How to Replace a Honeywell Thermostat Battery

Quick fix 

• Most Honeywell thermostats use two AA or two AAA alkaline batteries. 

• If your thermostat shows “Lo Batt” or a battery icon, replace both batteries now. 

• If new batteries don’t fix it, follow the quick troubleshooting flow below (power → batteries → wiring → C-wire). 

• For long-term peace of mind, consider a C-wire upgrade; Honeywell explains why C-wires matter on their support page. 

Quick Fix Box (do this first) 

1. Buy two fresh AA or AAA alkaline batteries (check your compartment). 
2. Turn thermostat system OFF (if manual instructs) or flip the HVAC breaker. 
3. Remove the thermostat, replace both batteries (match +/−), snap back on, turn power on. 
4. If still not working, follow “Troubleshooting” (page X). 

Table of contents 

1. How to identify & open your thermostat 
2. What batteries to buy & expected life 
3. Step-by-step battery replacement (by model group) 
4. Troubleshooting flow (quick steps + flowchart) 
5. Common mistakes (quick chart) 
6. Stop changing batteries: C-wire & upgrades 
7. FAQ (ready for schema) 

1) How to identify & open your thermostat (so you know what you have) 

Why this matters: Different Honeywell units use different batteries and open differently. Don’t force anything, use these safe steps. 

1. Turn system OFF if your manual suggests (or flip the furnace breaker). 
2. Look at the front display — some models show the model number in Settings (if accessible). 
3. Try removing the cover: 
4. RTH (retail) series — usually snaps off the wallplate. Grip bottom and pull straight out. 
5. FocusPRO / TH / T series — usually lift or pull the front panel; some have one small screw at the bottom. If so, remove the screw first with a small Phillips screwdriver. 
6. Find the battery compartment — it will show AA or AAA and polarity (+/−). 

Quick tip: If there’s a terminal block with wires and a labeled C terminal with a wire connected, your thermostat may use the C-wire (continuous power) rather than replaceable batteries; Honeywell explains C-wire basics here. 

2) What batteries to buy & expected life 

• Type: Use 1.5V alkaline batteries (AA or AAA depending on model). Honeywell’s manuals (e.g., the FocusPRO N100 manual) specify fresh alkaline. 

• Do not mix old + new or mix sizes. 

• Do not use NiMH (rechargeable) unless manual allows — NiMH are 1.2V and may not supply enough voltage. 

• Lifespan: Expect roughly 6–24 months depending on model and usage (many Honeywell users see ~1 year). Replace at the first “Lo Batt” warning. 

3) Step-by-step battery replacement — simple, by model group 

Always match polarity (+/−). Replace both batteries at once. 

RTH series (common retail models — typically AAA) 

1. Turn system OFF (if your manual advises). 
2. Pull the thermostat straight off the wall plate — it should snap free. 
3. Open battery compartment; remove two AAA batteries. 
4. Insert two fresh AAA alkaline batteries, observing polarity. 
5. Snap thermostat back on the wall plate. Turn system ON and verify operation. 

TH & FocusPRO series (home models — typically AA) 

1. Turn system OFF. 
2. Pull the front cover off or lift the thermostat off the base. (Some have one screw — remove if present.) 
3. Remove two AA batteries. 
4. Insert two fresh AA alkaline batteries, match polarity. 
5. Replace cover, power on, check time and operation. 

T-series / older Honeywell models (AA) 

1. Turn the system OFF. 
2. Pull or open the faceplate; some units have a latch. 
3. Replace two AA batteries with fresh alkalines. 
4. Close the faceplate and power on. 

4) Troubleshooting — quick flow to follow 

1. Did you insert batteries with correct polarity? Yes → go to 2. No → reinsert correctly. 
2. Try another new pair of batteries. If still no power → go to 3. 
3. Is the HVAC breaker or furnace switch ON? If no, turn on and test. If yes → go to 4. 
4. Inspect wiring behind thermostat (only if comfortable). If wiring looks damaged or loose, call a pro. If wiring looks fine → go to 5. 
5. Does your stat have a connected C-wire? If yes and display blank → call pro. If no C-wire and battery-only stat blank after fresh batteries → consider C-wire options or replacing the stat. 

5) Common mistakes  

6) Stop changing batteries: C-wire & upgrade options 

• What is a C-wire? The C (common) wire supplies continuous 24VAC power to the thermostat so it doesn’t rely on replaceable batteries. Honeywell explains C-wire basics here. 

• If you don’t have a C-wire: Options: (A) run a new wire from furnace to thermostat, (B) install a C-wire adapter/power extender kit, or (C) replace the thermostat with a model that supports a power kit. 

• Benefits: No more battery changes, more stable Wi-Fi and smart features. 

7) FAQ 

• How do I know what battery my Honeywell thermostat uses? Open battery compartment — the size is printed. Manuals list the size too. 

• Can I use rechargeable batteries? Usually no. Honeywell manuals call for 1.5V alkaline; rechargeables are 1.2V. 

• How often should I change batteries? Annually or when the “Lo Batt” message appears. Typical life: 6–24 months. 

• My thermostat still doesn’t work after new batteries — what now? Re-check polarity, test another new set, confirm HVAC power, inspect wiring, check for C-wire. Call pro if still broken. 

• What happens if thermostat batteries die? You may lose temperature control, schedules, or the display; HVAC may stop running until batteries or power are restored.