Virtually everyone can identify your home’s Thermostat. We know that it is usually mounted on a wall and controls how much heat is produced throughout the house (or that specific location). But most of us are not sure where to turn when we suspect the thermostat is malfunctioning. In this article we’ll cover troubleshooting the standard low voltage thermostat found in most homes (made by manufacturers such as White-Rodgers, Robertshaw, Honeywell, and Lux – just to name a few).
First, let’s cover some basics. Your thermostat is a device which controls your gas furnace, specifically determining when and how much heat will be produced.
Many folks don’t know that a transformer reduces the voltage on the incoming line from 120 volts to somewhere between 12 and 24 volts. This is what allows your thermostat to operate.
When your furnace is not running, most homeowners assume it’s a problem with the furnace. This may very well be the case, but it’s important to realize that your furnace problems could just as easily be a problem with the thermostat.
PROBLEM #1:
The furnace is NOT producing any heat.
Possible Issues
- Your digital (electronic) thermostat has a drained battery.
- The Circuit breaker is tripped or a fuse is blown.
- The thermostat components are dirty.
- You have an open connection or loose wires.
Solutions
- Verify that the display is working correctly and indicating an appropriate level of power on your digital (electronic) thermostat. If it is not working or the power is low, replace the batteries in the thermostat.
- Check your circuit breaker/fuse box. Reset the circuit breaker or replace the fuse if necessary.
- In order to clean the components, you’ll need to start by removing the thermostat cover. Gently clean the components, including the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch (mercury switches are common with older thermostats).
- To clean the bimetallic coil, set the thermostat to its lowest possible level and use a soft brush or can of compressed air. Next, set the thermostat to its highest possible level and clean the bimetallic coil once again. Be sure to set the thermostat at your preferred temperature when you have completed this drill.
- Use a screwdriver to tighten any loose screw terminals. If a wire is loose from its terminal, you must place the wire back on the terminal before tightening the screw.
- Be sure to check for any disconnected wires or loose connections on the low-voltage transformer. The low-voltage transformer is usually mounted on the furnace itself.
PROBLEM #2:
The heat being produced either does not reach OR exceeds the temperature set on the thermostat.
Possible Issues
- Verify that your thermostat is mounted level on your wall. A thermostat which is mounted crooked will cause problems.
- Verify that the heat anticipator is set correctly. What is the heat anticipator? It is a device which is mounted in the center of many mechanical thermostats. It signals the furnace to shut OFF by fine tuning the amount of time necessary to do so.
Solutions
- Remove the cover of the thermostat and loosen the screws which anchor the unit to the wall. Use a level to adjust the positioning of the thermostat. Tighten screws once again and replace the cover.
- If you need to adjust the heat anticipator, this is a little more complex and tricky. Let’s step through the detailed instructions:
Adjusting the Heat Anticipator
As we mentioned previously, the heat anticipator is a device (specifically an electrical resistor) which is mounted in the center of many mechanical thermostats. Its purpose is to fine tune the point in time in which the thermostat signals the furnace to shut OFF. It accomplishes this by anticipating the flywheel effect of the room (or area) heating up and therefore shuts OFF the burners a short amount of time before the space reaches the temperature as set on the thermostat. This saves energy and money.
The heat anticipator is composed of thin wire mounted to a disc. That disc is directly attached to the bimetallic coil. An adjustment arm touches the wire and serves to vary the electrical resistance of the wire, which in turn varies the temperature of the wire itself. This process results in warming the bimetallic coil, thus causing it to shut OFF the gas burners earlier than they would be otherwise.
If the heat anticipator is out of adjustment, this often results in the furnace short cycling (frequently turning ON and OFF). Alternatively, it may cause the furnace to exceed the desired temperature…or never reach the temperature as set on the thermostat. You adjust the heat anticipator as follows:
- Remove your thermostat cover
- Double check to make certain that your thermostat is level. Otherwise this will cause the mercury switch to work improperly.
- Look for a small disc with calibration marks in the center of your thermostat; it will likely have the word LONGER printed on it. Attached to the disc will be a lever arm and indicator relating to the given calibration marks. This is known as the heat anticipator adjustment lever arm.
- Is your furnace cycling ON and OFF too frequently? If so, go ahead and move the heat anticipator adjustment lever closer to the LONGER setting by exactly one calibration mark.
- Perhaps your furnace is not reaching the temperature as set on the thermostat…or exceeding that temperature. If this is your issue, just move the adjustment lever away from the LONGER setting – again by exactly one calibration mark.
- It’s important to let the furnace run for a period of 2 to 3 hours after you’ve made your adjustment. This will allow the temperature to stabilize and help you determine if further adjustments are necessary.
- Repeat the above procedure (starting at step #1), if necessary.
- If you were unable to resolve the problem after following these instructions, this unfortunately means that you may need to replace the thermostat. The good news is that it’s far less expensive than having to replace your furnace!
PROBLEM #3:
The furnace turns ON and OFF too rapidly/frequently (a condition known as Short Cycling)
Possible Issues
- The thermostat components are dirty.
- Verify that the heat anticipator is set correctly.
Solutions
- In order to clean the components, you’ll need to start by removing the thermostat cover. Gently clean the components, including the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch (mercury switches are common with older thermostats).
- To clean the bimetallic coil, set the thermostat to its lowest possible level and use a soft brush or can of compressed air. Next, set the thermostat to its highest possible level and clean the bimetallic coil once again. Be sure to set the thermostat at your preferred temperature when you have completed this drill.
- Adjusting the heat anticipator was covered in detail above under Problem #2. Please carefully read the section titled Adjusting the Heat Anticipator.
