Troubleshoot and Fix a Thermostat

Don't Call a Heating or Cooling Serviceman for a AC or Heat Problem

© Kelly Smith

Jan 4, 2008

Troubleshoot and Fix a Thermostat

Thermostats are the physical interface between a home’s occupants and the home’s heating and cooling system. Too often when the air conditioner or heater is malfunctioning or just not doing what it should, the homeowner immediately suspects the air conditioner, furnace, or boiler. It may just be a simple problem with the thermostat or humidistat.

A thermostat that is not working properly will raise energy bills. This article outlines how to troubleshoot a thermostat and hopefully avoid an expensive visit from a heating and cooling service person. It's as easy as changing an electrical light switch!

Heating Problem Symptoms and Fixes

• No Heat

1. No power to system? Reset circuit breaker or change the fuse.
2. Thermostat dirty? Clean the thermostat.
3. Thermostat faulty ? Replace the thermostat.
4. Thermostat batteries dead (if applicable)? Replace the batteries.

• Temperature Does Not Reach or Exceeds Setting

1. Thermostat is not level ? Use a level to realign thermostat.
2. Anticipator not set correctly ? Adjust the anticipator.
3. Thermostat not calibrated ? Recalibrate line voltage or low voltage thermostat.
4. Range stop incorrect ? Replace thermostat or call serviceman.

• Heater Turns Off and On Repeatedly

1. Thermostat dirty ? Clean the thermostat.
2. Anticipator not set correctly ? Adjust the anticipator.

• Heater Won’t Turn Off

1. Thermostat is not level ? Use a level to realign thermostat.

Cooling Problem Symptoms and Fixes

• Air Conditioner Won’t Turn On

1. No power to air conditioner ? Reset circuit breaker or change the fuse.
2. Thermostat dirty ? Clean the thermostat.
3. Thermostat faulty ? Replace the thermostat.
4. Thermostat batteries dead (if applicable) ? Replace the batteries.

• Temperature Does Not Reach or Exceeds Setting

1. Thermostat not calibrated ? Recalibrate line voltage or low voltage thermostat.
2. Thermostat is not level ? Use a level to realign thermostat.

• Humidifier Won’t Turn On

1. No power to system ? Reset circuit breaker or change the fuse.
2. Humidistat faulty ? Replace the humidistat.

How to Clean a Thermostat

1. Kill the power at the circuit breaker panel.
2. Remove the thermostat cover.
3. Clean the bimetal coil with a soft, clean brush.
4. Remove the thermostat body by unscrewing the retaining screws (on a round thermostat). Slide a piece of bond paper between the lever and contacts to clean them.
5. Reassemble the thermostat.

Adjusting a Thermostat Anticipator

1. Kill the power at the circuit breaker panel.
2. Remove the thermostat cover.
3. Find the label listing the ampere setting for the thermostat and write it down. It may be in the manual, on the furnace service panel, or on a boiler transformer.
4. Set the anticipator on its scale. On an air distribution system, set it right on the ampere setting located in the last step; on a water distribution system, set it 1.4 times the ampere setting.

A Honeywell Chronotherm III Thermostat

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