Honeywell CM921


The Honeywell CM921 is a wireless programmable thermostat that is designed for use in domestic and light commercial heating systems. It is a popular choice for homeowners and installers due to its flexibility, ease of use, and energy-saving features.

Some of the key features of the Honeywell CM921 include:

  1. Wireless operation – The CM921 uses radio frequency (RF) signals to communicate with the receiver unit, giving you more flexibility in terms of where you can install the thermostat.
  2. 7-day programming – The unit allows you to set different heating schedules for each day of the week, providing flexibility and energy savings.
  3. 6 time and temperature settings per day – The CM921 allows you to set up to six different temperature settings for each day, giving you greater control over your heating system.
  4. Large LCD display – The CM921 features a large, easy-to-read display that shows the current temperature, the time, and the status of the heating system.
  5. Pre-set heating schedules – The CM921 comes with pre-set heating schedules that can be easily customized to suit your specific needs.
  6. Holiday mode – If you’re going away, the CM921 can be set to holiday mode, ensuring that your heating is turned off while you’re away.
  7. Frost protection – The CM921 has a built-in frost protection feature that will turn on the heating if the temperature drops below a certain level, preventing pipes from freezing.

Overall, the Honeywell CM921 is a reliable and user-friendly wireless programmable thermostat that can help you save money on your heating bills while keeping your home comfortable.

How to operate

  1. Ensure that the receiver unit is installed and wired correctly to your heating system.
  2. Install the batteries into the thermostat.
  3. Press the ‘SET’ button to set the clock, then use the ‘+’ and ‘-‘ buttons to adjust the hour and minute. Press ‘SET’ again to confirm.
  4. To set the heating schedule, press the ‘PROGRAM’ button. You can then use the ‘+’ and ‘-‘ buttons to select the day of the week you want to program. Once you have selected the day, press ‘PROGRAM’ again.
  5. Set the ‘ON’ and ‘OFF’ times for each temperature setting using the ‘+’ and ‘-‘ buttons. Once you have set the times, press ‘PROGRAM’ to move onto the next time period.
  6. Repeat this process for each temperature setting for each day of the week until you have set your complete schedule.
  7. Once you have set your schedule, you can use the ‘+’ and ‘-‘ buttons to adjust the temperature settings for each time period.
  8. If you need to override the schedule and turn the heating on outside of the programmed times, simply press the ‘Override’ button until the heating comes on. To turn it off again, press the ‘Override’ button again.
  9. To enter holiday mode, press and hold the ‘Override’ button for 5 seconds. The CM921 will then switch to holiday mode, turning off the heating until you return and switch it back to normal mode.

That’s a basic overview of how to operate the Honeywell CM921. For more detailed instructions, you should refer to the user manual that came with your unit.


Sending a binding signal.

Before the wireless programmer will work, you will need to send a binding signal to the wireless relay box…


Operating the CM921 or CM927

Technical information

Central heating control typeWireless programmable thermostat
Heating system compatibilitySuitable for use with most gas, oil and LPG boilers
Control operation7-day programmable with up to 6 daily time and temperature changes
Temperature range5°C to 35°C
Display typeBacklit LCD with temperature display and status icons
Hertz (Hz)Designed to operate on a 230V AC, 50Hz power supply.
Switch rating5 (2) A, 230V AC
Max. load (Inductive)The maximum inductive load that the Honeywell CM921 can handle is 230V AC, 3 (1) A (IEC 60730-1). This means that the thermostat is designed to handle an inductive load of up to 690 VA (230V x 3A).
Max. load (Resistive)The maximum resistive load that the Honeywell CM921 can handle is 230V AC, 3 (1) A (IEC 60730-1). This means that the thermostat is designed to handle a resistive load of up to 690 W.
Power consumptionThe thermostat consumes less than 1.8mW in standby mode and less than 27mW in active mode with backlight on. It is important to note that the power consumption may vary depending on factors such as the frequency of use, ambient temperature, and signal strength.
Dimensions100mm x 100mm x 27mm (H x W x D)
Battery backupThe thermostat is designed to operate on two AA alkaline batteries, which have an estimated lifespan of up to two years.
Programmer Voltage230V AC
Smart compatibilityThe CM921 is not a smart thermostat and does not have compatibility with smart home systems or voice assistants. It is a wireless programmable thermostat that uses a plug-in receiver to communicate with the heating system.
Terminal capacityThe Honeywell CM921 wireless programmable thermostat has a terminal capacity of 3A (1) at 230V AC, which is suitable for most gas, oil and LPG boilers.
The thermostat has three terminal connections: Live (L), Neutral (N), and Switch (S). The Live and Neutral terminals are used to power the thermostat, while the Switch terminal is used to control the heating system.
It is important to note that the terminal capacity of the thermostat should be checked against the ratings of the heating system that the thermostat will be controlling to ensure compatibility and safe operation. If you are unsure about the compatibility or installation of the Honeywell CM921 thermostat, it is recommended that you consult a qualified electrician or heating engineer.

User guide