Combustion Air Inducer Prove Switch; Primary Limit Location - Lennox G61MPV Service Manual

PRIMARY LIMIT LOCATION

FIGURE 16
13. Combustion Air Inducer
Prove Switch (S18)
All G61MPV units are equipped with combustion air prove
switches located on the combustion air inducer housing.
G61MPV−070, −071, −090, −091, −110, −111 and −135 model
units are equipped with two dual prove switch assemblies"
consisting of two switches. See figure 17.
G61MPV−045 units have two single swithes as shown in fig-
ure 18. The switches are connected to the cold end header
box by means of a flexible hose that monitors negative air
pressure in the cold end header box.
The switches are a single-pole single-throw proving switch
electrically connected to the furnace control. The purpose of
the switch is to prevent burner operation if the combustion
air inducer is not operating or if the flue becomes obstructed.
On heat demand (first or second stage) the switch senses
that the combustion air inducer is operating. It closes a cir-
cuit to the furnace control when pressure inside the cold
end header box decreases to a certain set point. Set points
vary depending on unit size. See tables 26 through 31. The
pressure sensed by the switch is negative. If the air intake
vent pipe or outlet vent pipe becomes obstructed during
operation, the switch senses a change of negative pres-
sure and opens the circuit to the furnace control and gas
valve. A bleed port on the switch allows relatively dry air in
the vestibule to purge switch tubing, to prevent condensate
build up.
The switch is factory set and is not field adjustable. If switch
is closed or jumpered, the control will not initiate ignition at
start up.
limit shield
090, −110 and
−135 only
limit faces shield
The prove switch is a safety shut−down control in
the furnace and must not be jumpered for any rea-
son.
DUAL COMBUSTION AIR PROVE SWITCH
(−070, −071, −090, −091, −110, −111 −135 SHOWN)
second
stage
first stage
To troubleshoot the prove switches, temporarily jumper them.
The unit will not fire with the switches jumpered. Therefore, the
prove switches must be bypassed after the combustion air in-
ducer is activated. This will determine if the prove switches
and furnace are operating properly. However, this may not in-
dicate if the sealed combustion system is operating properly.
Checks of pressure differential can aid in troubleshooting.
The
When measuring the pressure differential, readings should be
taken at the prove switch. Lack of differential usually indicates
problems in the intake or exhaust piping, but may indicate
problems in the heat exchanger, condensing coil, head-
er boxes, combustion inducer or other components.
Page 44
WARNING
negative
pressure
FIGURE 17
positive
pressure