How to Change a Honeywell Thermostat From Celsius to Fahrenheit

If you’re looking at your Honeywell thermostat and thinking, “Why is my house set to 21°C instead of 70°F,” you’re not alone. I’m Nathan. I’ve worked as an HVAC technician for over 10 years, and this is one of those quick fixes I help homeowners with all the time especially after a thermostat replacement, power outage, reset, or when a home has been set up for a different region.

The good news: on most Honeywell thermostats, switching from Celsius to Fahrenheit takes under a minute once you know where to look.

Below are the exact methods I use in the field, broken down by the most common Honeywell models and menu types.

Quick Answer (Works on Many Honeywell Models)

Most Honeywell thermostats change temperature units in one of these places:

1. Menu > Preferences / Settings > Temperature Scale (F/C)
2. Installer Setup (often called “Function 14”)
3. A physical DIP switch on the back (some non-programmable models)

If you can’t find the setting in the normal Menu, you usually need to go to Installer Setup, and that’s where Function 14 comes in.

Before You Start (2 Things I Always Check)

1) Take note of your thermostat model

Look on the thermostat faceplate, behind the cover, or inside the battery compartment. You’ll often see something like:

• RTH2300 / RTH221
• RTH3100C
• PRO 4000
• FocusPRO 5000
• RTH6500WF
• T9 / T10 / 8000 series

2) Don’t confuse “set temperature” with “temperature units”

If you change the setpoint (like 21 to 22), that doesn’t change Celsius to Fahrenheit. You need the units setting (F vs C).

The Most Common Way: Use the Menu (If Your Thermostat Supports It)

Many Honeywell thermostats let you switch units directly in the regular settings.

Steps

1. Press Menu
2. Scroll using + (or arrow buttons) until you see F/C or Temp Scale
3. Press Select
4. Choose F (Fahrenheit)
5. Press Home (or Done) to exit

If you don’t see F/C anywhere in the Menu, go to the installer setup instructions below.

The Installer Setup Method (Function 14) — The One I Use Most Often

On many Honeywell models, the Celsius/Fahrenheit toggle lives in Installer Setup Function 14.

What does Function 14 usually mean

In a lot of Honeywell thermostat families, Function 14 controls the temperature display units:

14	0	Fahrenheit (°F)
14	1	Celsius (°C)

If yours is showing Celsius, you typically want 14 = 0.

Important: The button combinations to enter installer setup vary by model. I’ll break down the most common ones next.

Model-Specific Steps (Most Popular Honeywell Thermostats)

RTH2300 / RTH221 Series (Very Common Basic Models)

1. Press and hold Up + Down together until the display changes (setup mode)
2. Tap Up + Down to advance through setup functions until you reach Function 14
3. Set Function 14 to 0 for Fahrenheit
4. Press Run to save and exit

If you exit without pressing Run, some units won’t keep the change.

Honeywell RTH3100C

1. Press and hold Up + Down until the display changes (about 5 seconds)
2. Press Up to advance through functions until Function 14
3. Use Down to change the setting to Fahrenheit.
4. Press and hold Up + Down again to exit and save

This series feels a little different, but the pattern is the same: enter setup, find 14, switch to °F.

Honeywell PRO 4000 Series

1. Press Up + Down at the same time until the screen changes.
2. Press Up to move through setup functions until Function 14
3. Use Down to change:
   • 0 = Fahrenheit
   • 1 = Celsius
4. Exit using Done or by waiting for it to time out (I recommend using Done/Exit if available)

This is one of the most common thermostats I see in residential service calls.

Honeywell FocusPRO 5000 Series

1. Press and hold Up + Fan until the display changes (setup mode)
2. Press Next until you reach Function 14
3. Use Up/Down to change to Fahrenheit
4. Press Done to save and exit

If “Up + Fan” doesn’t work, double-check the exact FocusPRO model number because Honeywell has multiple sub-versions.

Honeywell RTH6500WF Smart Series (Wi-Fi, Button Style)

This model uses a setup screen with two numbers (function on the left, setting on the right).

1. Press and hold Fan + Up together for about 3 to 5 seconds
2. You’ll see two numbers appear on screen.
3. Press Next until the left number matches the function you need (often where units are configured)
4. Use Up/Down to change the smaller right-side number (the option)
5. Press Done to save and exit.

Smart Touchscreen Honeywell Thermostats (T9, T10, 8000 Series)

These usually don’t rely on “Function 14.” Instead, you’ll find it under Preferences.

Steps (typical)

1. Press Menu
2. Go to Preferences or Settings
3. Select Temperature Scale or Fahrenheit/Celsius
4. Choose Fahrenheit (°F)
5. Press Save or Done

If you can’t find it, it may be under Installer Options depending on firmware.

Honeywell Home App / Lyric App (App-Connected Thermostats)

If your thermostat is connected to Wi-Fi, the fastest way is often the app.

Steps

1. Open the Honeywell Home app
2. Select your thermostat
3. Tap Settings (gear icon)
4. Find Thermostat Settings or Temperature Units
5. Switch to Fahrenheit

Some Honeywell Models Use a Physical Switch (Example: TL7135A)

A few non-programmable Honeywell thermostats use a physical DIP switch on the back.

Honeywell TL7135A (Non-Programmable) — DIP Switch Method

1. Remove the thermostat/control module from the wall mount (power off if you’re uncomfortable)
2. Locate DIP switch S1 on the back
3. Flip it to:
   • F for Fahrenheit
   • C for Celsius
4. Reinstall the module and confirm the display changed

This is common on certain line-voltage or specialty Honeywell models.

Quick Conversion Help (So You Know You’re Setting the Right Temp)

When someone switches to Fahrenheit, they often ask me, “What is 21°C in Fahrenheit again?”

Here’s a quick cheat table:

18°C	64°F
20°C	68°F
21°C	70°F
22°C	72°F
24°C	75°F
26°C	79°F

Formula if you need it: °F = (°C × 9/5) + 32

My Practical Temperature Recommendations

Homeowners usually switch units and then realize their schedules feel “off.” These are widely used energy-saving baselines I see across the industry:

Heating (winter)	68°F when home
Cooling (summer)	78°F when home

If you’re using a schedule and your setpoint keeps changing, you may need to use Hold (more on that below).

Troubleshooting: If It Still Shows Celsius After You Changed It

1) You changed the wrong function

On many models, Function 14 is the units, but if you weren’t actually on 14, the display won’t change. Go back into setup and confirm the function number.

2) You didn’t save properly

Look for Done, Run, or Home depending on model. On some units, you must exit the installer menu correctly for the setting to stick.

3) The thermostat is locked

Some Honeywell thermostats can be installer-locked. If buttons don’t respond normally or settings won’t save, check for a lock setting in the menu or refer to your model’s manual.

4) Try a reset (last resort)

If nothing sticks, a reset often clears the glitch. After the reset, repeat the steps for units.

FAQ (The Questions Homeowners Ask Me Most)