A thermostat can be ‘wrong’ in that it only represents the temperature in one area, and it’s a poor representation of the overall building temperature or it can be wrong in that it thinks a 100 degree house is currently 0 degrees. Over time, the temperature sensor inside a thermostat can begin to fail. It might be a small error or something entirely bonkers.
Diagnosing the Issue
Thermostat issues can be caused by calibration errors, dirt, or complete hardware failure. In the case of dirt, there’s some build up over the sensor, insulating it from the real temperature. Calibration issues can develop over time as the system wears, reading differently as the sensor becomes less resistive or components oxidize, or other issues. There’s also those few occasions where the whole thermostat just loses the ability to read the correct temperature, and that could be caused by a million things, including age, wear and tear, or a power surge.
Before we run off and panic, we first need to confirm there is an issue and just how big it is. We need to be sure the thermostat is the cause of the problem and not just a victim of some other failure. The first step in this process is to measure the actual air temperature by the thermostat. You can generally just hold up a thermometer by the thermostat and see what it reads. I’m a bit lazy, I just use a digital meat thermometer. As long as you’re not touching the metal parts, it’ll read the temperature with reasonable accuracy (as long as it’s not ancient and uncalibrated itself).
If the difference between your thermometer and thermostat is more than a degree, we need to do a little maintenance.
Cleaning, Calibrating, and Leveling
First, always follow your manufacturer’s guidelines. Second, we’re going to talk about the general maintenance here, the procedures will vary by manufacturer. Many thermostats may not be user-maintainable. Third, always just leave this to your HVAC Contractor, they know what they’re doing. If you’re thermostat is really old, don’t open it. It might contain a mercury temperature sensor. If that mercury gets on you, you will experience a long, slow, painful death. It goes without saying: do not touch.
In order to get at the temperature sensor, we will need to disassemble the thermostat. For most household thermostats, this is probably pretty straight forward. For smart thermostats, good luck, those are going to be much trickier. Inside the thermostat, you should find markings on the circuit board for where the temperature sensor is. When you find it, gently clean it off with a can of compressed air.
Some thermostats can be easily calibrated. There will be a thin, curled piece of metal with a screw in it as part of a contact. You need to loosen that screw until the contact opens, remove your screwdriver to take off the pressure, then tighten it until the contact closes. Reassemble the thermostat and see what happens.
For older thermostats, don’t disassemble them. They may contain mercury. Again, mercury will not only kill you, it will be a slow and painful death. We recommend safely disposing of these thermostats and replacing them. If a mercury thermostat is not reading correctly, it may not be correctly leveled. The temperature element in these thermostats must be completely level. To adjust them, get a level and a screw driver, carefully loosen the thermostat mount, and adjust it until level.
Modern thermostats may or may not be capable of being calibrated. It depends on the manufacturer design. We’ve never looked inside a Nest Thermostat, so we’re not sure what it uses to read the temperature. With digital systems, the calibration can be hidden all in software, not accessible to you, the end user. These will most likely require replacement.
What If It’s Not The Thermostat?
So, your thermostat reads the correct temperature, but your HVAC system is creating entirely the wrong temperature. This can be a long rabithole of issues. In brief:
- The Thermostat’s relays may have failed or the logic-components of the thermostat.
- A zone-controller may have failed or is mis-configured.
- The furnace or air conditioner control board is malfunctioning.
- A fan, blower, or other piece of equipment has worn out and is not properly circulating the air.
These issues will require further examination and diagnostic to find and repair. As always, it’s going to be quicker and easier to consult your HVAC Professional and let them do the hard work.