Thermostat Doesn't Work

Problem: Thermostat for home heat and cooling system does not work, or does not work properly.

Background: A properly functioning thermostat is the brain that regulates heating and cooling to maintain comfortable temperatures in a home. The complexity of your thermostat depends on whether your home has a furnace only, or a furnace and central air conditioner, and whether it is a programmable set-back model. Set-back thermo stats automatically adjust the system at certain periods to save energy, either by dropping temperature during the heating season, or increasing it during the cooling season. Thermostats are quite reliable, but may not work because they are mis-installed, fall out of adjustment, need cleaning, or because components malfunction.

 

What to do: You can use the simple tests outlined below to determine whether your thermostat is defective. See Thermostat Malfunctions for malfunctions caused by dust or poor contacts; see Thermostat Needs Adjustment for adjustment-related problems.
   If the problem is that the system fails to produce heat, turn off the power to the furnace, then remove the thermostat from wall. Disconnect I wire from the back of the thermostat. Turn on the power to the furnace. Touch the loose wire to the wire still connected. If the burner comes on, the thermostat is defective and needs to be replaced.
   If the system is not cooling, turn off the power to the cooling system. Remove the thermostat from wall. Disconnect the wire from the Y terminal. Turn on the power to the cooling system. Touch the loose wire to the R terminal. If the compressor starts, the thermostat is defective and should be replaced. (Some systems have a time delay, so allow at least 2 minutes.)

Special advice: If you suspect some thing is wrong with your thermostat, make sure the problem is not with the furnace or air-conditioning equipment (see Furnace Doesn’t Work and Central Air Conditioner Quits). Make sure the equipment is receiving power by checking for a blown fuse or tripped circuit breaker at both the main service panel and any secondary circuit boxes. Also check the furnace power switch and the air-conditioner condenser switch (located outdoors on the unit) to see that they are on. If the equipment has been working, check your owner’s manual for any special start-up procedures—especially if an electronic-ignition furnace has “locked out” (or become temporarily disabled), or if the pilot has gone out on equipment with pilot lights.

Helpful hint: Before testing the thermostat, make sure the connections inside of the unit are making contact. With the power to the furnace turned off, tighten all mounting and terminal screws and repair any broken wires.