Honeywell RTH221B Programmable Thermostat


This is a legacy product document supported by Resideo. It is no longer manufactured




Display screen

display screen

When Cool On is flashing, this means the compressor protection is engaged (see section 1.7).

Setting the clock

  1. Press the Set button. The message Set Time will appear on the screen and the time display will flash.
  2. Press the up or down button to set the time.
  3. Press the Run button to save the changes & exit. (The changes are also saved if you do not press any button for 60 seconds.)

NOTE: The clock has lost its settings when the time display flashes. If the thermostat is following the programmed schedule, it will use the temperature settings programmed for the Wake period until you readjust the clock.

System switch
Use the System switch to place the system in one of the following modes.

  • Cool: System is in Cooling mode.
  • Off: System is off.
  • Heat: System is in Heating mode.

Do not use Cooling mode when the outdoor temperature is below 50 °F (10 °C).

Fan switch
Use the Fan switch to select the fan operation.

  • Auto: The fan runs only when heating or cooling is actually taking place (commonly used setting).
  • On: The fan runs continuously.

Temperature setting

Press the up or down button once to display the setpoint temperature.
Press either button more than once to change the setpoint.
The ambient (measured) temperature is normally displayed. To view the setpoint temperature, press either of the temperature adjustment buttons once. The setpoint temperature will appear for five seconds.

Built-in compressor protection

Your thermostat has a feature that helps prevent damage to the compressor of your air conditioning system. Because damage can occur if the compressor is restarted too soon after shutdown, this feature forces the compressor to wait five minutes before restarting.
During the wait time, the message Cool On flashes on the screen.
When the safe wait time has elapsed, the message stops flashing and the compressor turns on.

Programmed schedule

Using the schedule
Press the Run button to follow the programmed schedule. You can have up to four periods in one day. Each period can be programmed with a different temperature setting.

WakeMorning temperature
LeaveDaytime temperature
ReturnEvening temperature
SleepNight temperature

To program a period, you need to set its start time and its two temperature settings (one when the thermostat is in cooling mode and another one when it is in heating mode). The same period and settings are repeated every day.

For maximum energy savings without sacrificing comfort, you can program the schedule such that the temperature is at an economical level when you sleep or are away during the day and at a comfortable level when you are awake and at home.

Factory settings

PeriodStart timeHeatCool
Wake6:00 a.m.70 °F (21 °C)75 °F (24 °C)
Leave8:00 a.m.62 °F (16.5 °C)83 °F (28.5 °C)
Return6:00 p.m.70 °F (21 °C)75 °F (24 °C)
Sleep10:00 p.m.62 °F (16.5 °C)78 °F (25.5 °C)

Modifying the schedule

  1. Press the Set button twice until the message Set Schedule appears. The start time of the first period (Wake) will flash.
  2. Press the up or down button to set the time.
  3. Press the Set button. The temperature setting for that period will flash.
  4. To program the temperature settings for the cooling mode, place the System switch in Cool. To program the temperature settings for the heating mode, place the switch in Heat.
  5. Press the up or down button to set the temperature.
  6. Press the Set button. The start time of the next period will flash.
    NOTE: To skip a period, press the Hold button for four seconds while that period is displayed. When a period is skipped, dashes appear instead of the time and temperature display for the period. Only the Wake period cannot be skipped.
  7. Repeat steps 2 through 6 for each of the remaining periods.
  8. Once all adjustments are made, press the Run button to save them and exit. (The changes are also saved if you do not press any button for 60 seconds.)

Temporary override of schedule

To temporarily ignore the programmed schedule, press up or down until the desired temperature is displayed. The word Temporary will appear on the screen and the period display will flash to indicate that the current period has been overridden. The new temperature will be used until the next period begins.

NOTE: Pressing up or down once will display the setpoint temperature. To change the setpoint temperature, press either button more than once. Press the Run button if you wish to cancel the temporary setting before the beginning of the next period.

Permanent override of schedule

To permanently ignore the programmed schedule, press up or down until the desired temperature is displayed. Then, press the Hold button to maintain the new temperature indefinitely. The word Hold will appear on the screen.
Press the Run button if you wish to cancel the override and resume the programmed schedule. The word Hold will disappear and the thermostat will be set to the temperature programmed for the current period.

Battery replacement

Set the System switch to Off before removing the batteries. Otherwise, the heating/cooling system could remain activated even when batteries are removed.

Install two fresh AAA batteries when Lo Batt (low batteries) flashes on the screen. The icon appears for 60 days before the batteries are depleted. Replace batteries once a year, or when you will be away for more than a month.
Most settings are stored in non-volatile (permanent) memory. Only the clock must be set if the thermostat was without batteries.

In case of difficulty

Screen is blank: Ensure two fresh AAA alkaline batteries are correctly installed.

Temperature settings cannot be changed:

  • The System switch is off.
  • You are setting the temperatures outside their allowable ranges:
    • Heat: 40 °F to 90 °F (4.5 °C to 32 °C).
    • Cool: 50 °F to 99 °F (10 °C to 37 °C).

Cool On is flashing:  The compressor protection delay has been activated. Cooling will not be activated until the delay ends (maximum of five minutes).

HI appears on the screen:  The ambient (measured) temperature is above 122 °F (50 °C).

LO appears on the screen: The ambient (measured) temperature is below 32 °F (0 °C).

E1 appears on the screen:  The temperature sensor circuit is opened. Replace the thermostat.

E2 appears on the screen: The temperature sensor circuit is shorted. Replace the thermostat.

Technical Data

Power Supply
• 2 AAA batteries
Maximum Load
• 1 A @ 24 VAC per output
Temperature Ranges
• Heat: 40 °F to 90 °F (4.5 °C to 32 °C)
• Cool: 50 °F to 99 °F (10 °C to 37 °C)
Operating Ambient Temperature
• 32 °F to 122 °F (0 °C to 50 °C)
Shipping Temperature
• -40 °F to 130 °F (-40 °C to 55 °C)
Operating Relative Humidity
• 5% to 90% (non-condensing)
Physical Dimensions (H x W x D)
• 4.7” x 2.9” x 1.1” (120 mm x 74 mm x 28 mm)


Honeywell warrants this product, excluding battery, to be free from defects in the workmanship or materials, under normal use and service, for a period of one (1) year from the date of purchase by the consumer. If at any time during the warranty period the product is determined to be defective or malfunctions, Honeywell shall repair or replace it (at Honeywell’s option).
If the product is defective,

  1. return it, with a bill of sale or other dated proof of purchase, to the place from which you purchased it, or
  2. call Honeywell Customer Care at 1-800-468-1502. Customer Care will make the determination whether the product should be returned to the following address: Honeywell Return Goods, Dock 4 MN10-3860, 1885 Douglas Dr N, Golden Valley, MN 55422, or whether a replacement product can be sent to you.

This warranty does not cover removal or reinstallation costs. This warranty shall not apply if it is shown by Honeywell that the defect or malfunction was caused by damage which occurred while the product was in the possession of a consumer.
Honeywell’s sole responsibility shall be to repair or replace the product within the terms stated above. HONEYWELL SHALL NOT BE LIABLE FOR ANY LOSS OR DAMAGE OF ANY KIND, INCLUDING ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES RESULTING, DIRECTLY OR INDIRECTLY, FROM ANY BREACH OF ANY WARRANTY, EXPRESS OR IMPLIED, OR ANY OTHER FAILURE OF THIS PRODUCT. Some states do not allow the exclusion or limitation of incidental or consequential damages, so this limitation may not apply to you.


This warranty gives you specific legal rights, and you may have other rights which vary from state to state.

If you have any questions concerning this warranty, please write Honeywell Customer Relations, 1985 Douglas Dr, Golden Valley, MN 55422 or call 1-800-468-1502. In Canada, write Retail Products ON15-02H, Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V4Z9.

Customer Assistance

If you have any questions regarding your thermostat, please go to, or call Honeywell Customer Care toll-free at 1-800-468-1502.


Documents / Resources

Honeywell RTH221B Programmable Thermostat [pdf] Installation Guide
RTH221B, Programmable Thermostat


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