Open the thermostat cover and note which battery type it uses
Most Honeywell thermostats run on two AA or AAA batteries, though some models use a single 9-volt battery. The fastest way to find out is to open the cover — usually by pressing a small tab on the bottom or side, or by gently prying with a flat tool — and look at what is currently installed. The battery compartment is almost always on the back of the unit, behind the faceplate.
If you cannot see the batteries clearly, check the model number on the back of the thermostat and search for the manual online. Honeywell publishes PDFs for most models on their support site. The manual will tell you the exact battery type and how to access the compartment.
Write down the battery type before you close the cover again. This saves a trip back to the store if you grab the wrong size.
Key Takeaways
- Most Honeywell thermostats use two AA or AAA batteries, which you can replace in under two minutes without tools.
- Open the cover by pressing the tab on the bottom or side, or by gently prying with a flat screwdriver.
- Remove the old batteries and note their orientation — the positive end (+) usually faces up or toward the spring.
- Insert new batteries in the same direction, close the cover, and check that the display lights up and shows the correct time.
- If the thermostat does not respond after a battery change, the unit may have lost its settings and may need to be reprogrammed.
Remove the old batteries and note their direction
Once the cover is open, gently pull or slide the old batteries out of their slots. Most compartments have a small spring at one end to help eject the battery — push down on the battery if it does not slide out easily.
Before you insert new ones, look at how the old batteries were oriented. In most Honeywell thermostats, the positive end (marked with a +) faces toward the spring or toward the top of the compartment. Take a photo with your phone if you are unsure, so you can match it exactly.
Discard the old batteries according to your local rules. Many retailers, including Best Buy and Home Depot, accept used batteries for free recycling.
Insert new batteries in the same direction
Slide or press the new batteries into the compartment, matching the direction of the old ones. The positive end should face the same way it did before. You will feel a slight click or resistance as the battery seats into place — do not force it.
If you are using AA batteries and the compartment seems loose, the batteries may be slightly worn or the contacts may be dirty. Wipe the metal contacts inside the compartment with a dry cloth before inserting new batteries. This improves the connection and prevents the thermostat from losing power.
Close the cover and verify the display
Press or snap the cover back into place. The thermostat should power on when ready, and the display should light up. Check that the time is still correct — if it is not, the unit may have lost its settings during the battery change.
If the display does not light up, open the cover again and make sure the batteries are seated all the way in and facing the correct direction. If it still does not respond, try a different brand of battery. Some thermostats are sensitive to battery quality, and a fresh battery from a different manufacturer sometimes works when a cheaper brand does not.
Reset the thermostat if it lost its settings
If the time is wrong or the thermostat is not responding to your adjustments, it has lost its programming. Look for a small reset button on the back or inside the battery compartment — it is usually labeled "Reset" or "Clear" and requires a paperclip or pen to press.
Hold the reset button for three to five seconds. The display may go blank for a moment, then return to a default state. You will need to reprogram the time, day, and any schedules you had set. Refer to the manual for the exact steps, as the process varies by model.
If there is no reset button, turn off the thermostat at your breaker or furnace switch, wait 30 seconds, and turn it back on. This sometimes restores the unit without erasing your settings, though it depends on whether your thermostat has a backup power source.
When to replace batteries before they die
Honeywell thermostats usually show a low-battery warning on the display — often a small battery icon or the words "Low Batt" — before the batteries are completely dead. When you see this warning, you have a few days to replace them before the thermostat loses power.
If you wait until the batteries are completely dead, the thermostat will stop responding and your heating or cooling will not work until you replace them. In winter or summer, this can be uncomfortable. Replace batteries as soon as you see the warning.
Mark your calendar to check batteries every 12 months, even if no warning appears. Most AA and AAA batteries last one to two years in a thermostat, depending on the model and how often the thermostat switches the heating or cooling on and off.
Frequently Asked Questions
Do I need to turn off the thermostat before changing the battery?
No. Honeywell thermostats are designed to keep running on battery power alone, and changing the batteries while the unit is on will not damage it. You may lose your time and schedule settings if the power drops between removing the old battery and inserting the new one, so work quickly.
What if my thermostat has a 9-volt battery instead of AA?
The process is the same: open the cover, note the orientation, remove the old battery, and insert a new one facing the same direction. 9-volt batteries have a single connector on top, so there is only one way they fit. Make sure the connector is fully seated before closing the cover.
Can I use rechargeable batteries in my Honeywell thermostat?
Rechargeable batteries work but are not ideal. They hold less charge than alkaline batteries and may not last as long between charges. If you use them, check the thermostat more often for the low-battery warning, and keep fresh alkaline batteries on hand as a backup.
Why does my thermostat keep losing time after I change the battery?
The thermostat may have a weak internal backup battery or capacitor that is failing. If the time resets every time you change the main batteries, the unit may be nearing the end of its life. Contact Honeywell support with your model number to see if a repair or replacement is available.
Is it normal for the thermostat to beep when I change the battery?
Yes. Many Honeywell models beep when power is interrupted or restored. This is normal and does not indicate a problem. The beep should stop after a few seconds once the new batteries are fully seated.