The T8400 and T8401 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, 4, 5, 6, 9, or
12 cph. Cooling cycle rate is fixed at 3 cph. Temperature
indication can be set for °F or °C.
The T8400 Thermostat is powered through the heating/
cooling system controls. The T8401 requires a common
wire for operation. Batteries are not required because
setpoints are held permanently by non-volatile memory.
The T8400 and T8401 models include a thermostat,
wallplate (for wiring and mounting thermostat) and
If this control is replacing a control that contains
mercury in a sealed tube, do not place your old
control in the trash.
Contact your local waste management authority
for instructions regarding recycling and the
proper disposal of an old control containing
mercury in a sealed tube.
When Installing this Product...
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a hazard-
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable for
3. Installer must be a trained, experienced service
4. After installation is complete, check out product
operation as provided in these instructions.
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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 temperature.
See Fig. 1. Do not install the thermostat where it can be
— 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.
Mounting Wallplate to Wall
Level only for appearance. The thermostat func-
tions normally even when not level.
Mount the T8400 and T8401 with the screws provided
(see Fig. 2) as follows:
1. Place the wallplate at the desired location on the
2. Pull the thermostat wire through the entrance hole
on the wallplate.
3. Fasten the wallplate to the wall using the anchors
and screws provided.
4. After wiring the wallplate, plug the hole to prevent
drafts from affecting the thermostat; see Wiring