How to Replace an Old Honeywell Thermostat

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You can replace an old analog thermostat.
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You're presumably replacing your old dial-type Honeywell thermostat with a programmable or smart thermostat. They're both relatively similar to install, but be aware some programmable thermostats are becoming obsolete compared to the energy-saving capacities of smart thermostats. Of course, your heating system may have needs that only specific types of thermostats can address, so do your homework before you buy.


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Before You Start

Your Honeywell thermostat is wired electrically, which means there's the risk of playing with live wires. You must turn off the breaker for the thermostat before you start.

Whether you were successful at turning off the breaker won't be apparent on an older dial thermostat. While you can turn on the heat and wait a half hour to see if the heat is coming on, it's safer to check for juice with a voltmeter or circuit tester before you start tampering with wires.


While the following steps will work for many digital thermostats, they may not be compatible with yours. Refer to local and national codes regarding any needed electrical connections. Call a professional if you're uncomfortable working with a breaker or wiring.

Removing and Replacing a Thermostat

Step 1: Remove the Cover

With the breaker turned off, the thermostat's cover will either pop off without much effort or, if needed, come off easily with a screwdriver. Be careful when you remove it as some older thermostats will have mercury-filled glass tubes you'll have to handle carefully and dispose of safely since mercury is highly toxic. Leave the wall plate where it is so you can label wire connections before proceeding. First, use the circuit tester to ensure the power is off.


Step 2: Label the Wires

Your new thermostat may include wire-labeling stickers to help you keep track of where each wire goes. If not, use tape and label each wire with the letter for where it connects. And because more info is good, take a photo with your smartphone for reference in case it helps you later in the installation process.

Step 3: Remove the Wall Plate

Use a screwdriver to disconnect the now-labeled wires. Unscrew the wall plate and remove it. Tape the wires to the wall with low-tack painter's tape, being careful not to cover the lettered labels as you might accidentally tear them off. If your new thermostat is smaller than the old one, you may need to remove drywall anchors or do some putty repairs. (Don't skip taping them to the wall — they could slip back into the wall if you don't!)


Step 4: Install the New Wall Plate

Make sure to mount the plate level by dry fitting it and marking where to install it. Remove the tape holding the wires to the wall and feed them through the new plate, then attach the plate to the wall. Tape the wires to the wall again so they don't fall through.

Step 5: Wire the New Thermostat

Refer to the instructions the thermostat came with and check your wire labels. Attach them to the correct terminals as instructed and then tighten the terminal screws. If using a programmable thermostat, you may have a metal jumper between the "R" and "Rc" terminals that needs to be left in place or removed; your unit's instructions will clarify this for you. A smart thermostat or Wi-Fi-enabled one may need a "C" wire to operate. If your older model didn't need a "C" wire, the manual may have suggestions, but this might be time to call an electrician or HVAC pro.


Step 6: Check for Proper Installation

If batteries are needed, install them. You can now put the new thermostat cover in place and turn the breaker back on to see if everything works correctly.

Final Points: Smart Thermostats

A smart thermostat, like the Honeywell Prestige, usually has an outdoor sensor to install that monitors the outdoor climate. This sensor is easy to install — it's a wireless unit that connects via Wi-Fi with the base thermostat and is battery-operated. The sensor won't need professional installation.


What may require a professional are any further electrical connections needed for installing a smart thermostat. Honeywell offers a "compatibility check" quiz on their website to see if your existing system is an easy switch to their Prestige smart thermostat.



Steffani Cameron is the daughter of a realtor and interior decorator mother and a home contractor father. Steffani is a professional writer with over five years' experience writing about the home for BuildDirect and Bob Vila. Raised with a mad love for decorating, Steffani gave up her Art Deco apartment to travel and work remotely for five years. She's in love with experiencing traditional decor around the world, including stays in Thai teak plantations on the Mekong River and cave homes in Turkey.