What’s Up with My Thermostat?

broken thermostat

So, your thermostat is acting wonky. Don't assume that your furnace is broken. In many instances, simply repairing the thermostat or replacing it with a new one will save on unnecessary service calls.

Thermostat Basics

How a thermostat senses the temperature and how it responds varies by type — electromechanical or electronic.

Electromechanical Thermostats:

  • Considered somewhat old-school, electromechanical thermostats operate on basic mechanical principles. A bimetallic strip, fancy talk for a joined pieces of two different metals, rests, sometimes in a coil, under the thermostat cover. As the room temperature fluctuates, the coil expands or contracts, activating the contact. Think of a contact as a light switch: When flipped, an electrical circuit closes and the light — or the furnace or central air, in this case — operates. Without the contact, the electrical circuit is open and nothing happens.
  • Sometimes, the bimetallic switch is coupled with a mercury switch, a small glass tube or “ampoule” filled with mercury, that tilts as the coil expands or contracts. A liquid conductor of electricity, it either closes or opens the contact, as the mercury moves from one side of the tube to the other.
  • If the thermostat controls both a heating and air unit, it has contacts on both sides. The mercury or bimetallic switch tilts in one direction for heat and the opposite for cooling. As the temperatures fluctuate, contact is made or broken and the circuit opened or closed on the given side, activating the corresponding equipment.

Electronic Thermostats:

  • Instead of mercury-filled tubes and strips of metal, electronic thermostats use heat-sensing receptors to monitor the room temperature and electrical circuits to respond to the digital data, turning on or off your heating and cooling equipment. Similar to a small computer, electronic thermostats will store data, allowing you to program settings. Wake up to a warm house, cool it while you sleep and set it to different temperatures on weekends when you're out and about. All it takes is a few pushes of the buttons.
  • Digital sensors and circuitry provide greater temperature accuracy. Bimetallic strips and mercury switches allow “deadband,” a temperature range between which it fails to activate or shut down the furnace or air. This means even if you set your thermostat to 70 degrees Fahrenheit, the switch may not activate to turn equipment on or off until it’s much lower or higher.
  • Thermostats may wear out. Since electronic thermostats involve electronic equipment instead of purely mechanical components, these thermostats are more complex, meaning there are more things to wear out. Consult the manufacturer’s thermostat literature or contact a qualified technician to troubleshoot. Given the relatively low replacement cost, in many cases buying and installing a new thermostat may prove a more cost-effective option.

Most thermostat failures aren’t dramatic. Chances are, no shooting sparks, puffs of smoke or piercing alarms will signal that you have a problem. It may be obvious, like the furnace refuses to come on or the display isn’t lit, or you may notice that the house is cooler than the temperature setting.

Common Thermostat Problems

Most thermostat failures aren’t dramatic. Chances are, no shooting sparks, puffs of smoke or piercing alarms will signal that you have a problem. It may be obvious, like the furnace refuses to come on or the display isn’t lit, or you may notice that the house is cooler than the temperature setting. All you know is something’s wrong and you want it fixed. Always start with the thermometer, troubleshooting according to the symptom. Many symptoms and causes overlap, and the general solutions narrow down to one of a handful of fixes.

Furnace or Central Air Doesn’t Start:

  • Check for power. A blown fuse, tripped circuit breaker or dead batteries will prevent the thermostat from turning on the appliance.
  • Is the thermostat excessively dirty? Buildup of dust, dirt, spiderwebs and nicotine, for example, can coat the inside of the thermostat, interfering with both electrical and mechanical components. It’s easy to overlook the inside of your thermostat during spring cleaning, so a dirty thermostat is nothing uncommon. Simply use a soft, clean brush (an artist’s paintbrush works well) to brush the interior components gently. Move parts to ensure you reach every portion of the thermostat. A can of compressed air, such as is used for electronics, also works well.
  • Are there loose wires or terminal screws inside the thermostat? Are the wires corroded? Never remove the thermostat cover without removing the batteries or cutting the power at the fuse or breaker box. Tighten screws and secure loose wires when necessary. Consult the manufacturer’s wiring schematics or have a qualified technician rewire the thermostat, if needed.
    Replace the thermostat if other troubleshooting methods fail. The most expensive programmable electronic thermostat costs around a couple hundred dollars, and inexpensive mechanical ones cost around $20. Worrying over a thermostat you can’t make work — or paying a professional more than a new one costs — just isn’t worth it.

The Room Temperature Doesn’t Match the Thermostat Setting:

  • Is the inside of the thermostat clean? A dirty thermostat is an inaccurate thermostat.
  • Is the thermostat level? Hold a level underneath or above it and check. Careless installation or a forceful bump, knocking it off level, may alter the accuracy of the components.
  • Is the thermostat in a poorly chosen area of the home? Thermostats located in direct sunlight, in front of cold or drafty windows and doors, or isolated from the main living areas may not regulate the temperature correctly. Consider relocating the thermostat, if possible, or taking other measures to solve the problem. Consult a professional for further information.
  • The anticipator may not be set correctly. The anticipator is a little metal tab inside mechanical thermostats, mounted to a round dial with a printed scale. Pushing lightly on the anticipator, in either direction, may solve the issue. More complex adjustments require moving the anticipator to indicate the ampere setting needed for the furnace involved. Typically, this is specified in the owner’s manual or on the unit’s service panel. Consult a qualified technician for further assistance.

The Unit Turns On and Off Constantly or Won’t Turn Off:

  • Is the thermostat clean, inside and out? Build-up of any type may interfere with proper thermostat operation.
  • Does the anticipator need adjustment? Simply moving the anticipator arm one notch toward longer should cause a furnace to run longer during its cycle. Moving it away helps if the room temperature fails to reach the thermostat settings. Wait two or three hours after any adjustments to see if the problem is solved.
  • Is the thermostat completely level? Use a level to adjust it until correct.

Updating Your Thermostat

If all else fails, consider completely replacing your old thermostat with a new digital, electronic thermostat. With a wide range of features and price tags, there’s one to fit most any budget. Touch screen operation, digital displays, alarms to remind you when it’s time to change your HVAC filters and other options make these thermometers very convenient.

Better yet, electronic thermostats help you save money. With old thermostats, every adjustment requires your attention. Programmable electronic thermostats, in contrast, allow you to maintain settings, letting you heat or cool your home only when you need it. The Department of Energy estimates you can save about 10 percent on your heating and cooling bill merely by lowering your thermostat 7 to 10 degrees for eight hours a day during the heating season and raising it similarly during the cooling season. When you think about it, that’s enough savings to pay your utility bill for almost an entire month.

Installing an electronic thermostat is, however, more complicated than installing a mechanical one due to the programming needed. An improperly installed thermostat can, of course, cause significant problems. If you are uncertain about your ability to install or program your own, contact a specialist to do it for you.