Lennox G27M Series Unit Information

Page 11
FIGURE 15
PROVE SWITCH
COMBUSTION AIR BLOWER
PROVE SWITCH
Normally Open
Closes on Negative Pressure
PROVE
SWITCHES
sensing tube
Ts the two switches
together and attaches
to side of blower
Top
Bottom
Left
Right
Back
Front
7--Combustion Air Blower Prove
(Pressure) Switches (S128 and S102)
G27M series units are equipped with two combustion air
prove switches. S128 (low fire) and S102 (high fire) are lo-
cated on the vestibule panel. The switches T togeth er
and are connected to the combustion air blower housing
by a flexible silicone hose. The switches monitor air pres-
sure in the combustion air blower housing.
Both switches are N.O. SPST auto-reset pressure
switches electrically connected totheintegratedcon-
trol A3. The switches prevent burner operation if the
combustion air blower is not operating properly and
in cas e of flue block age.
On s tart-up , the S 128 and S102 s witc hes sens e w he --
ther the combu st ion air blower is o perati ng. The
switches close a circuit to the burner control when pres-
sure inside the combustion air blower decreases below a
given setpoint (see table 5). The pressure sensed by the
switches is relative to atmospheric pressure. If the flue be-
comes obstructed durin g operation, and either switch
senses a loss of negative pressure (pressure approaches
atmospheri c pressure) the switch opens the circuit to the
burner control and gas valve. Either switch, if opened, will
open the circuit. The switch trip pressures are differen t de-
pending on unit size. The trip pressures are printed on the
side of the switches.
All the switches prov ide factory set and are not field ad-
justable. The switches provide a safety shut-down control
in the furnace and must not be bypassed for any reason.
TABLE 5
MODEL
NUMBER
S102
HIGH
S128
LOW
G27M3--60 .52W.C. + 0.05W.C.
(129.37 Pa +
12.43Pa)
0.29W.C + 0.05W.C.
(72.10Pa +
12.43Pa)
G27M--75 0.56W.C. + 0.05W.C.
(139.23Pa +
12.43Pa)
0.29W.C. + 0.05W.C.
(72.05Pa +
12.43Pa)
G27M--100 0.65W.C. + 0.03W.C.
(161.61Pa +
7.5Pa)
0.43W.C. + 0.05W.C.
(107Pa +
12.43Pa)
G27M--120 0.68W.C. + 0.03W.C.
(169Pa +
7.53Pa)
0.53W.C. + 0.05W.C.
(131.77Pa +
12.43Pa)
NOTE: All measurements are negative.
II--PLACEMENT AND INSTALLATION
Make sure unit is installed in accordance with the G27M
installation instructions.
III--START-UP
A--Preliminary a nd Seasonal Checks
1 -- Inspect electrical wiring, both field and factory
installed for loose connections. Tighten as required.
2 -- Check voltage at disconnect switch. Voltage must be
within range listed on the nameplat e. If not, consult
the powe r c ompany and have voltag e co nditi on
cor rec ted b efore s tarti ng unit.
B--Heating Start-Up
1 -- Set thermostat to OFF position. Close manual knob
on gas valve.
2 -- Wait 5 minutes.
3 -- Open manual knob on gas valve, replace the front
access panel and turn on unit electrical supply.
WARNING
Shock and burn hazard.
G27M units are equipped with a hot surface
ignition system. Do not attempt to light manually .
4 -- Set fan switch to AUTO or ON and move system
selection switch to HEAT. Adjust thermostat to a
setting above room temperature.
5 -- If unit does not light the first time, it will attempt
four more ignitions before locking out.
6 -- If lockout occurs, repeat steps 1, 2, 3 and 4.
7 -- On start-up check operation of both high and low
fire. Refer to section IV item E for manifold pressure
check procedure.
C--Safety or Emergency Shutdown
Turn off unit power. Close manual and main gas valves.
D--Extended Period Shutdown
Turn off thermostat or set to UNOCCUPIED mode.
Close all gas valves (both internal and external to unit)
to guarantee no gas leak into combustion chamber.
Turn off power to unit. All access panels, covers and
vent caps must be in place and secured.

Lennox G27M Series Specifications

General

Brand	Lennox
Model	G27M Series
Category	Furnace
Language	English