Lennox F8AUH series Service Manual page 13

LIMIT CONTROL FOR
F8AUH SERIES UNITS
THIS TYPE AUTO-RESET LIMIT
IS USED FOR THE PRIMARY LIMIT (S10) AND
SECONDARY LIMIT (S21)
FIGURE 12
10−Gas Valve
The F8AUH uses a gas valve manufactured by White
Rodgers (figure 13) or Honeywell (figure 14). The valve 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 top of
the valve. All terminals on the gas valve are connected to
wires from the electronic ignition control. 24V applied to the
terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A regu-
lator adjustment screw is located on the valve.
LPG change over kits are available. Kits include burner ori-
fices and a gas valve regulator conversion kit. See optional
accessories in the SPECIFICATIONS section of this manual
for kit number.
WHITE RODGERS 36E SERIES GAS VALVE
GAS VALVE SHOWN IN OFF POSITION
FIGURE 13
HONEYWELL VR8205 SERIES GAS VALVE
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
GAS VALVE SHOWN IN OFF POSITION
FIGURE 14
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
MANIFOLD
PRESSURE
OUTLET
MANIFOLD
PRESSURE
OUTLET
ON
OFF
11−Combustion Air Blower Prove
(Pressure) Switch (S18)
F8AUH series units are equipped with a combustion air
prove switch located on the vestibule panel. The switch is
connected to the combustion air blower housing by means
of a flexible silicone hose. It monitors air pressure in the
combustion air blower housing.
The switch is a single-pole single-throw pressure switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the combus-
tion air blower is not operating or if the flue becomes ob-
structed.
COMBUSTION AIR BLOWER
PROVE SWITCH
Normally Open
Closes on Negative Pressure
PROVE
SWITCH
On start-up, the switch senses that the combustion air
blower is operating. It closes a circuit to the furnace control
when pressure inside the combustion air blower de-
creases to a certain set point. Set points vary depending
on unit size. The pressure sensed by the switch is relative
to atmospheric pressure. If the flue becomes obstructed
during operation, the switch senses a loss of negative
pressure (pressure becomes more equal with atmospher-
ic pressure) and opens the circuit to the furnace control
and gas valve. A bleed port on the switch allows dry relativ-
ity air in the vestibule to purge switch tubing, to prevent
condensate build up. The switch also has an internal inline
orifice, designed to prevent nuisance shut downs due to
erratic vent pressure fluctuations.
The switch is factory set and is not field adjustable. It is a
safety shut-down control in the furnace and must not be
by−passed for any reason. If switch is closed or by−passed,
the control will not initiate ignition at start up.
Page 13
PROVE SWITCH
Sensing Tube
Attaches to Top
Side Of Blower
Back
Left
Bottom
FIGURE 15
Top
Right
Front