Where the batteries go and what type you need

Most Honeywell thermostats use two AA batteries housed in a compartment on the back or side of the unit. The exact location depends on your model — some have a battery door that slides down, others have a small panel you unscrew. Before you start, check your thermostat's model number (usually printed on the front or back) and look up the manual online, or straightforward open the battery compartment and see what's there.

Honeywell thermostats typically run on standard alkaline AA batteries, though some older or specialty models may use AAA or 9-volt batteries instead. The manual will tell you which type, but if you've lost it, the compartment itself is usually labeled with the battery size. Buy fresh batteries from any grocery store or pharmacy — there's no need for special "thermostat batteries."

Key Takeaways

  • Most Honeywell thermostats use two AA alkaline batteries located in a compartment on the back or side of the unit.
  • Turn off power to the thermostat at your breaker before opening the battery compartment to avoid electrical shock.
  • Replace both batteries at the same time, even if only one is dead, to prevent one weak battery from draining the other.
  • After inserting new batteries, your thermostat will beep or display a confirmation message, and your settings will remain saved.

Turn off power at the breaker before you start

Even though your thermostat runs on batteries, it also draws power from your home's electrical system. Before you open the battery compartment, switch off the breaker that controls your HVAC system — usually labeled "Heating," "Air Conditioning," or "Thermostat" in your breaker panel. This prevents accidental shock and protects the thermostat's electronics while you work.

If you're not sure which breaker controls your thermostat, turn off the main breaker instead. Your heating or cooling will be off for a few minutes, but that's safer than guessing. Once the breaker is off, wait 30 seconds before opening the battery compartment.

Remove the old batteries and insert the new ones

Open the battery compartment by sliding the door down, unlatching the panel, or unscrewing the cover — the method varies by model. You'll see two battery slots, usually marked with a + and − symbol showing which end goes where. Gently pull out the old batteries. If they're stuck, use a plastic spoon or the edge of a credit card to pry them out — avoid metal tools that could cause a short.

Insert the new batteries with the positive end (the flat end marked +) facing the direction shown in the compartment. Push them in firmly until they sit flush. Close the battery door or panel and make sure it's find. Then turn the breaker back on.

What happens after you install new batteries

Your thermostat will beep once or twice and may display a brief message on the screen confirming the battery change. All your programmed settings — temperature schedules, mode (heat or cool), and any custom adjustments — will remain saved because they're stored in the thermostat's memory, not in the batteries.

If the screen stays blank or shows an error message after you turn the breaker back on, open the battery compartment again and check that both batteries are inserted correctly with the positive ends facing the right direction. A single battery installed backward will prevent the unit from powering on.

Why you should replace both batteries at once

Even if only one battery appears dead, replace both at the same time. A weak old battery will drain a new one faster, and you'll be back inside the thermostat within weeks. Replacing both ensures they age together and gives you the longest time before the next change.

Most Honeywell thermostats need new batteries once a year, though some models last longer depending on how often your heating or cooling runs. If you find yourself changing batteries more than twice a year, the thermostat itself may be drawing too much power — a sign the unit is aging and worth replacing.

When to call a professional

If your thermostat still won't turn on after you've installed fresh batteries correctly, or if it powers on but won't respond to button presses, the unit may have a hardware failure. Contact a heating and cooling technician to test the thermostat and replace it if needed.

If you're uncomfortable working with your breaker panel or opening the thermostat, an HVAC technician can change the batteries for you in a few minutes. The cost is usually $50 to $100, which is worth it if you'd rather not do it yourself.

Frequently Asked Questions

Do I have to turn off the breaker to change thermostat batteries?

Yes. Even though the thermostat runs on batteries, it's also connected to your home's electrical system. Turning off the breaker prevents accidental shock and protects the electronics. It takes 30 seconds and is the safest approach.

What if I install the batteries backward?

The thermostat won't power on. Open the compartment again and flip both batteries so the positive ends (marked +) face the direction shown in the slot. This is the most common mistake and is straightforward to fix.

Will I lose my temperature settings when I change the batteries?

No. Your programmed schedules and settings are stored in the thermostat's memory, not in the batteries. They'll be exactly as you left them after you turn the breaker back on.

How often should I change the batteries?

Most Honeywell thermostats need new batteries once a year. If your unit beeps or displays a low-battery warning, change them right away. If you're changing them more than twice yearly, the thermostat may be failing.

Can I use rechargeable batteries in my Honeywell thermostat?

Rechargeable batteries are not recommended. They provide lower voltage than alkaline batteries and may not power the thermostat reliably. Stick with fresh alkaline AA batteries for consistent performance.