The thermostat setting is quite simple: Set a temperature, and AC or furnace will reach the set thermostat. That’s the theory at least. In practice, we experience a problem of the thermostat not reaching the set temperature quite often.

Here are just a few ways how homeowners express this temperature setting problem:

“Why does my thermostat not match my home’s temperature?”
Why is digital thermostat reading higher than setting?” This is primarily a problem with the AC thermostat setting (cooling).
“AC not reaching the set temperature. What’s the issue here?”
“Furnace is not racing the set temperature. Am I missing something?” This is primarily a problem with the furnace thermostat setting (heating).

To illustrate practically what is happening, here is what many folks are experiencing in the summer and in the winter.

When working as it should, the indoor temperature should be the same as the temperature set on the thermostat. If this is not the case, you do have a problem.

Thermostat not reaching temperature set temperature in the summer (AC and cooling problems):

Start indoor temperature is 80°F.
You set an air conditioner’s thermostat to 75°F.
After a few hours, the indoor temperature as measured by a thermometer is 78°F.

Thermostat not reaching temperature set temperature in the winter (furnace and heating problems):

Start indoor temperature is 65°F.
You set an air conditioner’s thermostat to 72°F.
After a few hours, the indoor temperature as measured by a thermometer is 68°F.

Needless to say, if your thermostat setting does match your home’s temperature, you do have a problem.

 

We are going to look at 7 common reasons why your thermostat is not reaching the set temperature. Most of these culprits for thermostat set temperature not reached are valid for both air conditioners and furnaces (cooling and heating).

These imbalances can only have 2 possible roots, namely:

Thermostat issues. This includes wiring issues, for example.
AC or furnace issues. Namely, something is wrong with your HVAC unit itself.

If you are experiencing furnace or AC not reaching the set temperature, you should just go one by one through these causes, eliminating them as you go.

When you hit the real reason why the set temperature is not adequately reached, you can check what you can do to fix these problems. Once you fix the problem, the thermostat’s set temperature should match the real temperature in your home.

Of course, some of these causes are harder to fix than others. Having a smaller than needed AC unit is harder to fix than recalibrating the thermostat, for example.

Let’s look at what might be preventing your thermostat from reaching the set temperature:

Why Does My Thermostat Setting Not Match My Home’s Temperature?

As we have mentioned above, if the thermostat is not reaching the set temperature, the root cause is in either:

Thermostat.
AC or furnace.

Example: Let’s say that a thermostat reads higher than the actual temperature in the summer. That means that AC is not reaching the set temperature. This problem is caused either by the thermostat or by the air conditioning unit. In the summer, if the furnace is not reaching the set temperature, the cause will either be the thermostat or the furnace.

We are going to list all the culprits here, starting with the faulty thermostat and then checking if the AC or furnace might be causing this temperature difference:

1. Thermometer Is Not Calibrated Correctly (Just Recalibrate It)

All thermostats need to be calibrated. In many situations where we see a temperature deviation between the set temperature and home temperature, the culprit is an incorrectly calibrated thermometer.  Example of a thermometer not being correctly calibrated: Let’s say you set a thermostat to 74°F. However, when you measure the indoor temperature, the thermometer reads 78°F. Chances are that the thermostat considers this 78°F temperature to be 74°F. This is a case of the incorrectly calibrated thermometer with a 4°F deviation.

Cases of incorrect thermostat calibration predominantly happen if:

You have a new thermostat. New thermostats have to be properly calibrated. If they are not, they might be causing this temperature inequality problem.
You have just experienced a power outage. After a power outage, the thermometer might go back to the default calibration.
You have a digital thermostat. If a digital thermostat is reading a higher temperature than the set temperature, the fault might be in the setting of the digital thermostat.

If the temperature is set to 74°F but the thermometer shows 78°F, you will have to recalibrate the thermostat to account for this 4-degree deviation.  Even with older AC units or furnaces, you might have to recalibrate your thermostat from time to time.

If the temperature is set to 74°F but the thermometer shows 78°F, you will have to recalibrate the thermostat to account for this 4-degree deviation.

Solution: Recalibrate the thermostat to measure the correct temperature.

Here is a step-by-step way how to properly recalibrate the thermostat:

Use a thermometer to measure the room temperature near the thermostat.

Note the temperature difference between the thermometer and the thermostat. Example: If the thermometer shows 80°F and the thermostat setting is 76°F, you have a 4-degree anomaly.
Recalibrate the thermostat by offsetting the thermostat by the measured temperature difference. Here is a good resource about how to calibrate a programmable thermostat, for instance.

After the thermostat is adequately recalibrated, you should see that the home temperature matches the thermostat setting exactly or to 1 or 2 degrees deviance.  If that is not what the preventing the thermostat from reaching the set temperature, go further along this list:

2. Dirty Thermostat Heating Up (Leading To Incorrect Temperature Reading)

The thermostat not reaching the set temperature might be explained as easily as this:

There is dirt accumulated within the thermostat box.

Like any other thing in your house, the thermostat can accumulate hair, fur, dust, and so on. A yearly HVAC maintenance always includes cleaning the internal parts of the thermostat for a good reason.  If your thermostat is dirty (you find dirt coating on the wires and sensors), it may heat up just a bit too much (excess heat), leading to incorrect temperature measurements. The end result is that the thermostat setting does match the home’s temperature.

Solution: Clean the thermostat. This is one of the easiest fixes. Take off the thermostat faceplate and use a toothbrush to clean the thermostat. You will have to use it very gently, especially on the sensors.  After the thermostat is clean, the thermostat sensors should read the correct temperature again.

3. Thermostat Is Old, Broken, Or Misaligned

Every new thermostat should match the home’s temperature exactly to about 1 or 2 degrees. However, with the older thermostat misreading the correct temperature, we have 3 probable reasons why the thermostat set temperature is not being met:

Thermostat is just old. Years of even mild wear-and-tear can damage the thermostat – including the wires and sensors – and the thermostat doesn’t work as well as it used to.
Thermostat is broken. Individual components in the thermostat can break down over time. This will lead to the thermostat set temperature being met.
Thermostat is misaligned. Every thermostat should be attached close to the wall in a perfectly horizontal position. Over years, the thermostat might detach from the wall or we will see that the left or right side of the thermostat is sinking a bit, ruining the perfect horizontal position.

Thermostat should be aligned horizontally and pressed neatly against the wall.

All of these situations can lead to the thermostat having issues. Namely, it will not meet the set temperature as reliable as when it was new.

Solution: Old thermostat has to be replaced. A broken thermostat can be fixed (quite difficult) or, more often, it should be replaced. You can, however, realign the thermostat yourself.  For thermostat replacement, you will need an HVAC professional to help you out. He or she will also diagnose if indeed the culprit for a thermostat not meeting the set temperature is a broken or old thermostat.

The easy part you can DIY here is to realign the thermostat and hope that will fix the temperature anomaly issue.

4. Incorrect Or Loose Thermostat Wiring Causing A Higher Or Lower Than Thermostat Temperature

Needless to say, a correctly working thermostat has to be correctly wired and the wires have to be attached correctly. You can learn exactly how to wire any thermostat here.

If a newly wired thermostat does not meet the set temperature, it is likely not wired correctly. The incorrectly wired thermostat will, of course, lead to temperature mismatching problems.

Example of a correctly wired 5-wire thermostat.

In both new and older thermostats, the wires that need to be fixed in the thermostat sockets can become loose. In the new thermostat, they have not been fixed correctly. In older thermostats, the wire fixes just got loose over time.

Solution: Check if the thermostat is wired correctly. You can use the article cited above to check if every wire is in its correct socket.  Also, check if all the wires are fixed in their sockets. If you see a wire that is now wholly fixed in the socket (the end of the wire should be entirely in the socket), unscrew the socket, reapply the wire, and screw the socket back.  If this was the issue causing a digital thermostat reading higher than the setting or lower than the setting, the problem will now be fixed and you should see 75°F temperature on the thermostat as well as on the thermometer again, for example.

5. New Thermostat Is Exposed To The Sun, Heating Source, Etc.

Where you position the thermostat in your house is important. For example, if it is exposed to the sun or directly above a heating source, the localized temperature surrounding the thermostat will be higher than in other parts of the now.  Now, it’s key to understand that higher or lower temperatures are accounted for by the calibration process. What is really problematic – and can lead to the thermostat not reading the correct temperature – is temperature variations.

Example: Let’s say the thermostat is located on the wall that is exposed to the sun. You calibrate it in cloudy weather. When you want to use AC because the sun is shining, the AC will not reach the set temperature. That’s because the sun heating up the thermostat messes it up.

In order to avoid this problem, you should know where it is best to place your thermostat. Keep it away from all heating sources (fireplaces, space heaters, badly ventilated spaces) and places where there is a lot of natural temperature variation (near a window, in the kitchen, next to a door, and so on).

Solution: If you already have an installed thermostat in a place with highly variable temperatures, you do have a problem. That’s because relocating a thermostat is painfully difficult. All those wires have to be relocated as well. You will most definitely need professional help for this.

The most optimum option in most cases is to keep the thermostat where it is. This sounds wrong, right? Well, because repositioning a thermostat is difficult (and costly), most homeowners can deal with lowering the thermostat temperature setting in the summer (Example: Set thermostat to 70°F if you want 74°F ) and increasing it in the winter (Example: Set thermostat to 76°F if you want 72°F).

This is not ideal but once you get a feeling about how faulty your thermostat is, you will get a grip on how to control the indoor temperature even with an incorrectly placed thermostat.

Now, these issues were all connected with the thermostat. Let’s look at how a faulty AC or furnace can cause the set temperature to not be met:

6. AC Or Furnace Is Too Small (Undersized HVAC System)

To adequately reach the temperature set temperature, you need an adequately sized HVAC unit. This goes without saying. In many cases involving the set temperature from being met, the problem is an undersized HVAC unit.

If you have an air conditioner or furnace that is just too small for your home, it will run at 100% output and still won’t be able to produce enough cooling or heating output to meet the set temperature.

Example: You have a house that requires a 3-ton AC unit but you have installed a 2-ton AC unit. You set the temperature to 72°F. Obviously, an undersized AC unit will not be able to cool your house down to 72°F. You might set the thermostat to 72°F but the temperature measured by a thermometer will be something like 76°F.  The same thing goes for furnaces. If you require a 4-ton furnace but your furnace is capable of only producing 2.5-ton of heating output, it just won’t be enough for the furnace to heat your home up to the set temperature.

For reference, if you indeed have a too-small HVAC unit, you can check what size air conditioner you need here and what size furnace you need here. If the tonnage you get with these two calculators deviates a lot from the HVAC unit tonnage you have installed, the thermostat temperature setting is likely not being met due to an undersized HVAC unit.

Solution: ‘Just get a bigger AC or furnace’ is easy to say but it is a disaster in practice. You can’t just upgrade a 2-ton unit to a 3-ton unit, for example.

What you can do, however, is to:

Keep the existing HVAC unit.
Get another (smaller) HVAC unit.

For example, if you need a 3-ton AC but you have a 2-ton AC, you can always buy another smaller 1-ton AC (12,000 BTU). You can choose between a mini split, window unit, and portable air conditioner.  The same goes for a furnace. If you have a smaller than needed furnace, just get 1 or a couple of space heaters. Each 1,500W space heater produces about 5,200 BTU of heating output. These will produce the supplemental heating your undersized furnace needs in order to meet the thermostat temperature setting.

7. HVAC Unit Issues (Dirty Filters, Ducts Issues, Frozen AC, Etc.)

If you have checked all of these issues that are known to cause a thermostat not to reach the temperature, and you haven’t found a culprit, the only other thing that can be causing this problem is a problem with an HVAC unit.  If your adequately-sized air conditioner or furnace cannot reach the set temperature, there is an issue with performance. In practice, the HVAC unit is not capable of achieving its 100% specified output (either cooling or heating).

There is a myriad of problems that can cause this, including:

Dirty air filters. If the filters are dirty, they restrict the airflow. Less airflow equals the lower-than-specified output of an HVAC unit.
Airflow restriction caused by the ducts. Another reason why airflow might be too low are ducts that are too small or leaking (they have a hole).

Bad capacitors. This will lower or even completely diminish the capability of the HVAC unit to reach the temperature set by the thermostat.
Bad fans. If the AC fan is not spinning as it should, for example, it will produce less airflow and therefore less heating/cooling output.
AC is frozen.
Cracked furnace exchanger. You can check for the symptoms of a cracked heat exchanger in your furnace here.

These are just some problems that your HVAC unit might have, preventing it from meeting the set temperature.