Are you finding that your room temperature doesn’t match your thermostat setting, rising above or dropping below it? If so, your thermostat is likely experiencing a broken or damaged sensor, preventing it from thoroughly reading the room temperature.
However, while this is the most common reason for a thermostat reading higher than setting or lower, there are a number of other potential issues that can cause your thermostat to misread the room temperature. See our extensive list of these problems and how you can troubleshoot them below.
Keep in mind that some of these answers are dependent on the specific thermostat model you own. With how expansive the thermostat market is, we want to ensure that we provide all the possible problems and their solutions for our readers.
Note: The focus of this post is thermostats. If your AC runs “all the time” and the room temperature doesn't match the thermostat setting – or if the furnace can’t get it up to the set point – you’ve got problems with those components. Before you call an HVAC technician, make sure the filter is clean and, in the case of an AC, the indoor and outdoor coils are clean. An HVAC technician can do the cleaning for the cost of a service call - $75 to $200. Here’s how to clean the indoor coil.
Types of Home HVAC Thermostats
Thermostats come in various kinds these days:
Programmable mechanical thermostats are quite scarce these days, as they are an older thermostat model. They are non-digital programmable devices that provide basic HVAC user settings. You can purchase one at a cheap price, but this also means that the models are more cheaply constructed. They are also known for using a mercury switch to complete their circuits, so several states have banned this type of thermostat.
Non-programmable digital thermostats require you to manually change the HVAC system temperatures each time.
Programmable digital thermostats allow you to regulate the amount of energy you use throughout the day by scheduling the time you want your HVAC system to start/end and the temperature you want your house to be. The set time and temperature will only change when you reset the program.
Smart thermostats are more complex than the other three. They consist of multiple, convenient features, such as Wi-Fi compatibility, zone control, touch screen and voice command options, all while conserving the amount of energy you use up in a day.
Broken or Damaged Sensors
A thermostat is able to determine the temperature in a surrounding environment through its sensor. If the sensor is damaged or nonfunctional, then you’ll get a thermostat reading higher than setting or lower than the setting.
Each type of thermostat carries its own kind of sensor:
Mechanical thermostats use bi-metal strips, which consist of two different metals, such as copper and nickel, that are bonded together. They can also use an alternative sensor, called gas-filled bellows.
Programmable and non-programmable digital thermostats use a solid-state temperature sensing device, known as a thermistor. Thermistors are a type of electrical resistor that change their physical resistance when exposed to temperature changes.
Smart thermostats typically have remote controlled sensors that are handled through Wi-Fi and smartphone apps.
How to fix this: Unfortunately, repairing a thermostat isn’t easy. Typically, they are replaced.
You will need to switch your thermostat from “auto” to its manual mode. Once you have done this, contact an HVAC professional to fix or replace your thermostat, or do it yourself. This Thermostat Replacement Guide discusses your options as does our Thermostat Buying Guide. We’ve reviewed all major brands. The reviews can be found using the Search box.
Thermostat is Unlevel
A thermostat can have incorrect temperature readings if it is not flush with the wall. Whether the wall itself is uneven or the thermostat is not properly mounted, the result is a poor pin connection between the thermostat and the backplate. Until the pin connection is fixed, your thermostat will not be able to correctly gauge the room temperature or properly heat or cool your home.
How to fix this: The backplate needs to be squared to the wall. To do this, remove the thermostat faceplate and loosen the screws that attach the backplate to the wall until the backplate is squared. Replace the thermostat faceplate, and see if the repair allows for proper sensing.
Depending on where your thermostat is located, the area may have a considerable effect on it. For instance, if the thermostat is around a particularly warm area of the house, such as a fireplace, a room with several lights, a window that sunlight regularly shines through, then the thermostat temperature will be exaggerated.
The same thing applies for a thermostat located in a drafty hallway, on an exterior wall, or near an exterior door, which will make the thermostat register a colder temperature than what it is in the house.
How to fix this: Some practical things you can do is keep your window shades closed, regulate how often you go in and out an outside door, and make sure appliances such as hair dryers are not regularly used near your thermostat.
If your thermostat is mounted on an exterior wall, where the outside climate will have a serious impact on your device, it would be in your best interest to call an HVAC professional to relocate it.
Needs to be Cleaned
Sometimes your thermostat simply needs to be cleaned. Having dust or lint collect inside the thermostat can block the thermostat’s sensors, heating them up so that the room temperature doesn’t match the thermostat setting.
How to fix this: Remove the thermostat faceplate and use a soft tip brush or cloth to wipe away the dust. Every thermostat type looks different, however, so you will need to be careful not to damage the delicate components inside the thermostat while wiping.
Do not use a vacuum cleaner for this process. If you want to try something other than a brush or cloth, use canned air.
While a vacuum may sound harmless, the suction will be too powerful for the more delicate thermostat components. You may not only damage the components, but you may also suck one up.
If your thermostat is dirty inside, there is a chance the thermostat wiring is not securely connected. This happens when corrosion and lint can collect where the wiring is plugged; the buildup can hinder or entirely prevent the electrical current from passing through.
How to fix this: A brush can sweep away dust, but you will need to use a cotton swab for the smaller, harder-to-reach places. A swab is especially good for wiping away corrosion.
After cleaning, make sure the wiring is securely plugged in or screwed tight.
If your room temperature still doesn’t match the thermostat setting in a short period of time, your thermostat may also be experiencing another issue on this list.
Needs to be Recalibrated
When buying new thermostats, they come with their own pre-adjusted settings. It is up to you to change those settings for your preferred heating or cooling once you have the thermostat installed. This does not always mean your settings will stay as they are, however.
There are four specific circumstances in which your device can lose its calibration.
If you had a thermostat recently installed. It is common for new, just-out-of-the-box thermostats to have a difficult time registering the room temperature, even after you have calibrated them.
If your batteries die. Not all thermostats take batteries, but if yours does, they may need replacing.
If you have a manual, non-programmable digital thermostat. Unfortunately, due to lacking programmable settings and being easily affected by external factors, such as opening an outside door, non-programmable thermostats are prone to having an inaccurate reading of the room after a while. It’s probably time to upgrade.
Electrical issues. It is unlikely to happen, but any type of thermostat can lose calibration in the case of an electrical issue.
Pro-tip: A practical way to gauge if your thermostat’s temperature sensor is correct is by taping an accurate thermometer next to your thermostat. Check the devices after fifteen minutes. If the thermostat has a different reading than the thermometer, you will know your thermostat will need recalibration.
How to fix this: Here are three possibilities:
If your thermostat uses batteries, switch them out and see how your device reacts.
Recalibrate your non-programmable or new thermostat. Every model has its own instructions on how to recalibrate, so you will need to refer to the owner’s manual. Just know that if you own a non-programmable, you will need to recalibrate your device every so often.
Consider replacing your non-programmable with a programmable digital thermostat. This will cost you money, of course, but programmable thermostats never lose their calibration, and it is unlikely your smart thermostat will lose calibration outside an electrical issue.
If you suspect electrical issues, contact an HVAC professional.
If you decide to recalibrate your device, then use your thermometer a second time to ensure your thermostat is officially reading the temperature correctly.
On the other hand, if you decide to purchase a new thermostat, make sure you check out the PickHVAC ultimate thermostat buying guide to ensure your purchase is compatible with your HVAC system. Our AC Repair Guide shows that replacing a thermostat will cost anything from $75 to $575 for one hour of labor plus the thermostat and depending on the thermostat type and company you choose.
Uneven Mercury Switch
Recalibration works differently for a programmable mechanical thermostat. If your mechanical thermostat is reading higher than the set temperature, it is because the mercury vial, known as the mercury switch, is not completely straight.
The mercury switch sits inside the thermostat and lays horizontally. If the switch is tilted at all, it affects your thermostat’s accuracy.
How to fix this: Remove your thermostat faceplate to look at the glass mercury switch. If it is not laying straight horizontally, you can use your hand—with a level if you prefer—to make it even.
However, be careful with how you handle the switch since it contains mercury, which can give you mercury poisoning if you come in contact with it.
Heat Anticipator Problems
Mechanical thermostats have an electrical resistor device inside them, known as the heat anticipator. This device looks like a small disc and includes a heat resistive wire and an adjustment arm with a pointer on one end. Here is an image with the labels of what we are talking about.
The anticipator properly adjusts the point at which the thermostat turns off the furnace burners, stopping them just prior to the room temperature reaching the set temperature.
If the anticipator is offset, your thermostat will not be able to settle on your preferred temperature, making the room temperature higher than you want.
How to fix this: Take the faceplate off to see the anticipator. The resistor disc should have the words “longer cycle” on it with calibration marks around the disc. Lightly push the adjustment arm one calibration mark closer to the word “longer.” You will need to let the furnace run several cycles to adjust to the change.
If the furnace still has not reached the set temperature by that time, being too hot or too cold, lightly move the adjustment arm one or two calibration marks in the opposite direction. You will then need to wait another two to three furnace cycles for the furnace to adjust.
Repeating this process may be necessary. If the room temperature and the set temperature still do not match after a few tries, it could be in your best interest to replace your thermostat.
Still in Trouble? Call an HVAC Professional
If you try our solutions and your thermostat is still not working, then your device may be experiencing a different or additional issue, such as damaged wiring. In this case, we recommend contacting an HVAC professional to diagnose and address the situation.
Of course, you could simply replace the thermostat, especially if its old, and see if that’s a solution. However, if none of these apply, the issue might with one of the HVAC system components and best diagnosed by a technician.