjusting the range of the deadband also helps. However, manufacturers don't make it easy to adjust the deadband range. Although the procedure varies from model to model, you usually have to go into configuration mode to do it.
How to Change the Deadband Range
Your owners' manual is the best place to look for instructions on changing the deadband range on your thermostat. On an Excel Air System thermostat, the procedure is to remove the thermostat cover, press the "config" button and scroll to "deadband" (Db) using the fan button. Once there, you can use the temperature set buttons to increase or decrease the deadband range.
Lowering the range below the factory setting is not recommended because it results in nuisance heating or cooling, which occurs when the system cycles on and off when it isn't really needed. Increasing the range may be desirable if you want to save energy but be sure it's OK with everybody in the house.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.