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Main Limit
Each unit comes with a high
temperature limit control switch to
monitor air temperature in the heat
,:,xchanger area. If the temperature
rises above its set point the limit
c:ontac1s open to shut down the
burners and operate the main blower
and IDB. This is a 24
volt control and
automatically resets,
Heat Assisted Limit Control
(HALC)
Downf1ow/horizontal units have a
heat assisted limit control on the
blower housing. It prevents
excessive temperatures in the blower
compartment. This control is a 3/4
inch open faced disc with a 24 volt
heater energized during normal
furnace operation to accelerate its
response. Eiectncally its contacts
were in series with the gas valve to
shut it down if the limit contacts
open. It automatically resets .
NOTE: Current units now have the
contacts in series with the induced
draft blower motor. This began with
date code F3198.
Blower Door Switch
All furnaces have a blower
compartment door switch to stop all
furnace operation if the blower door
opens, This control is a 120 volt
device and carries all electrical
power to the furnace.
Transformer
All furnaces use the same control
transformer. It is a 40 VA device with
a
120 volt primary winding and 24
volt secondary winding. It supplies all
the control voltage power for both
heating and cooling and mounts to
the side of the main blower housing.
►
Current (-)GRA/(-)GTA furnaces
have a 3 amp, slow blow fuse
mounted on the control board. Earlier
units had a fuse in one of the
secondary wires. The fuse protects
24 volt circuit components.
FIGURE 14
HEAT ASSISTED LIMIT CONTROL
FIGURE 13
LIMIT SWITCH
FIGURE 15
BLOWER DOOR SWITCH
17
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