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Form I-UDA, P/N 195673 R24, Page 21
8.0 Controls and
Operation
8.1 Thermostat
Use either an optional thermostat available with the heater or a eld-supplied 24-volt
thermostat. Install according to the thermostat manufacturer’s instructions, paying par-
ticular attention to the requirements regarding the location of the thermostat.
Make sure that if there is a heat anticipator setting on the thermostat, it is set at 0.6 amps
(or in accordance with the amperage value noted on the heater wiring diagram).
Make thermostat connections at the terminal strip on the back of the heater. The strip
has four terminals, R, G, W1, and W2; refer to the wiring diagram.
IMPORTANT: All units MUST be operated by a 24-volt thermostat. Never use a
line voltage disconnect switch as a means of operating the heater. Operating Model
Sizes 30-125 by means other than a 24-volt thermostat may result in the ame rollout
switch tripping.
Unit Mounted Thermostat, Option CM3 - If the heater was ordered with Option CM3,
a kit for mounting the thermostat on the rear of the heater is shipped separately. Follow
the instructions in the option package to attach the bracket.
Select a snap-action thermostat when using the unit mounted thermostat bracket. Do
not use a mercury switch thermostat because the vibration may cause excessive unit
cycling. Be careful with the thermostat leads; shorting the thermostat wires to a metal
surface will cause the transformer to fail.
Sizes 30-125 are equipped with a temperature activated manually reset ame rollout
switch. The ame rollout switch is located at the top of the burner assembly. It is fac-
tory set and is non-adjustable. If the setpoint is reached, the ame rollout acts to inter-
rupt the electric supply to the gas valve. If the ame rollout switch activates, identify
and correct the cause before resetting the switch. Refer to the Maintenance Section,
Paragraph 10.2.10, for information on probable causes and instructions on resetting
the ame rollout switch. (For location, see FIGURE 18, page 30.)
DANGER
If the manual reset ame rollout switch activates, identify and
correct the cause before resetting the switch. Never bypass the
ame rollout switch; hazardous conditions could result. See
Hazard Intensity Levels, page 2.
7.5.3 Door Switch -
Model UDAS only
All sizes of Model UDAS heaters are equipped with a door switch which prevents
the heater from operating when the service door panel is open. The service panel of
a Model UDAS is equipped with a pliable gasket material that fully seals the door to
provide added protection from building air entering the combustion zone of the heater.
(For switch location, see FIGURE 18, page 30.)
The main operating gas valve is powered by the 24-volt control circuit through the ther-
mostat and safety controls. The main control valve is of the diaphragm type providing
regulated gas ow preset at the factory. (For location, see FIGURE 18, page 30.)
WARNING
The operating valve is the prime safety shutoff. All gas supply
lines must be free of dirt or scale before connecting the unit to
ensure positive closure. See Hazard Levels, page 2.
7.5.4 Gas Valve
7.5.5 Fan Motor
The fan motor is equipped with thermal overload protection of the automatic reset type.
Should the motor refuse to run, it may be because of improper current characteristics.
Make certain that the correct voltage is available at the motor.
NOTE: If the unit is equipped with an optional totally enclosed motor or optional volt-
age, the horsepower may be larger than the standard motor. Refer to the motor name-
plate to verify horsepower.
7.5.6 Venter Motor
The venter motor is assembled to the venter wheel and operates to provide combus-
tion airow. Operation is controlled by the ignition control module (circuit board); refer
to ignition system in Paragraph 8.3.
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