5 min read • Updated October 2026 • By the CliQ Team — the team that created Blink
How do you change the battery in a Honeywell thermostat? Set the System switch to Off, then either slide out the battery holder (FocusPRO models) or gently pull the thermostat straight off its wall plate (T4 Pro and RTH models). Swap in two fresh alkaline batteries, AA or AAA depending on the model, and press it back on. On the T4 Pro and RTH models, your schedule stays saved.
Honeywell has made dozens of models, but the battery job usually comes in one of three versions. Below: the steps, how to tell which version you have, what "BATT" and "Lo Batt" mean, and what to do if it still won't run.
How to Change the Battery: 5 Steps
You need two new alkaline batteries. Check the table below for AA or AAA. No tools.
- Set the System switch to Off. Honeywell's RTH manual is blunt about why: otherwise the heating or cooling could stay on even after the batteries are out.
- Get to the batteries. On a FocusPRO, press and pull the battery holder out of the side of the thermostat. It stays on the wall. On a T4 Pro or RTH model, gently pull the thermostat straight off its wall plate. RTH models release from the bottom.
- Take out the old batteries. Lift each one out from the + end first.
- Put in two fresh alkaline batteries. Match the + and – marks in the holder. Seat the – end first, press down, then push the + end into place.
- Put it back. Line up the pins on the back of the thermostat with the wall plate and push it straight on until it seats. Or slide the battery holder back in. Set the System switch back to Heat or Cool.
The video below walks through a T4 Pro, which is the version that comes off the wall.
Video by Todd's Garage via YouTube, August 2021. Not affiliated with CliQ.
His thermostat showed "BATT," so he pulls it straight off the wall. He does it slowly for a reason: the back of the thermostat has a row of pins that bend easily. The trick for the batteries is the order. Out from the + end, in from the – end. Going back on, he lines the pins up with the wall plate first, then pushes straight in. No angle, no force.
Which Honeywell Do You Have?
The fastest tell is the warning on the screen. Each of the three families says it differently.
| Family (example model) | Low-battery warning | Batteries | How to reach them |
|---|---|---|---|
| T4 Pro (TH4110U) | "BATT" with an alert icon | 2 AA alkaline | Press gently to loosen it, then pull it straight off the wall mount. Batteries are on the back |
| RTH basic programmable (RTH2300, RTH221) | "Lo Batt" flashing | 2 AAA alkaline | Pull at the bottom to remove it from the wall plate. Batteries are on the back |
| FocusPRO (TH6220D) | "Replace Battery" | 2 AA alkaline | Press and pull the battery holder out of the side. The thermostat stays on the wall |
Still not sure? The model number is printed in the manual and on the thermostat itself. Search "Honeywell [model number] manual" for your exact steps. And if your thermostat is wired to the furnace for power, the batteries may only be a backup, or it may have none. The T4 Pro manual says batteries are optional on units wired that way.
What BATT, Lo Batt, and Replace Battery Mean
Honeywell gives you plenty of notice. The T4 Pro shows its warning about two months before the batteries run out. RTH models show "Lo Batt" for about 60 days.
On the T4 Pro, the warning gets louder as the batteries fade:
- BATT plus the alert icon: replace the batteries.
- Backlight stops working: the thermostat has turned it off to save power.
- Only BATT and the icons on screen: the batteries are critically low. Change them now.
Don't wait for the warning. Honeywell's T4 Pro and RTH manuals both say to replace the batteries once a year, and before you leave home for more than a month. A thermostat with dead batteries can't call for heat.
Changed the Battery and It's Still Not Working?
The screen is blank
Honeywell's T4 Pro troubleshooting list covers this: make sure fresh AA alkaline batteries are installed the right way, the power switch for the heating and cooling system is on, and the furnace door is closed securely. Many furnaces cut power when that door is loose. If the furnace still won't start, here's how to rule out the thermostat.
"Cool On" or "Heat On" is flashing
That's usually not a problem. It's built-in compressor protection: the thermostat makes the compressor wait a few minutes before it restarts, because restarting too soon can damage it. Switching the system off and back on for a battery swap can start that wait. The message flashes until the wait is over, then the AC or heat pump turns on. Give it a few minutes.
The time is wrong
Your schedule is safe. The T4 Pro and RTH manuals say settings live in permanent memory that doesn't need batteries. Only the clock and day can reset, and on the T4 Pro, only if the batteries were out for more than two minutes. Set the time and day again. If the schedule itself seems off, our guide to programming any thermostat walks through it.
Done with the old batteries? The EPA says that in most communities, alkaline batteries can go in the household trash. Its recommendation is to recycle them or check with your local solid waste authority.
Batteries weren't the problem? Our Honeywell troubleshooting guide works through every common symptom, from a blank screen to settings that won't stick.
Thinking about replacing your thermostat instead?
See the CliQ Smart Thermostat →Frequently Asked Questions
What batteries does a Honeywell thermostat take?
It depends on the model. The T4 Pro and FocusPRO take two AA alkaline batteries. The RTH2300 and RTH221 basic programmable models take two AAA alkaline batteries. Honeywell recommends alkaline.
What does BATT mean on a Honeywell thermostat?
The batteries need replacing. On the T4 Pro, BATT appears with an alert icon about two months before the batteries run out. RTH models say "Lo Batt" instead, and FocusPRO models say "Replace Battery."
How do I get a Honeywell thermostat off the wall?
Pull it gently and straight toward you. On the T4 Pro, press gently to loosen it first. On RTH models, pull from the bottom. Don't twist or pry: the pins on the back bend easily. FocusPRO models don't need to come off, since the battery holder slides out of the side.
How often should I change the batteries in my Honeywell thermostat?
Once a year, according to Honeywell's T4 Pro and RTH manuals, and before you leave home for more than a month. Replace them right away when a low-battery warning appears.
Will I lose my schedule when I change the batteries?
No, on the T4 Pro and RTH models. Honeywell stores their settings in permanent memory that doesn't need battery power. You may need to reset the time and day. On the T4 Pro, even that's kept if the new batteries go in within two minutes.
Why is "Cool On" flashing after I changed the batteries?
It's compressor protection. The thermostat makes the air conditioner or heat pump wait a few minutes before restarting, because restarting too soon can damage the compressor. "Cool On" or "Heat On" flashes during the wait and stops on its own when the system turns on.
Sources
Resideo — T4 Pro Programmable Thermostat TH4110U2005, TH4210U2002 User Guide (33-00188EFS-11, 2022) — PDF
Resideo — Programmable RTH2300/RTH221 Thermostat Series Owner's Manual — PDF
Honeywell — FocusPRO TH6220D Programmable Thermostat Installation Guide — PDF
US Environmental Protection Agency — Used Household Batteries — epa.gov
Photo by Landiva Weber via Pexels
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