Honeywell sells a variety of thermostats, and not all require batteries. A number of models do, though, and if you need to change your Honeywell thermostat battery, you need to know the model. You can usually find the procedure, which is always simple, by checking the Honeywell thermostat instructions for your model.
The thermostats most likely to have batteries are the programmable ones with an LED display, but some of these draw power from the transformer. Models that do this have a connection for a common wire, or C-wire, that completes the circuit initiated by the red power wire that connects to the R terminal. If your thermostat doesn't have a C-wire, it probably needs batteries.
Will the Thermostat Work if the Batteries Are Dead?
If your Honeywell thermostat has an LED display, and it needs batteries, it usually has a low battery warning. This warning appears on the screen as a flashing banner that says "Replace Battery" or "Batt Lo." If you ignore this, a blank screen is your last warning before the thermostat stops functioning.
In some models, the batteries function only as backup for when the thermostat isn't getting 24-volt power from the heating system. If you fail to change them when they go dead, you will lose the date and time settings, but the other programmable settings will remain intact.
Honeywell 2000 Series
The Honeywell 2000 series includes model numbers TH2110D and TH2210D. It has a LED screen and push-button controls, and it's suitable for conventional heating/cooling systems and heat pumps. The batteries are for backup when the thermostat is connected to 24-VAC power. You should replace them once a year.
- Switch the system to OFF.
- Pull the cover plate straight off the wall to disengage it from the wall plate.
- Turn the cover plate over and remove the two AAA batteries. Replace them with fresh ones.
- Snap the cover plate back onto the wall plate and turn the system back ON.
Honeywell 4000 Series
The Honeywell 4000 series includes the TH4110D and TH4210D models, which are programmable and designed to run heating/cooling systems with or without a heat pump. If the REPLACE BATT icon flashes, you have about two months to replace the batteries before they go dead.
- Pull at the bottom of the face plate to remove the thermostat from the wall plate.
- Turn it over, remove the two AA batteries and replace them.
- Snap the thermostat back onto the wall plate.
Honeywell 5000 and 6000 Series
The 5000 series or the FocusPRO 5000 consists of three models: the TH5110D, the TH5320U and the TH5220D. The former model takes AAA alkaline batteries, while the latter two take AA alkaline batteries. The battery compartment is on the side of the unit.
In terms of battery replacement, the 6000 series is virtually identical except that it runs on AA batteries.
- Depress the tab to open the battery compartment. Swing it out from the face plate to reveal the batteries.
- Remove the old batteries and replace them with new ones.
- Swing the compartment back into the face plate and push to snap it shut.
Honeywell 8000 Series
The 8000 series, which includes the TH8110U, TH8320U and TH8321U, is the company's most versatile offering. It can control up to three heating and two cooling systems at the same time, with dehumidification. Three AAA batteries are optional and are needed only if the thermostat is wired directly to AC power. To replace them:
- Pull off the cover plate.
- Turn over the cover plate and pull out the used batteries. Replace them with new ones.
- Snap the cover plate back onto the wall plate.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.