Honeywell T8400C1099 User manual

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OWNER’S GUIDE
Electronic
Thermostat
FAN SYSTEM
Auto On Cool Off Heat
Set Room
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Congratulations on the purchase of your new thermostat!
Over 100 years of Honeywell engineering expertise went
into the making of this thermostat in an effort to provide
you with a more comfortable and convenient living
environment.
•Simple and easy to use . The ▲ and ▼keys make it
easy for you to change your temperature setting.
•Large digital display. You can easily read the current
room temperature at a glance.
•No need for batteries. Batteries are not required
because your temperature settings are held perma-
nently in memory.

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Settings
Setting FAN and SYSTEM Switches
Fan and system settings are controlled by using FAN and
SYSTEM switches located on the bottom of the thermostat
case.
First set the FAN switch:
Auto: Normal setting for most homes. In cooling, the
fan goes off and on with the cooling equipment.
In heating, the fan may start a few minutes after
the heating equipment comes on.
On: The fan runs continuously. Use for improved air
circulation and air quality.
FAN
Auto On
FAN
SWITCH M10273

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Then set the SYSTEM switch:
Cool: Normal setting for months when your home
needs cooling (air conditioning). Digital display
shows a snowflake when cooling is on.
Off: Both heating and cooling are off.
Heat: Normal setting for months when your home
needs heating. Digital display shows a flame
when heating is on.
SYSTEM
Cool Off Heat
SYSTEM
SWITCH
M14228
IMPORTANT
Do not operate the T8400 and T8401 in cooling
mode when the outdoor temperature is below
50
° F (10 ° C), or damage to compressor could
result.

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Displaying Temperature Setting
NOTE: Your temperature setting can be set anywhere
from 40°to 99°F (5°to 37°C).
The temperature setting and the room temperature are
shown separately on the digital display. The ▼ indicator
points to Set when the temperature setting is displayed
and to Room when the room temperature is displayed.
1. To display the temperature setting on the digital
display, press either the ▲or ▼key once.
Set Room
INDICATOR
M18441

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Changing Temperature Setting
1. To lower the temperature setting, press the ▼ key.
When in the heating mode, lowering the temperature at
night or during the day can lower your energy costs.
2. To raise the temperature setting, press the ▲ key.
When in the cooling mode, raising the temperature
before you leave for work or school also lowers your
energy costs.
Set Room
M18442
Set Room
M18443

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Troubleshooting Guide
CAUTION
Operating at too low of an outdoor temperature
may cause compressor damage.
Damage to compressor possible.
Do not operate cooling if outdoor temperature is
below 50°F (10°C). Refer to manufacturer’s
recommendations.
Heating or cooling does not come on.
•Check power to the heating or cooling equipment.
For compressor protection on initial startup and during
power interruptions, the T8400 and T8401 Electronic
Thermostats go into a five-minute delay.During this
delay, the cooling icon, a snowflake flashes.
•Check that the SYSTEM switch on the thermostat is set
to Heat or Cool.

•Check the system fuse or circuit breaker and replace or
reset if necessary.
•Check for air movement at the register.
Temperature setting is too warm or cool
•Adjust the temperature by pressing either the ▲or ▼
key.
If you have questions concerning this thermostat, call
Honeywell Customer Assistance at 1-800-468-1502.
69-1491-1 G.H. Rev. 11-02
Printed in U.S.A.
Copyright © 2002 Honeywell
All Rights Reserved ® U.S. Registered Trademark
Honeywell Limited- Honeywell Limitée
35 Dynamic Drive
Scarborough, Ontario
M1V 4Z9
Automation and Control Solutions
Honeywell
1985 Douglas Drive North
Golden Valley, MN 55422
www.honeywell.com/yourhome