Thermostat Not Working After Changing Batteries? (Fix It Now)

Dennis Howard
by Dennis Howard

Thermostats have come a long way. They have gone from the dark ages of manual cranking all the way to smart technology that sometimes even comes complete with voice recognition. Modern thermostats make controlling your home’s HVAC system seamless, which is why it can be particularly frustrating when your thermostat stops working when you change the batteries.

If your thermostat is not working after you change your batteries you most likely installed the wrong type of batteries or installed the batteries incorrectly. Consult your thermostat manual to make sure you have the right kind of batteries. If the thermostat still does not work try to reset the system, and then check the thermostat itself for any damage or visible wire issues.

While there are many reasons your thermostat has stopped working after you changed the batteries, it is best to start with the most likely solutions. This means before you open the thermostat up and tear it apart, you should first look at the batteries.

Did You Install The Batteries Correctly?

While it may seem impossible, one of the most common reasons a thermostat stops working after you change the batteries is because the batteries were incorrectly installed. In order to rule out battery installation as the cause for your thermostat not working, follow the steps below.

  • Open the thermostat in order to access the battery chamber 
  • Locate the battery indication instructions on the thermostat. A plus and minus sign in the battery tray or on the cover will indicate the proper position for the battery or batteries.
  • Make sure your batteries are in the same position as the diagram on the thermostat.
  • Ensure the batteries are fully in place and not askew or out of line.

Make Sure You Used The Correct Type Of Batteries

Another common mistake is purchasing the exact correct type of batteries. While it might seem like the battery you purchased is the same size as the one that was in the thermostat, it does not mean it is the right type.

There are many types of batteries, and thermostats (especially modern smart thermostats) require a very specific type of battery. Consult your user manual in order to find out the type of battery your specific thermostat takes.

The Batteries Are in Correctly But Still No Display?

Some thermostats require the body of the thermostat to be removed from the base to change the batteries. When you remove the case, it is quite easy to loosen to dislodge one of the small wires. These wires connect to the HVAC unit allowing the thermostat to communicate with the HVAC unit. These steps should help you diagnosis a connection problem.

Use Reset Button Or Factory Reset

Some electronic thermostats have a reset button. The best place to find information about resetting your thermostat is in the user manual. Generally, the reset button is small in a recessed housing. Follow the instructions in the user manual to perform a reset on your thermostat.

If you cannot locate the reset button, you can also try to perform a factory reset. In order to perform a factory reset, you need to ensure the thermostat is disconnected from all sources of power. This means the batteries must be removed, and any it must be disconnected from any other power source. Then, wait 5 seconds to upwards of one minute, and then reinstall the power source. This will drain the thermostat of any existing power and reset it to its factory defaults.

Check All The Wires

You should have four or five wires connected to your thermostat. The wires are color-coded.

  • Blue
  • Yellow
  • Red
  • Green
  • White

The Blue Wire – Common Connection

Blue wires are the common wire, typically identified by a C on the transformer connection. Any AC system requires two wires to operate properly. If your thermostat doesn’t have a blue wire, it usually means that the thermostat doesn’t need power from the HVAC system. When this is the case, your thermostat may need replacing.

If this wire is connected and not broken, you may need to check the 24-volt transformer in your HVAC system. You can check for voltage from your HVAC system transformer with a multimeter

The Red Wires – The Heart of the Transformer

The red wires may supply the power to operate your thermostat. This power is typically 24-volt AC power coming from a transformer in either the heater or air conditioning system. If the red wire is broken or loose on the thermostat connection, the thermostat will not work.

Check the Power of The Red Wire

You can check for the correct power from the transformer using a multimeter.

Step 1 – Set your Multimeter Correctly

Set your multimeter to read AC current. If you aren’t sure how to do this, read the manual that came with your multimeter.

Step 2 – Attach the Multimeter leads

The leads on your multimeter may be color-coded. It doesn’t matter which lead you place on which terminal. Attach the multimeter lead to the red wire on the thermostat. Place the other lead on the blue wire. Your meter should read around 24-volts.

If there is no reading, or if the voltage difference is considerable, you may need to check the output directly from the transformer on the HVAC system. Checking the transformer requires removing the cover from the unit and poking around inside. We recommend the services of a qualified HVAC technician at this point.

Assess the Base Plate

Most new electronic thermostats have a base plate to which the wires are attached. The thermostat itself snaps onto the base plate and a small multi-plug engages the baseplate. There is a possibility that the plug is not engaging or is damaged.

Check the back of the thermostat and the base plate. Make sure that none of the small pins on the plug are bent or missing. Take care when reinstalling the thermostat to the base plate. It is easy to get the small pins on the plug misaligned and damage them.

If the pins or the plug are damaged, you must replace the entire thermostat. It is almost impossible to repair this problem.

Time for A New Thermostat?

If there is power to the thermostat and it still doesn’t work after a reset, you may have a bad thermostat. While not common, this can happen. Fortunately, new electronic thermostats for most HVAC systems are moderately priced. A quick trip to the home improvement store will soon have your HVAC up and running again.

Choosing a Replacement Thermostat

If you determine that your thermostat needs replacing, there are some things to consider as you choose your new thermostat. The type of heating and cooling system you have is a major factor. The type of thermostat you want is also important to consider.

Types of Thermostats

Thermostats come in three varieties

  • Manual
  • Programmable
  • Smart

Most major differences with each thermostat are found in their specific features and capabilities.

The Manual Thermostat

The cheapest of all the options, the manual thermostat is just what it says. Manual thermostats are also known as “mechanical” or “non-programmable” thermostats. Once installed, you set the temperature at which you want the heat or air-conditioning system to keep you home.

Although these thermostats may be called mechanical, most of them are electronic. These new manual thermostats feature digital display but don’t have any memories or pre-set functions.

Programmable Thermostats

As the name suggests, programmable thermostats allow programming for different functions for more economical operation of your HVAC system. Some programmable thermostats allow you to program individual routines by day or week.

Some of these thermostats may be labeled “set-back” thermostats. The programming functions allow you to tell the heater to raise or lower the temperature when you are at work. Conversely, the program can bring the temperature back to the normal settings a few minutes before your return home.

Smart Thermostats – Thinking Machines

Think of a programmable thermostat with a mind of its own. No, it won’t take over your home, but as you set and reset the thermostat, some of these smart thermostats will begin to recognize patterns in your settings and automatically follow that pattern.

Most of these smart thermostats also connect to your wi-fi. Using the connectivity feature, you can control the thermostat from your smartphone using an app. Remotely accessing your thermostat allows you to make changes to the thermostat remotely if the need arises.

Is the Thermostat Compatible?

HVAC systems have different requirements for thermostats and not all thermostats with work with all HVAC systems. There are several things you need to check to make sure the thermostat you want is compatible with your HVAC system.

  • Does the thermostat work with the voltage your HVAC system delivers?
  • Is your HVAC system using a heat pump?
  • How many stages does your HVAC system have?
  • Is your HVAC system zoned? Do you need more than one new thermostat?

Summing Up Your Thermostat Issue

When you try to replace the batteries in your thermostat but your thermostat does not work, the first place you look is the batteries themselves. First make sure you have installed the battery or batteries correctly. Next, ensure you have purchased the exactly correct type of battery for your thermostat.

If this is not the issue, try to reset the thermostat, and then check the thermostat itself. If nothing seems to work, it may be time to upgrade to a new and improved thermostat.

Dennis Howard
Dennis Howard

Dennis is a retired firefighter with an extensive background in construction, home improvement, and remodeling. He worked in the trades part-time while serving as an active firefighter. On his retirement, he started a remodeling and home repair business, which he ran for several years.

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