Honeywell RTH8500D Owner's Manual

Turn thermostat over, insert 

fresh AAA alkaline batteries, 

then reinstall thermostat.

Battery replacement

M28391

Batteries are optional (to provide 

backup power) if your thermostat 

was wired to run on AC power when 

installed.
Install fresh batteries immediately when 

the LO BATT warning begins flashing. 

The warning flashes about 30 days 

before batteries are depleted.
Even if the warning does not appear, 

you should replace batteries once a 

year, or before leaving home for more 

than a month.

Grasp thermostat and pull to 

remove from wallplate

M28337

Special features

 This feature allows the thermostat to 

“learn” how long the furnace and air conditioner take to reach programmed 

temperature settings, so the temperature is reached at the time you set. For 

example: Set the Wake time to 6 am, and the temperature to 70°. The heat will 

come on before 6 am, so the temperature is 70° by the time you wake at 6. 

The message “Recovery” is displayed when the system is activated before a 

scheduled time period.
Compressor Protection:

 This feature forces the compressor to wait a few 

minutes before restarting, to prevent equipment damage. During the wait time, 

the message “Wait” is displayed on screen.
Auto Changeover:

 This feature is used in climates where both air conditioning 

and heating are used on the same day. When the system is set to Auto, 

the thermostat automatically selects heating or cooling depending on the 

indoor temperature. Heat and cool settings must be at least 3 degrees apart. 

The thermostat will automatically adjust settings to maintain this 3-degree 

separation.

To avoid possible compressor damage, do not use Auto 

Changeover if the outside temperature drops below 50°F (10°C).