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A homeowner adjusts a thermostat

Thermostat not Working?

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Tim H.Verified

Lead Building Engineer

Residential and Commercial HVAC

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5,194 positive ratings
Cleaning your Thermostat

Dust and dirt can cause inaccurate temperature readings. Open the thermostat housing and take a small soft brush to lightly dust the metal coils and contact plates.

The thermostat controls the heating and cooling of your home. It measures the temperature in the home and compares it to the desired temperature. If the current temperature is above or below this, then the thermostat will signal the HVAC system to heat or cool, depending on the season. A broken thermostat can be confused for a broken furnace, but try some of the following troubleshooting steps first.

Troubleshooting thermostat problems

The following is a list of thermostat problems that you can troubleshoot yourself.

No warm or cool air coming from the vents

A hand in front of a furnace

First, for a digital thermostat, check for dead or corroded batteries. Change the batteries with brand new ones or if they are corroded then clean the wires that touch the batteries. Check the components inside the thermostat for dust, dirt and debris; clean if necessary. Next, check for any broken wires or loose connections. Dust the thermostat’s interior. Gently push the anticipator, the small metal tab, in either direction. Set the thermostat to its lowest setting and dust the bimetallic coil. Set it to the highest setting and do the same. Retighten the screws that may have come loose at the base.

Next, if there is still no power, check the fuse or circuit breaker in the power panel to see if the breaker has blown or has tripped. You should replace the breaker or if it is tripped, flip the switch OFF then back to ON and check for power.

Indoor temperature does not match thermostat setting   

If the indoor air is not matching the temperature set on the thermostat, you may need to make sure the thermostat has not been bumped and is level on the wall. Also, check the circuit breaker and make sure it is switched to ON.  Make sure the setting is correct for HEAT or COOL.

Next, adjust the thermostat anticipator or control knob to five degrees warmer or cooler, depending on the season. This should signal the furnace or air conditioner to come on.

Turning a knob on the thermometer

If you have a warm air heating system, the system will turn on after the thermostat calls for heat. It does this to avoid damage to the heat exchanger, air handler or blower motor. Thermostats with hot water and steam heating systems will do this to cool down the heating baseboards, convectors or radiators.

Check thermostat wiring

First, turn the power off to the thermostat by flipping the breaker to OFF. Remove the face of the thermostat from the back section, called the subbase, to expose the wiring. On most digital thermostats it should pull straight off. Sometimes you may have to use a butter knife in the slot that goes all the way around to get it started. On other older types, there may be some screws that hold it on.

Find the R terminal and the W terminal. Turn furnace power off, and go back up to the thermostat and take both the R wire and the W wire off the terminals and twist the metal ends together, then turn power back on. If the heat now works, you have a bad thermostat and need to replace.

Similarly, if you are testing air conditioning, you will look for the R, Y, and G terminals and follow the procedure above (power off, twist the ends of these three wires together, power on). If AC now works, you know that you have a bad thermostat.

Once you’ve determined that you have a bad thermostat, purchase a new one. But before you replace, make sure you label all the wires and which terminals they go into. A good practice is to take a picture of the wires before you disassemble the old thermostat. Turn power off and replace with a new thermostat.

Check location and balance

If your thermostat is near a heat source, the temperature reading will be off and send wrong commands to your furnace or air conditioner unit.

A technician takes a temperature reading of a furnace.

If your thermostat is not level, the mercury switch will not work properly. You can level your thermostat by using a leveling tool.

Adjust the anticipator

Adjust the heat or cooling anticipator if your furnace or air conditioner is cycling on and off too frequently or not often enough. To do this, look for a small metal tab next to a scale that measures from shorter to longer.

If your HVAC system is cycling too often, the anticipator should be moved closer to the “longer” setting. If the system never seems to reach the desired temperature, move it away from the “longer” setting. Adjustments should be made one mark at a time. Wait a couple hours after you have made these small changes to see if the system stabilizes itself.

A diagram of the inside of a heat anticipator

If you have tried the troubleshooting tip above, and your thermostat is still not working properly, it may be time to consult an HVAC professional.

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