508146-01
Issue 2051
Page 28 of 7
0
Primary Limit Control (S10)
Figure 15 shows the primary limit (S10) used on
GUH97M
units located in the heating vestibule panel. When excess
heat is sensed in the heat exchanger, the limit will
open. Once the limit opens, the furnace control
energizes the supply air blower and de-energizes the
gas valve. The limit automatically resets when unit
temperature returns to normal. The switch is factory set
and cannot be adjusted. In the event of restricted air
flow, the integrated control will reduce firing rate and
indoor blower airflow in 10% increments until a
sustainable air flow is reached. If the furnace reaches
35% firing rate, and adequate air flow is not available,
the furnace will shutdown and enter one hour
watchguard. For limit replacement remove wires from limit
terminals and rotate limit switch 90 degrees. Slowly
remove from the vestibule panel.
install limit face down
Figure 15.
Gas Valve (GV1)
The
GUH97M
uses a variable capacity gas valve
(Figure 16) and is applicable for two-stage or
variable capacity settings. See “Thermostat selection
modes” in the integrated control section for more
details The valve is internally redundant to assure safety
shut-off. If the gas valve must be replaced, the same type
valve must be used.
A 24VAC 2-pin plug and gas control switch are located on
the valve. 24V applied to the pins enables valve
operation. Inlet and outlet pressure taps are located on
the valve.
LPG change over kits are available from
Comfort-Aire/
Century
. Kits include burner orifices and an LP gas valve.
Danger of explosion.
There are circumstances in which odorant
used with LP/propane gas can lose its scent.
In case of a leak, LP/propane gas will settle
close to the floor and may be difficult to
smell. An LP/propane leak detector should
be installed in all LP applications.
DANGER
The burner box is sealed and operates under a negative
pressure. A pressure hose is connected from the burner
box to the gas valve. The gas valve senses the pressure
in the burner box and uses that to set the maximum
manifold pressure while the pressure switch with pressure
conditioning device adjusts the gas flow. This will
compensate for different vent configurations which can
greatly affect the rate of the unit.
Figure 16.
Combustion Air Inducer (B6) & Pressure Switch (S18)
All
GUH97M
units are equipped with a combustion air
inducer (B6) and dual pressure switch assembly (high fire
and low fire). The pressure switch (Figure 17) serves four
functions. First it establishes calibration points for the vent
calibration routine. The combustion air inducer’s speed at a
given firing rate is a function of the vent system resistance.
The calibration routine establishes the inducer speed
required to make low and high fire switches for a given
vent pipe installation and interpolates the speeds required
to achieve all intermediate rates between these two points.
The setting for lowfire switch on the assembly is such that
it does not normally enter into the vent calibration routine.
Second, the switch proves combustion air inducer
operation by sensing a vacuum energizing the control
circuit and allowing ignition. The low fire pressure switch
provides this function.
Third, the switch interrupts the combustion process in the
event vent outlet or combustion air intake blockage.
