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D−Gas Pressure Adjustment
1 − Operate unit at least 15 minutes before checking gas
flow. Determine the time in seconds for one revolu-
tions of gas through the meter. A portable LP gas me-
ter (17Y44) is available for LP applications.
2 − Compare the number of seconds and the gas meter
size in table 39 to determine the gas flow rate. Multiply
the gas flow rate by the heating value to determine the
unit input rate. If manifold pressure is correct and the
unit input rate is incorrect, check gas orifices for proper
size and restriction.
3 − Remove temporary gas meter if installed.
NOTE − To obtain accurate reading, shut off all other gas
appliances connected to meter.
TABLE 39
Gas Flow Rate (Ft.
3
/Hr.)
Seconds for 1
Revolution
Gas Meter Size
1/2 cu ft Dial 1 cu ft Dial
10 180 360
12 150 300
14 129 257
16 113 225
18 100 200
20 90 180
22 82 164
24 75 150
26 69 138
28 64 129
30 60 120
32 56 113
34 53 106
36 50 100
38 47 95
40 45 90
42 43 86
44 41 82
46 39 78
48 38 75
50 36 72
52 35 69
54 33 67
56 32 64
58 31 62
60 30 60
E−Check Manifold Pressure
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the threaded
plug, install a field−provided barbed fitting and connect a
manometer to measure supply pressure. Replace the
threaded plug after measurements have been taken.
Manifold Pressure Measurement
To correctly measure manifold pressure, the differential
pressure between the positive gas manifold and the nega-
tive burner box must be considered. Use pressure test
adapter kit (available as Lennox part 10L34) to assist in
measurement.
1 − Remove the threaded plug from the outlet side of the
gas valve and install a field−provided barbed fitting.
Connect test gauge +" connection to barbed fitting to
measure manifold pressure.
2 − Tee into the gas valve regulator vent hose and connect
test gauge −" connection.
3 − Start unit on low heat (40% rate) and allow 5 minutes
for unit to reach steady state.
4 − While waiting for the unit to stabilize, notice the flame.
Flame should be stable and should not lift from burner.
Natural gas should burn blue.
5 − After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in table
40.
6 − Repeat steps 3, 4 and 5 on high heat.
NOTE − Shut unit off and remove manometer as soon as an
accurate reading has been obtained. Take care to remove
barbed fitting and replace threaded plug.
CAUTION
Do not attempt to make adjustments to the gas valve.
TABLE 40
Manifold Pressure (Outlet)
Manifold Pressure at All Altitudes (in. w.g.)
Low Fire High Fire
Natural Gas LP/Propane Natural Gas LP/Propane
1.7 4.9 3.5 10.0
F− High Altitude
NOTE − In Canada, certification for installations at eleva-
tions over 4500 feet (1372 m) is the jurisdiction of local au-
thorities.
The manifold pressure may require adjustment to ensure
proper operation at higher altitudes. Refer to table 40 for
manifold pressure and table 41 for required pressure
switch changes and conversion kits at varying altitudes.
The combustion air pressure switches are factory−set and
require no adjustment.
NOTE − A natural to L.P. propane gas changeover kit is nec-
essary to convert this unit. Refer to the changeover kit
installation instruction for the conversion procedure.
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