508183-01
Issue 2114
Page 12 of 52
Heating Components
Ignitor
The ignitor is made of durable silicon nitride. Ignitor
longevity is enhanced by controlling voltage to the ignitor.
The integrated control provides a regulated 120 volts to the
ignitor for a consistent ignition and long ignitor life. Ohm
value should be 39 to 70. See Figure 9 for ignitor location
and Figure 10 for ignitor check out.
NOTE:
The
GUH96T
furnace contains electronic
components that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.
Flame Sensor
A flame sensor is located on the left side of the burner
support. See Figure 9. The sensor tip protrudes into the
flame envelope of the left-most burner. The sensor can
be removed for service without removing any part of the
burners. During operation, flame is sensed by current
passed through the flame and sensing electrode. The
control allows the gas valve to remain open as long as
flame signal is sensed. See Table 5 for flame signal.
Normal
Low
Drop Out
1.5 or greater
1.0 or less
0.5
Table 5. Flame Signal in Microamps
Gas Valve
The valve (Figure 50) is internally redundant to assure
safety shut-off. If the gas valve must be replaced, the same
type valve must be used.
24VAC terminals and gas control knob are located on the
valve. A wire harness connects the terminals from the gas
valve to the electronic ignition control. 24V applied to the
terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the valve.
LPG change over kits are available from
Comfort-Aire/
Century
Air. Kits include burner orifices and a gas valve.
Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit located on
top of the burner box, one on each side.- See Figure 9.
The limit is a N.C. SPST manual-reset limit. When S47
senses rollout, the circuit breaks and the ignition control
immediately stops ignition and closes the gas valve.
Rollout can be caused by a blocked heat exchanger, flue
or lack of combustion air. The switch is factory set to trip
(open) at 210°F and cannot be adjusted. The switch can
be manually reset. To manually reset a tripped switch,
push the reset button located on the control.
Burners
All units use inshot burners. Burners are factory set and
require no adjustment. Always operate the unit with the
burner box front panel in place. Each burner uses an orifice
that is precisely matched to the burner input. Burners can
be removed as a one piece assembly for service. If burner
assembly has been removed, it is critical to align center
of each burner to the center of the clamshell when re-
installing. See more detail in Maintenance section.
Primary Limit Control (S10)
The primary limit (S10) is located in the heating vestibule
panel. When excess heat is sensed in the heat exchanger,
the limit will open. If the limit is open, the furnace control
energizes the supply air blower and closes the gas valve.
The limit automatically resets when unit temperature
returns to normal. The switch must reset within three
minutes or the control will go into Watch guard for one
hour. The switch is factory set and cannot be adjusted. The
switch may have a different set point for each unit model
number.
Combustion Air Inducer (B6) and Cold End
Header Box
All
GUH96T
units use a two-stage combustion air inducer
to move air through the burners and heat exchanger
during heating operation. The blower uses a 120VAC
motor. The motor operates during all heating operation
and is controlled by integrated control control A92. The
inducer also operates for 15 seconds before burner
ignition (prepurge) and for 5 seconds after the gas
valve closes (postpurge). The inducer operates on low
speed during first-stage heat, then switches to high
speed for second stage heat.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece made
of hard plastic. The box has an internal channel where the
combustion air inducer creates negative pressure at unit
start up. The channel contains an orifice used to regulate
flow created by the combustion air inducer. The box has
pressure taps for the combustion air inducer pressure
switch hoses. The pressure switch measures the pressure
differential across the combustion air inducer orifice or
difference in the channel and the box. If replacement is
necessary the gaskets used to seal the box to the vestibule
panel and the combustion air inducer to the box, must also
be replaced.
A proving switch connected to the combustion air inducer
orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different for each
model. The pressure switch measures the pressure
differential across the combustion air inducer orifice.
When the proving switch opens, the furnace control
(A92) immediately closes the gas valve to prevent burner
operation.
