When your HVAC thermostat is not calibrated correctly anymore the temperature adjustments in your home will not be effective. Your home will always be either too warm or too cool as a result of the incorrect calibration. It is good practice to check the calibration of your thermostat at least once a year. However, many homeowners have the problem that they don’t know how to check the calibration, and even if they can check it, they don’t know how to rectify any issues that may occur.

It is easy to check whether your digital or mechanical thermostat is still calibrated correctly. You can do it yourself and all you need is an accurate thermometer and it will take you only 15 to 20 minutes. To calibrate the thermostat when the check reveals that the calibration is not correct will take a bit of time but you can also do it yourself if you follow a few easy steps. 

In this article, we’ll look at 

  1. how you can quickly check whether there might be a calibration issue,  
  2. how a thermostat that is not correctly calibrated anymore affects your home,
  3. how to calibrate a mechanical thermostat, and 
  4. how to calibrate a digital thermostat

If you already know your thermostat needs to be calibrated, you can skip directly to section 3.

How to quickly check whether there might be a calibration issue

It is easy to check the calibration of your thermostat and it is good practice to check it regularly.  Many homeowners do a calibration test once a year, normally in the fall. With an annual inspection, you should be able to pick up any calibration problem in time. 

To do the checking, take a glass tube thermometer and wrap a paper towel around it. Find your thermostat and tape the wrapped thermometer to the wall a few inches away from the thermostat.  Ensure that both devices are away from any exterior influences, such as an open window or door. 

Leave the glass tube thermometer on the wall, for 15 to 20 minutes.  Then check the readings on the thermostat and the thermometer. 

The ideal should be that both readings are exactly the same, but a variance of +/- 3˚F is generally acceptable, although many HVAC technicians will not accept any variance more than 1 degree Fahrenheit.  If the readings are outside these parameters, it means your previously accurate thermostat may require calibration. 

How a thermostat that is not correctly calibrated anymore affects your home

Effect on the comfort in your home

If your thermostat is not correctly calibrated, it can’t sense the temperature in your home accurately. The result is that your HVAC system either makes your home too warm or too cool. 

As an example, if you’ve preset your system to maintain a temperature of 72 degrees Fahrenheit your thermostat should activate the cooling system when the temperature rises above 72 degrees. If, however, the thermostat has a calibration problem and reads the real temperature 3 degrees lower, it will have to be 75 degrees in reality before the thermostat will trigger the cooling system. 

If the calibration issue is the other way round, your cooling system might be running all day. If the AC should cut out when the temperature is 68 degrees and your thermostat reads the real temperature 3 degrees higher, the real temperature has to drop to 65 degrees before the thermostat cut the power to the AC. 

Thus, a thermostat that is not calibrated correctly, can make your home uncomfortable warm or cold.

Effect on your energy bill

According to a study by the U.S. Energy Information Administration, 65% of the energy used in homes is used for heating and air conditioning. The majority of this energy consumption is based on the HVAC system. Therefore energy consumption must efficiently be regulated in the home. 

The HVAC system is regulated through a thermostat that can sense the room temperature and regulate the system which keeps the temperature of the room at a preset comfort level.

As we’ve described earlier in this article, a thermostat that is not calibrated correctly can keep your AC or furnace on when it is not needed. This unnecessary running of the AC or furnace provides you with a high energy bill. 

Thus, to save money you have to ensure that your HVAC thermostat is always calibrated correctly.

How to calibrate a mechanical thermostat

Because you calibrate mechanical and digital thermostats differently, we’ll have to look at both types separately.  Let’s firstly look at what you have to do to calibrate a mechanical HVAC thermostat. 

If you’ve done the comparison between the temperature readings on the HVAC thermostat and the glass tube thermometer and they are not the same, you have to start taking steps to calibrate the thermostat. 

A mechanical thermostat can lose calibration after some time. There are various reasons why it can happen. It can be something like dirt and dust or accidental bumping. 

1. Determine the type of mechanical thermostat

Before you start to rectify the issue, determine what type of thermostat you have. There are two types of mechanical thermostats – one with magnetic contacts and a calibration screw, and another that uses a mercury switch. During the calibration process, you’ll have to know what type of thermostat you have. 

2. Clear around the mechanical thermostat

Clear the area around the thermostat and remove anything that may influence the thermostat’s reading. A candle or lamp may be too close to the thermostat and then you have to remove it. Or there can be a cold draft from an open window or door and you have to close the window or door.

3. Clean the mechanical thermostat

Remove the faceplate of the thermostat and clean around the edges and at the top. Use a brush or a cotton swab to clean the connections and to remove any dust and dirt. Also, look for any other obstruction and remove it. 

Do not use a vacuum to clean. A vacuum can cause the wires to pull out or it can damage the thermostat.

4. Calibrate a mechanical thermometer equipped with a calibration screw

Use a wrench to keep the dial straight and then slowly turn the calibration screw. You’ll find the calibration screw in the middle of a thin, curled piece of metal. Turn the screw left and right until the metal contacts open and then remove the screwdriver. After you’ve waited for a second, insert the screwdriver again. Turn it until the contacts close. Then remove the wrench. You’ve now returned the calibration settings to the original setting of the manufacturer. 

Remember, different products have different calibration techniques, so consult your manual before doing anything.

5. Calibrate a mechanical thermometer equipped with a Mercury switch 

Ensure that the thermostat is completely level. You can use a standard spirit level to adjust the thermostat until it is level.  Sometimes it may be necessary to readjust the screws that are holding the mounting plate to the wall. When the mercury switch is level, the calibration should be correct again.

How to calibrate a digital thermostat

Let’s now look at how you calibrate a digital thermostat. Usually, you don’t need to calibrate a digital thermostat too often, but occasionally, your system might be a little off. Calibrating the thermostat can make it more accurate again. 

Most digital thermostats are calibrated at the manufacturer, and you don’t need to calibrate them again. But there can be circumstances that can cause the thermostat to lose its calibration. They can lose their calibration when there is, for instance, too much dust in the thermostat or when somebody or something bumps into it.

If you’ve done the test with a glass tube thermometer as we’ve explained earlier in this article and you’ve found different readings on the thermostat and the thermometer, you have to attend to the thermostat’s calibration. 

1. Clean the digital thermostat

The first step will be, as in the case of a mechanical thermostat, to clean the thermostat. This may solve the problem. 

Remove the cover of the thermostat and gently clean the inside by using a soft brush, cotton swab or compressed air on the contact points and all the internal parts. 

Replace the cover and check the calibration again with your thermometer. If the two readings are the same, you don’t have to do anything more – the calibration is correct again. 

2. Calibrating the digital thermostat

If the readings still differ you have to follow the appropriate steps to calibrate the thermostat. 

But you must remember to always keep in mind that every brand or model of a digital thermostat might have slightly different ways in which you have to calibrate it. The main difference is often the way how you access the calibration option on the digital keypad. Check your owner’s manual to find out how you have to reach the calibration adjustment option.

Step 1: Determine the temperature difference

To know to what extent the calibration should be adjusted, you have to calculate the difference in temperature readings between the thermostat and thermometer. You must determine the number of degrees that the two readings differ and whether the thermostat’s reading is higher or lower than the actual temperature. 

If, for instance, your thermostat indicates it’s 70 degrees Fahrenheit but your accurate thermometer indicates 73 degrees, the thermostat is reading 3 degrees lower than the actual temperature. 

Step 2: Access the calibration setting

You now have to enter the calibration mode on the thermostat. Follow the instructions in your thermostat’s manual to get to the calibration screen. (Remember, if you don’t have a printed manual, most manufacturers offer an online manual where you can check how to enter the calibration mode on your specific brand and model thermostat.)

For some Lux thermostat models, for instance, you have to turn the thermostat to “off” and hold the “up” and “down” buttons simultaneously until a number displays on the screen. 

On other brands and models, it can take a little bit longer to get to the appropriate screen. With some other Lux models, for instance, you have to press “menu” and then scroll to “set review swing value.” Then you press “ok,” and press “next” to skip swing and finally press “next” again to reach “adjust value cal.” 

If you can’t get to the calibration mode of your thermostat, you’ll have to contact your local electrician or HVAC technician.

Step 3: Adjust the calibration

After you’ve reached the calibration mode of your thermostat, press the “up” and “down” arrows to adjust the temperature, based on your earlier calculations. 

The thermostat can be adjusted to negative or positive numbers. To lower the temperature shown on the screen, you have to choose a negative number.  If, for example, you adjust it to -3, the thermostat will lower what the screen reads by 3 degrees. If it previously read 72 degrees, it will now read 69 degrees. 

If you’ve chosen a positive number such as +3, it would change the original display of 72 degrees to 75 degrees.

Step 4: Exit the calibration setting

Go back to the main screen on your digital thermostat. Usually, you’ll find an “exit” or “back” button that will return you to the main screen. 

Step 5: Recheck the calibration

Use the same thermometer you used previously for the original comparison test and check whether the calibration has worked. Wait at least 15 minutes and see if the thermostat and thermometer have the same readings. 

If you are still receiving two different readings, continue adjusting the calibration until you’re satisfied with the result. If you’ve followed all the steps and you still can’t calibrate the thermostat correctly, contact your local electrician or HVAC technician. 


It is easy and quick to check whether your HVAC thermostat is still calibrated correctly and it is also relatively easy to rectify it. As an inaccurate calibrated thermostat can affect your energy bill negatively, it is good practice to check the accuracy at least once a year. 

The steps to calibrate your thermostat is straight forward and the whole procedure can be done by yourself. If, however, you cannot get the thermostat accurate again, there may be a problem with the thermostat and you might even have to replace it.  


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