Most Honeywell thermostats that use batteries take two fresh AA or AAA alkaline batteries, and you change them by setting the system to Off, pulling the thermostat off its wallplate (or sliding out the battery holder), and swapping in new cells. Which Honeywell thermostat battery you need depends on the model: T6 Pro, T4 Pro and FocusPRO models use AA, while RTH2300, RTH221 and PRO 1000 models use AAA. Many Wi-Fi models, like the T5, T6 Pro Smart and T9, have no batteries at all and run on a C wire.
Quick facts
| Task | Replace the batteries in a Honeywell Home thermostat or its wireless room sensor |
|---|---|
| Appliance | Honeywell Home battery and dual-powered thermostats (T6 Pro, T4 Pro, FocusPRO 6000, RTH2300, RTH221, RTH8500, PRO 1000, Lyric Round) and T9 room sensors |
| Time needed | 5 minutes |
| Difficulty | Easy |
| Tools needed | Fresh batteries of the type your model lists (usually AA or AAA alkaline); no tools for most models |
| Most likely fix | Fresh alkaline batteries put in the right way around clear a blank screen or low-battery warning |
Honeywell thermostats (sold under the Honeywell Home brand, made by Resideo) use a few different battery sizes. This guide uses each model’s official manual to show which battery yours takes, where it goes, and how to change it without losing your settings. If your screen is blank or frozen even with new batteries, see our Honeywell thermostat reset guide for every model.
What battery does a Honeywell thermostat take?
It depends on the model, so check the model number on the back of the thermostat first. The table below lists what Honeywell’s manuals say for common models. Honeywell’s manuals call for alkaline batteries on every model below except the Lyric Round, which uses a lithium AAA.
| Model family | Battery | Are batteries required? | Low-battery warning |
|---|---|---|---|
| T6 Pro (TH6210U, TH6220U, TH6320U) | 2 AA alkaline (included) | Optional backup if wired to a C wire; required if not | Low-battery icon about two months before the batteries run out; BATT shows when they need changing |
| T4 Pro (TH4110U, TH4210U) | 2 AA alkaline (included) | Can run on AA batteries, a C wire, or both | Not described in the installation guide |
| FocusPRO 6000 programmable | AA alkaline in a pull-out battery holder | Optional backup if wired for AC power | “Replace Battery” flashes about two months before they run out |
| RTH2300 and RTH221 | 2 AAA alkaline | Yes, batteries are the power source | “Lo Batt” flashes for 60 days before they run out |
| PRO 1000 (TH1100DV, TH1110DV, TH1210DV) | 2 AAA alkaline | C wire is optional when batteries are installed | “Lo Batt” shows for 60 days before they run out |
| RTH8500 touchscreen | 3 AAA alkaline | Optional backup if wired for AC power | “LO BATT” flashes about 30 days before they run out |
| Lyric Round (Smart Round) | 1 AAA lithium | Not stated in the support article | Not stated in the support article |
| T5, T5+, T6 Pro Smart, T9 | None in the thermostat | Need 24 VAC power from a C wire | Not applicable |
| T9 Smart Room Sensor | 2 AAA alkaline | Yes | Thermostat warns about 60 days before; sensor light flashes red when critically low |
The T9 user guide states plainly that the T9 thermostat does not use any batteries and runs on 24 volts from the C wire. The T5+ and T6 Pro Smart guides both list a C wire (24 VAC power) as required, and neither lists batteries. If one of these models has a blank screen, the problem is power from your HVAC system, not a battery.
How do you know your Honeywell thermostat battery is low?
Look for a low-battery word or icon on the screen. The wording varies: “Replace Battery” on FocusPRO 6000 models, “Lo Batt” on the RTH2300, RTH221 and PRO 1000, “LO BATT” on the RTH8500, and “BATT” with a battery icon on the T6 Pro.
Honeywell gives you plenty of warning on most models, from about 30 days on the RTH8500 to about two months on the FocusPRO 6000 and T6 Pro. The T6 Pro manual adds that when battery power is very low, the backlight turns off to save power. When it is critically low, the screen shows only BATT and the alert and battery icons.
Even with no warning, Honeywell’s manuals for the FocusPRO 6000, T6 Pro, RTH2300 and RTH8500 say to replace the batteries once a year, or before you leave home for more than a month.
How do you change a Honeywell thermostat battery? (step by step)
These steps work for most wall-mounted Honeywell battery models. FocusPRO 6000 owners can skip removing the thermostat, because the batteries sit in a holder that slides out.
Total time: 5 minutes
Buy the right batteries
Check the table above or your manual for the size (AA or AAA) and use fresh alkaline batteries. The T6 Pro manual says alkaline batteries are recommended.
Set the system to Off
Switch the thermostat mode to Off. Honeywell’s RTH2300 manual warns that the heating or cooling system could stay on even with the batteries removed if you skip this.
Remove the thermostat or the battery holder
On most models, grip the thermostat and pull it straight off the wallplate; the RTH2300, RTH221 and PRO 1000 manuals say to pull at the bottom. On a FocusPRO 6000, press and pull the battery holder out instead.
Swap the batteries quickly
Take out the old batteries and put in the new ones, matching the + and – marks. On FocusPRO 6000 and T6 Pro models, Honeywell says that if the new batteries go in within two minutes, you will not need to reset the time and day.
Reinstall the thermostat
Line the thermostat up with the wallplate and push it gently until it snaps in place, or slide the FocusPRO battery holder back in. Do not force it or bend the wires behind it.
Check the time and set your mode
Set the clock and day if they reset, then switch back to Heat, Cool or Auto. The RTH2300 manual says only the clock and day need to be set after a battery change, because other settings are stored in permanent memory.
Write the battery change date on a small piece of tape inside the battery door or on the back of the thermostat. Since Honeywell suggests a yearly change, it makes the next one easy to plan.
How do you change the battery in a Lyric Round or T9 room sensor?
The Lyric Round takes one lithium AAA battery, and the T9 wireless room sensor takes two AAA alkaline batteries. Neither change needs tools.
Lyric Round (Smart Round)
- Pull the thermostat face straight off the wall. Honeywell says the face plate disconnects when you pull it straight out, as long as the sub base is firmly attached to the wall.
- Remove the old battery and insert a new AAA lithium battery, matching the polarity marks on the sub base.
- Press the face plate back on and make sure it is firmly seated.
T9 Smart Room Sensor
- Remove the cover from the sensor.
- Install 2 fresh AAA alkaline batteries. The T9 guide says a green flashing light means the batteries are good, and red means you need fresh ones.
- Snap the cover back on. The sensor reconnects to the thermostat a few seconds later.
If the T9 shows “Indoor Sensor Connection Error,” the guide says to replace the batteries and make sure the sensor is in an acceptable location.
What should you do if the screen is still blank after new batteries?
Check that the batteries face the right way and are fresh alkaline cells, then check power to your HVAC system. Honeywell’s troubleshooting lists the same steps for a blank display on its battery models:
- Check the circuit breaker and reset it if needed.
- Make sure the power switch at the heating and cooling system is on.
- Make sure the furnace door is closed securely.
- Make sure fresh alkaline batteries are correctly installed.
If the thermostat has a C wire, a blank screen can also mean a wiring or transformer problem. Our Honeywell thermostat wiring guide explains the C terminal. For the RTH2300, Honeywell’s advice is to replace the batteries, and if it still will not power up, replace the thermostat.
⚠️ Safety first
- Set the system mode to Off before removing batteries, so your heating or cooling does not keep running.
- Only pull the thermostat off its wallplate. Do not pull wires out of the wallplate. If you need to touch any wiring, turn off power to the HVAC system at the breaker or furnace switch first.
- Disconnect power at the breaker before removing panels on the furnace or air handler.
- Keep batteries away from children and pets. EPA says lithium single-use batteries should not go in the trash or curbside recycling; recycle them. Check with your local solid waste authority about alkaline batteries.
- If you have a gas furnace and you smell gas, leave the home and call your gas utility or 911 from outside. Do not operate switches, including the thermostat.
When should you call a technician?
Call an HVAC technician when new batteries do not fix the problem and the issue points to your equipment or wiring. A thermostat battery only powers the thermostat itself, not your furnace or air conditioner. Signs you need help:
- The screen stays blank on a C-wire model after you check the breaker, furnace switch and furnace door.
- The display works but the heating or cooling does not start after 5 minutes.
- The furnace breaker trips again after you reset it.
- You see loose, burned or corroded wires behind the thermostat.
- You want to add a C wire so the thermostat no longer depends on batteries.
Honeywell Home technical support (1-800-633-3991) can help confirm the right battery and steps for your model.
How can you prevent Honeywell thermostat battery problems?
A simple routine keeps the screen alive and your schedule intact.
- Replace the batteries once a year, even with no warning, as Honeywell recommends.
- Change them before you leave home for more than a month.
- Use fresh alkaline batteries in the size your model lists.
- Change batteries in under two minutes on FocusPRO 6000 and T6 Pro models to keep the time and day.
- If your thermostat has a C wire, keep the batteries in as backup on models where they are optional.
Frequently asked questions
What kind of battery does a Honeywell thermostat take?
Most take AA or AAA alkaline batteries. T6 Pro, T4 Pro and FocusPRO 6000 models use AA, while the RTH2300, RTH221, PRO 1000 and RTH8500 use AAA. The Lyric Round uses one lithium AAA, and the T5, T6 Pro Smart and T9 thermostats use no batteries.
Will I lose my settings when I change the batteries?
Usually not. Honeywell’s manuals say settings are stored in permanent memory. On some models only the time and day reset, and on FocusPRO 6000 and T6 Pro models even those stay if you swap the batteries within two minutes.
Where is the battery in my Honeywell thermostat?
On most models the batteries are on the back of the thermostat, so you pull it off the wallplate to reach them. On the FocusPRO 6000, a battery holder slides out of the thermostat, so you do not need to remove it from the wall.
Does my Honeywell thermostat need batteries if it has a C wire?
On models like the T6 Pro, FocusPRO 6000 and RTH8500, batteries are optional backup when the thermostat is wired for AC power. The T9, T5 and T6 Pro Smart have no batteries and need a C wire.
Why is my Honeywell thermostat blank after changing the batteries?
Check that the batteries are fresh alkaline cells facing the right way. Then check the breaker, the furnace power switch and the furnace door. If it still will not power up, see our Honeywell thermostat reset guide or call a technician.
How often should I change my Honeywell thermostat battery?
Honeywell recommends once a year, or right away when the low-battery warning appears. Also change them before you leave home for more than a month.
Related error codes
- How to reset a Honeywell thermostat (every model)
- Honeywell thermostat blinking “Cool On”: what it means and how to fix it
- Honeywell thermostat wiring: terminal and color code guide
- Honeywell T6 thermostat: setup, settings and troubleshooting
- How to program a Honeywell thermostat step by step
- How to set a Honeywell thermostat (heat, cool and schedule)
- All thermostat guides
Sources
- Resideo: T6 Pro Programmable Thermostat User Manual (33-00182EFS)
- Resideo: FocusPRO 6000 Series User Guide (69-1921EFS)
- Resideo: RTH2300/RTH221 Series Owner’s Manual (33-00100EF)
- Resideo: PRO 1000 Series Non-Programmable Thermostat Operating Manual (69-2798EFS)
- Resideo: RTH8500 Touchscreen Programmable Thermostat Owner’s Manual (33-00179ES)
- Resideo: T4 Pro Thermostat Installation Guide (33-00187EFS)
- Resideo: T9 Smart Thermostat with Smart Room Sensor User Guide (33-00478)
- Resideo: T5+ Smart Thermostat Product Data (33-00474)
- Honeywell Home: How Do I Replace the Lithium AAA Battery on the Lyric Round?
- U.S. EPA: Used Household Batteries
