Lennox EL180UHE Series Manual page 15

11. Combustion Air Inducer
Pressure Switch (S18)
EL180UHE(X) series units are equipped with a combustion
air pressure switch located on the combustion air inducer
orifice bracket. The switch is connected to the combustion
air inducer housing by means of a flexible silicone hose. It
monitors negative air pressure in the combustion air induc­
er housing.
The switch is 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 combus­
tion air inducer is not operating or if the flue becomes ob­
structed.
On start‐up, the switch senses that the combustion air in­
ducer is operating. It closes a circuit to the integrated con­
trol when pressure inside the combustion air inducer de­
creases to a certain set point. Set points vary depending on
unit size. See table 7. The pressure sensed by the switch is
negative relative to atmospheric pressure. If the flue be­
comes obstructed during operation, the switch senses a
MULTI−METER
SET TO MEASURE OHMS
loss of negative pressure (pressure becomes more equal
with atmospheric pressure) and opens the circuit to the in­
tegrated control and gas valve. A bleed port on the switch
allows relatively dry air in the vestibule to purge switch tub­
ing, to prevent condensate build up.
EL180UHE(X)
045E36A
070E36B
090E48B, 090E60C
110E60C
135E60D
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
integrated control will not initiate ignition at start up.
See figure 10 for measuring operating pressure and check­
ing resistance in the pressure switch.
To Pressure Switch
+
FIGURE 10
Page 15
TABLE 7
inches w.c.
Make
Break
-0.75
-0.83
-0.80
-0.83
-0.80
Troubleshooting
Remove tubing from CAI and
insert "Tee" and additional tubing.
Field Provided
Tubing To CAI
Port
High
-
or
Low
+ 0.05
-0.65
-0.68
-0.65
-0.68
-0.65