Lennox SLP98DF090XV48C

Page 55
Furnace should operate at least 5 minutes before checking
gas flow. Determine time in seconds for two revolutions of
gas through the meter. (Two revolutions assures a more
accurate time.) Divide by two and compare to time in
table 28. If manifold pressure matches table 33 and rate is
incorrect, check gas orifices for proper size and restriction.
Remove temporary gas meter if installed.
E-Testing Gas Supply Pressure
When testing supply gas pressure, connect test gauge to
supply pressure tap on the gas valve. See figure 46. Check
gas line pressure with unit firing at maximum rate. Low pres
sure may result in erratic operation or underfire. High pres
sure can result in permanent damage to gas valve or over
fire. See table 29 for operating pressure at unit gas connec
tion (line).
On multiple unit installations, each unit should be checked
separately, with and without units operating. Supply pres
sure must fall within range listed in table 29.
TABLE 29
All SLP98DFV Units
Natural LP
Line Pressure WC” 4.5 - 10.5 11.0 - 13.0
Manifold Pressure Measurement (Figure 48)
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 measuring device “+” connection to barbed
fitting to measure manifold pressure.
2 - Tee into the gas valve regulator vent hose and connect
measuring device “-” connection.
3 - Start unit on low heat (35% 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
33.
6 - Repeat steps 3, 4 and 5 on high heat.
7 - Shut unit off and remove manometer as soon as an ac
curate reading has been obtained. Take care to re
move barbed fitting and replace threaded plug.
8 - Start unit and perform leak check. Seal leaks if found.
Operating Pressure Signal (Delta P) Measurement
(Figure 49)
Operating pressure signal can be taken while the manifold
pressure pressure check is taken (using two measuring de
vices). Or, taken after the manifold pressure measurement
is complete.
1 - Tee into the negative line between the gas valve and
pressure switch and connect to measuring device neg
ative “-”.
2 - Tee into the positive line between the gas valve and
pressure switch and connect to measuring device posi
tive “+”.
3 - Start unit on low heat (35% rate) and allow 5 minutes
for unit to reach steady state.
4 - After allowing unit to stabilize for 5 minutes, record op
erating pressure signal and compare to value given in
table 33.
5 - Repeat steps 3 on 4 high heat.
CAUTION
Do not attempt to make adjustments to the gas valve.