Honeywell sells a variety of thermostats, and not all require batteries. If your Honeywell thermostat suddenly isn't working properly or it's giving you a low battery warning, it's time for some new batteries. Most Honeywell thermostats use standard AA batteries or AAA batteries. Changing the batteries in the thermostat is necessary whenever the display shuts off or when it indicates that batteries are low.
Does My Honeywell Thermostat Use Batteries?
The thermostats most likely to have batteries are the digital or programmable thermostat models with an LED display, but some of these draw power from the home's HVAC system. Models that do this have a wire connected to the "C" terminal (for common wire, or C-wire), which provides continuous power to the thermostat unit. If you have a digital thermostat or a programmable thermostat that doesn't have a C-wire, it probably needs batteries.
Many Honeywell thermostat models with C-wire connections also have a battery holder for providing battery power in the event of a power outage. This enables the thermostat to retain program schedule settings if the power goes out.
What If the Batteries Are Dead?
If your Honeywell thermostat does require batteries, it will also have a low battery notification function. This warning appears on the screen as a flashing banner that says "Replace Battery" or "Batt Lo," indicating the need for a battery change. If you fail to put in fresh batteries, eventually the LED screen will go blank; this 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 HVAC 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.
Opening Nonprogrammable Thermostats for Battery Replacement
For a non-programmable Honeywell thermostat, such as the RTH5100B, look for a battery-compartment latch on the upper-right portion of the thermostat. Press the latch down and slide the battery compartment out. Release batteries from the holder by pressing them out from the hole in the back of the compartment.
Align the positive and negative terminals of the two batteries with the diagram inside the compartment, to ensure the polarity is correct. The left battery goes upright, or plus side up, while the right battery goes downward, or minus side up. This type of thermostat permanently stores temperature settings, so there's no need to replace batteries within 30 seconds.
When to Change Honeywell Thermostat Batteries
A thermostat battery change is a recommended once a year, ideally before the cold-weather season kicks in. If you will be away from your home for a month or longer, it's a good idea to change the thermostat batteries beforehand. This helps ensure adequate climate control while you're away, especially during the winter.
If you live in a climate where outdoor winter temperatures drop below freezing, changing the thermostat batteries before you leave town can help prevent frozen pipes, as the indoor air temperature will stay moderate. If the heat is completely off during a number of below-freezing days, pipes could burst, resulting in expensive repair bills and potential property damage.
Signs of Low Battery Life
Most Honeywell thermostats provide some type of alert when the batteries are beginning to fail. The thermostat display will show "LOW BATT," "REPLACE BATT," "REPL BAT," or similar verbiage a month or two before the batteries will fail completely. Low or dead batteries are among the most common issues affecting thermostats. Fortunately, this requires only basic DIY troubleshooting.
If you don't regularly pay attention to the display but notice that it seems dim and hard to read, the batteries may be failing. If the thermostat isn't remembering the daily temperature settings or doesn't allow a change of temperature, the batteries may be to blame. If the display is completely blank, the batteries have already failed.
Will a Thermostat Work if the Batteries Are Dead?
A thermostat that uses batteries for its primary power won't work if the batteries within it are dead. The batteries are required for the thermostat to remember the temperature settings or to turn the central heating or air conditioner on or off. Changing the temperature via the thermostat is also impossible if the batteries in the thermostat are dead. The batteries also ensure the thermostat keeps its settings even during a power outage.
Changing the Batteries in Honeywell 2000 Series Thermostats
The Honeywell 2000 series of programmable thermostats includes model numbers TH2110D and TH2210D. These have an LED screen and push-button controls, and they are suitable for conventional heating systems, air conditioning, and heat pumps. The batteries are for backup when the thermostat is connected to 24-VAC power. You should install fresh batteries once a year.
- Switch the system to OFF.
- Pull the cover plate straight out to disengage it from the sub-base or base plate.
- Turn the cover plate over and remove the two AAA batteries. Replace them with new AAA alkaline batteries.
- Snap the cover plate back onto the base, and turn the system back to ON.
Changing the Batteries in Honeywell 4000 Series Thermostats
The Honeywell 4000 series includes the TH4110D and TH4210D models, which are programmable thermostats 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 faceplate to remove the thermostat from the wall plate.
- Turn the faceplate over.
- Remove the two AA batteries and replace them.
- Snap the faceplate back onto the wall plate.
Changing the Batteries in Honeywell 5000 and 6000 Series Thermostats
The 5000 series, or the FocusPRO 5000, consists of three models: the TH5110D, which takes AAA batteries, and the TH5320U and TH5220D, both of which take AA batteries. The battery compartment is on the side of the unit. The 6000 series includes the TH6110D and TH6320U models. Both of these use AA batteries.
- Depress the tab to open the battery compartment.
- Swing the compartment out from the faceplate to reveal the batteries.
- Remove the old batteries and replace them with new ones.
- Swing the battery compartment back into the faceplate and push to snap it shut.
Changing the Batteries in Honeywell 8000 Series Thermostats
The Honeywell 8000 series, which includes the TH8110U, TH8320U, and TH8321U, is the company's most versatile offering. These home thermostats 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 not wired directly to AC power.
- Remove the cover plate or faceplate from the wall 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.