How To Use an Old Honeywell Thermostat

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Your old Honeywell thermostat is a workhorse with some nifty features, especially if it's programmable. Here's some help figuring out how to use it.

Honeywell — originally known as Minneapolis-Honeywell — introduced its iconic round thermostat, known as “The Round,” in 1953. It’s as old a Honeywell thermostat as you’re likely to come across, although it isn’t the first the company manufactured. That distinction goes to “The Jewell,” a clunky round model released in 1906 that was — amazingly — programmable!

Honeywell still makes The Round, along with other square and rectangular mechanical models. But the bulk of Honeywell’s current offerings are now digital.

If you’re having trouble getting an old Honeywell thermostat to work, it’s probably a digital one. It might even be programmable, because digital non-programmable thermostats are almost as easy to use as mechanical ones.

How do you know if your Honeywell thermostat is programmable? You can tell at a glance. If there’s a Prog, Set or Menu button on the front, it’s programmable.

Of course, cutting-edge smart thermostats are programmable and don’t have these buttons. But it’s difficult to mistake a smart thermostat for an older model, except for one: The Round Smart Thermostat. It looks almost the same as its mechanical counterpart. Even though it’s been discontinued, there are still plenty out there.

The Old Honeywell Thermostat

You won’t find any Honeywell-made thermostats manufactured before 1953 in a modern home. The Round and heat-only mechanical thermostats like it are simple in concept: You set the dial or lever to the desired temperature, and the thermostat does the rest.

Mechanical thermostats can also control a cooling system and fan. Those that do come with levers for toggling between heating and cooling, and turning the fan on and off.

Honeywell introduced digital thermostats in the 1980s. The first ones were non-programmable and worked in essentially the same way as mechanical ones.

The addition of programmable functions was inevitable, and many older programmable models are still in use today. Specific features may vary from model to model, but basic functions like programming room temperatures for specific times of the day are similar.

Old Honeywell Thermostat Models

There are three types of old Honeywell thermostats: mechanical, digital non-programmable and digital programmable. Wi-Fi enabled and smart thermostats are more recent developments so we won’t cover those, but that still leaves too many models to list here.

Here’s a representative from each category:

  • CT87N1001/U1: This is the version of The Round you need for a heating/cooling system. An analog dial sets the temperature. There’s also a fan toggle for switching the fan from On to Auto, and a mode toggle for Heat, Cool or Off.
  • RTH111B: This basic non-programmable model features up/down buttons for setting temperature, a fan toggle and a mode toggle.
  • RTH221B: The basic programmable model features the same toggle switches and up/down buttons as the non-programmable one. It also has programming buttons labeled Set, Hold and Run.

How To Use a Honeywell Thermostat

It’s easy:

  • Select the desired temperature by turning the dial, moving the lever or pushing the up/down buttons.
  • Turn the fan to Auto to make it come on when the furnace or air conditioner does. If you want the fan without heat or air conditioner, turn it On.
  • Set the mode to Heat or Cool depending on the season.

If the thermostat is programmable, you must set the time before engaging the programming features. After that, use the Set, Prog or Menu button to advance through programming options for different times of the day and select the ones you want.

How To Set an Old Honeywell Thermostat

This depends on the model, but as an example, here’s how you do it with the RTH221B. If you have a different model, consult your product manual or look online.

To set the time:

  • Press the Set button and the time will flash on the screen.
  • Press the up/down buttons to choose the correct time.
  • Press the Run button to save changes and exit.

To program a schedule, move the mode toggle to Heat or Cool as necessary, then do the following:

  • Press the Set button twice. The message “Set Schedule” appears on the screen along with the start time of the first period.
  • Use the up/down buttons to set the start or wake time.
  • Press the Set button and the temperature for that period will flash. Use the up/down buttons to select the temperature.
  • Press the Set button again and the start time for the next period will flash. Change it with the up/down buttons, follow the procedure to select the temperature, then press Set to move on to the next period.
  • When you’ve set all the periods available on the device, press Run to save your settings.

How To Reset an Old Honeywell Thermostat

There’s a procedure to unlock a programmable thermostat and restore factory settings using the buttons or touch screen. This varies by model, so it’s best to consult the product manual.

However, if the thermostat has batteries, there’s an easier way to restore its original settings. Take out the batteries, reverse the polarity and reinsert them. Leave them for 10 seconds, then remove them and put them back in the correct orientation.

How To Replace an Old Thermostat

You can replace an old thermostat with virtually any modern one— with one exception.

If the old thermostat was mechanical or had batteries, it didn’t need a common wire. Usually blue, it supplies power to the thermostat itself. If your new thermostat needs a common wire, you’ll need an HVAC pro to install it. Otherwise, choose a battery-powered model.

To do the replacement, this simple procedure usually works.

  1. Shut off the circuit breaker that powers the heating/cooling system.
  2. Remove the cover from the old thermostat and take a picture of the wires with your phone (you’ll see why in a sec).
  3. Disconnect the wires and remove the old thermostat baseplate.
  4. Install the baseplate for the new thermostat.
  5. Use the image on your phone to connect the wires to the correct terminals.
  6. Install batteries if necessary, snap on the cover plate and turn the circuit breaker back on.

Chris Deziel
Chris Deziel has been active in the building trades for more than 30 years. He helped build a small city in the Oregon desert from the ground up and helped establish two landscaping companies. He has worked as a carpenter, plumber and furniture refinisher. Deziel has been writing DIY articles since 2010 and has worked as an online consultant, most recently with Home Depot's Pro Referral service. His work has been published on Landlordology, Apartments.com and Hunker. Deziel has also published science content and is an avid musician.