Common Furnace Thermostat Problems

When the Furnace Stops Working, First Check the Thermostat

A closeup of a thermostat on a orange wall set to 68 degrees
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If your furnace isn’t functioning properly and you’ve ruled out the usual suspects—like a dirty furnace filter or bad flame sensor—it may be that the thermostat is to blame. This is usually a good thing because thermostat problems are usually easier (and cheaper) to correct than problems with the furnace itself. In fact, it's best to first rule out thermostat issues, which are often relatively simple to fix, before looking at the major furnace components.

Sometimes, faulty or aging wiring, dust accumulation, extreme temperature changes, and other issues can cause your thermostat to malfunction, so that it can't properly communicate with the furnace and regulate the temperature of your home. Although some of these problems can also be caused by furnace issues, the thermostat is the first place to look if you notice one of three common problems:

  • Your furnace stops producing heat.
  • The furnace puts out erratic amounts of heat.
  • The furnace cycles on and off more frequently than is necessary.

How Furnace Thermostats Work

The vast majority of standard forced air furnaces or furnace/ AC units are controlled by low-voltage thermostat units mounted somewhere in the house, usually in a central location. Even modern high-tech wifi or "learning" thermostats operate on principles similar to the classic older dial thermostats. Powered by 24-volt transformers that step-down 120-volt line voltage, the thermostats monitor room temperature and send ON-OFF signals to the furnace through small-gauge wires when temperatures drop below or rise above the temperature set by the user.

Even when a thermostat is controlled wirelessly by a smartphone, has sophisticated programming to automatically shut on and off, or is controlled wirelessly by a smartphone, the principles are still the same: Low-voltage wires inside the thermostat send control signals to the furnace through small-gauge wires when the thermostat's inner mechanism senses the room temperature has moved outside the user setting. Thus, many of the problems with thermostats involve the elements that are common to all thermostats: the temperature-sensing mechanism, the wires and their connections, and the low-voltage transformer.

Here’s a look at a few of the most common thermostat-related furnace problems and their solutions.

Simple Power Problems

A surprising number of thermostat-related problems stem from problems with the power supply that operates the thermostat.

  • Check the power switch on the furnace itself; it’s easy to mistake this for a light switch and turn it off. The furnace switch is usually mounted on a wall near the furnace, or sometimes even on the furnace itself. This switch usually powers the low-voltage thermostat as well as the furnace itself.
  • Try replacing its batteries if the display on a traditional low-voltage thermostat or wifi thermostat is not indicating power. Most programmable or "learning" thermostats have back-up batteries that are designed to maintain the time and program settings if the house power is interrupted; older standard thermostats do not have batteries. If the system is running at inconsistent times, ensure that you are using AA lithium batteries. Traditional alkaline batteries will run out of juice quickly and may cause inconsistencies and failures in your system.
  • Check the functioning of the transformer, using a multi-tester, and replace it if it is faulty. Low-voltage thermostats, including modern wifi thermostats, run on a small transformer that converts 120-volt line voltage to 12 or 24-volt current that runs the thermostat. If this transformer is faulty or has wire connection problems, it may cause the thermostat to stop functioning. The transformer is typically located inside the service panel on the furnace. You can identify it by the small-gauge wires (similar to telephone or doorbell wires) that run from the transformer to the thermostat. Make sure to shut off power to the transformer's circuit before testing or replacing it.
  • Look for a tripped circuit breaker or blown fuse which may be interrupting power to the furnace or to the thermostat. Resetting the circuit breaker or replacing the blown fuse may return the system to proper operation.

Mismatched Components

Proper operation of your furnace requires correctly matching your thermostat to your heating system. Low-voltage thermostats are the most common type in residential forced-air systems. Your thermostat must be matched to your heating system based on the type, capability, and capacity of your furnace. Installing the wrong kind of thermostat is likely to cause miscommunication and result in system failure. If your furnace stops working after a new thermostat is installed, it's quite possible that the wrong type of thermostat has been installed, or that its wiring has been connected improperly.

To ensure proper matching, consult with an HVAC professional or take your old thermostat with you when you’re shopping for new equipment.

Faulty Wiring

Loose or missing wire connections, and faulty or aged wires can cause your thermostat to lose its connection to your heating and cooling systems, thereby causing an interruption in service. If your furnace stops working properly, it’s a good idea to inspect your thermostat’s wiring. If you suspect faulty wiring, tighten connections and replace wires as necessary, or consult with a knowledgeable HVAC professional. Low-voltage thermostats use small-gauge wires that are easy and safe for DIYers to work with.

Dust and Debris

A dirty thermostat can cause erratic operation and sudden system failure. To address this issue, remove the cover from your thermostat and gently clean its interior components—including the bimetallic coil and switch contact surfaces. First, set the thermostat to its lowest setting and use a soft brush or compressed air to clean the bimetallic coil. Then, set the thermostat to its highest setting and clean the coil again. Finally, reset the thermostat to your preferred setting.

Heat Anticipator Issues

On older mechanical, non-digital (analog) thermostats, there is usually an electrical resistor device—a small metal tab—mounted in the center. This device, called a heat anticipator, tells the thermostat when to turn off the furnace burners. When it is not functioning properly, it can cause your furnace to cycle on and off more frequently than is necessary. If your heat anticipator is improperly set, it will require an adjustment.

Digital and programmable thermostats have built-in anticipators that set themselves automatically, requiring no manual adjustments. Older mechanical thermostats, however, must be manually adjusted—ideally, using an amp meter to determine the proper setting. Sometimes, giving the heat anticipator a light push in both directions is enough to solve the problem.

Inaccurate Temperature Readings

If your furnace fails to turn on when it should (or if it turns on when it shouldn’t), an inaccurate temperature reading may be the culprit. Check to ensure that your thermostat is installed in the proper location—away from outside doors and windows and heat sources such as fireplaces and radiant heaters. Exposure to direct sunlight, heat emissions, and outside temperatures will give your thermostat an inaccurate reading, thereby triggering your furnace to turn on and off when it shouldn’t. Experts recommend that thermostats be located close to the air return ductwork to allow for the most accurate temperature reading possible.

Drafts coming from the wall cavity behind your thermostat may also cause inaccurate temperature readings. Should you find a significant void in the wall behind your thermostat, try filling it with some insulation to curb the airflow. This is more common when the thermostat is mounted on an exterior wall, but it can also occur on interior walls if there is air movement from a basement or attic.