T8400B Thermostat provides single-stage, non-
programmable temperature control for 24V heat-only
applications. Heating cycle rate is selectable at 1, 3, 4,
5, 6, 9, or 12 cph. Temperature indication can be set for
°F or °C.
The T8400B Thermostat is powered through the heating
The T8400C and T8401C Thermostats provide single-
stage, non-programmable temperature control for 24V
heating-cooling systems with manual changeover from
heat to cool. Heating cycle rate is selectable at 1, 3, 6,
9, or 12 cph. Cooling cycle rate is fixed at 3 cph.
Temperature indication can be set for °F or °C.
The T8400C Thermostat is powered through the
heating/cooling system controls. The T8401C Thermo-
stat is powered directly from the system transformer.
Batteries are not required because setpoints are held
permanently by nonvolatile memory for all models.
All models include a thermostat, wallplate (for wiring
and mounting thermostat) and Owner's Guide. Two
decorator cover plates (for covering wall marks) are
included for select models.
If this control is replacing a control that contains
mercury in a sealed tube, do not place your old
control in the trash. Dispose of properly.
Contact your local waste management authority
for instructions regarding recycling and the
proper disposal of an old control.
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When Installing this Product...
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable
for your application.
3. Installer must be a trained, experienced service
4. After installation is complete, check out product
operation as provided in these instructions.
Electrical Shock or Equipment Damage
Can shock individuals or short equipment
Disconnect power supply before installation.
Install the thermostat about 5 ft (1.5m) above the floor in
an area with good air circulation at average tempera-
ture. See Fig. 1. Do not install the thermostat where it
can be affected by:
— drafts or dead spots behind doors and in corners.
— hot or cold air from ducts.
— radiant heat from the sun or appliances.
— concealed pipes and chimneys.
— unheated (uncooled) areas such as an outside wall
behind the thermostat.
This thermostat is a precision instrument and was
carefully adjusted at the factory. Handle it carefully.