Control of Air Conditioners

Air conditioning compressors are easiest to control by breaking the low-voltage signal from the thermostat to the compressor relay. This is normally the yellow wire (“Y” or “Y1”). Be sure to program a minimum OFF-TIME on the relay of at least 5 minutes to prevent short cycling of the compressor, and a minimum ON-TIME of 6 to 9 minutes for desired comfort level. In most cases, the fan should not be controlled - it should run when the compressor is shed.

Figure 1 relay installation on air conditioner

Hook-up

  1. Refer to the A/C schematic and locate the wire from the thermostat that signals the compressor to operate (not the common return line). This can be located in the air handler or at the condensing unit.
  2. Run a 2-conductor, 18-gauge wire from the control unit to the outdoor unit or the air handler and interrupt the wire located in Step 1 above.
  3. Program a minimum OFF-TIME on the relay of at least 5 minutes to prevent short cycling of the compressor. Program a minimum ON-TIME of 6 to 9 minutes for desired comfort level.