The Most Common Reasons for Inaccurate Thermostats

When your thermostat is inaccurate, it can lead to temperature inconsistencies and reduced heating and cooling efficiency. However, there are several things you can try before resorting to replacing it.
Miscalibration
If you suspect your thermostat’s temperature readings may be inaccurate, you can test it. All you will need is a reliable thermometer and some masking tape. Tape the thermometer next to your thermostat and leave it in place for 10 to 15 minutes. Next, compare the two readings. If the thermostat and thermometer readings are within a few degrees of each other, your thermostat is reading your home’s indoor temperature correctly. However, most thermostats need periodic calibration to maintain accuracy. Your HVAC technician can do this during a regular heating and cooling maintenance visit.
Mercury-based Thermostats
For older mercury-based thermostats, the recalibration process involves adjusting a small screw, usually located near the coil or bimetallic strip inside the unit. To begin, carefully remove the thermostat cover. Locate the calibration screw, which is often brass and near the base of the unit. Use a small flathead screwdriver, like the ones that come with eyeglass repair kits, to make the adjustments. Rotate the screw clockwise to increase the temperature reading or counterclockwise to decrease it. Make small adjustments until the reading matches the temperature displayed on the thermometer you have taped to the wall.
Programmable Thermostats
For programmable thermostats, you can use the settings menu to recalibrate your unit. Start by pressing the “Menu” or “Settings” button and navigating to the calibration or adjustment option, often labeled “Temperature Calibration.” Once there, compare the thermostat’s reading to the thermometer, using the up and down buttons to align the two readings. Be sure to save the updated setting to apply the changes.
Improper Placement
Faulty temperature readings from your thermostat may result from choices made long before you moved into your home. Often, these issues are due to the thermostat’s placement. To ensure the most accurate readings, your thermostat should be positioned on an interior wall near the center of your home, away from heat sources like lamps or appliances. It should also not be in direct sunlight and should be mounted 52 to 60 inches above the floor.
Relocating your thermostat is best handled by an HVAC technician. It involves moving control wires that connect the thermostat to your HVAC system. Improperly wired thermostats can cause a range of issues, including irregular heating or cooling cycles and damage to your heater’s or air conditioner’s control board.
Dust Accumulation
Believe it or not, dust can act as an effective insulator. So, if too much dust gets into the inner workings of your programmable thermostat, it may cause trouble. Dust may build up in layers on the thermostat’s temperature sensor, preventing it from working correctly. In most cases, a can of compressed air is all you need to solve the problem. After removing the thermostat’s cover, use short bursts of air from different directions to clean the sensor.
For mercury thermostats, dust poses a different issue. It can settle on the coil, which is part of the mechanical sensing mechanism that controls the temperature. The coil expands or contracts in response to temperature changes, triggering a switch that activates your heating or cooling system. If dust builds up on the coil, it can restrict movement, preventing your thermostat from working correctly. You can clean dust off the coil with a soft toothbrush. Be careful not to bend any part of the coil, as this may permanently damage your thermostat. If you are uncomfortable cleaning the coil, contact a professional.
Battery Issues
Dying backup batteries in a programmable thermostat do not typically cause it to give inaccurate temperature readings. However, the issue is that weak batteries can affect the thermostat’s ability to retain settings, schedules, and clock functions. If the backup batteries fail, the thermostat may “forget” your preferred temperature schedules or custom settings, leading to improper heating or cooling based on lost information. The good news is that replacing the batteries is usually a simple fix that restores proper function.
Leveling Issues
Mercury thermostats can provide inaccurate readings if they are not properly leveled. They rely on a mercury switch inside that moves in response to changes in temperature. The switch is designed to shift in a specific direction based on gravity, so it must be level to function correctly. If the thermostat is tilted, the mercury may not reach the correct part of the switch, leading to incorrect temperature readings.
While minor issues can sometimes be corrected using the thermostat’s calibration screw, it’s best to level the thermostat. If needed, loosen the mounting screws and carefully adjust the position to ensure the thermostat is level. This adjustment will help maintain accurate temperature readings.
How To Tell if You Need a New Thermostat
Faulty temperature readings can sometimes indicate a broken thermostat. If none of the issues discussed earlier are the cause, consider replacing your thermostat. A few other signs could also indicate the need for a replacement.
For instance, if you notice a sudden increase in your energy bills without any clear reason, it could be due to a malfunctioning thermostat. A thermostat that isn’t working properly may cause your HVAC system to run more frequently, wasting energy. Additionally, if your HVAC system turns on and off rapidly, this is another sign that the thermostat might not function correctly and is usually due to defective wiring. In these cases, replacing your thermostat is the most effective solution.
Another indication that your thermostat may need replacing is if it stops retaining your temperature settings. A malfunctioning programmable thermostat can change its settings on its own, often due to the wear and tear of its electrical components. As the internal circuitry deteriorates, it may also reset frequently, which is a major hassle because you must reprogram it every time it resets. The best solution in this case is usually to replace it.
When purchasing a new HVAC system, it’s also a good idea to consider installing a new thermostat. Older thermostats may not be fully compatible with newer HVAC systems, which can lead to excessive wear on the system and comfort issues. To ensure optimal performance, many HVAC manufacturers recommend specific thermostat models designed to work seamlessly with their systems. Upgrading your thermostat at the same time as your HVAC system can help maximize efficiency and comfort in your home.
Finally, replacing your thermostat is recommended once it reaches around 10 years of age, as this is the typical lifespan for most models. After this point, it becomes less reliable, leading to inaccuracies in temperature readings and other issues. Replacing your thermostat before it fails means you can schedule the upgrade for a time that does not interfere with your family’s daily routine.
Contact Your Local Experts
At Hoff Heating & AC, we have been serving the residents of O’Fallon, MO and the surrounding areas since 1983. We install, maintain, and repair heating systems and ACs. You can also count on us for all indoor air quality needs and thermostat upgrades. Contact us today to schedule an appointment with one of our experienced HVAC technicians.