Gas Pressure Adjustment - Lennox Merit Series Installation Instructions Manual

TURNING OFF GAS TO UNIT
1. Set the thermostat to the lowest setting.
2. Turn off all electrical power to the unit if service
is to be performed.
3. Remove the upper access panel.
4. Move lever on gas valve to OFF. See Figure 19.
5. Replace the upper access panel.
FAILURE TO OPERATE
If the unit fails to operate, check the following:
1. Is the thermostat calling for heat?
2. Are access panels securely in place?
3. Is the main disconnect switch closed?
4. Is there a blown fuse or tripped circuit breaker?
5. Is the filter dirty or plugged? Dirty or plugged filters will
cause the limit control to shut the unit off.
6. Is gas turned on at the meter?
7. Is the manual main shut off valve open?
8. Is the internal manual shut off valve open?
9. Is the unit ignition system in lock out? If the unit locks
out again, call the service technician to inspect the unit
for blockages.
10. Is pressure switch closed? Obstructed flue will cause
unit to shut off at pressure switch. Check flue and outlet
for blockages.
11. Are flame roll-out switches tripped? If flame
roll-out switches are tripped, call the service technician
for inspection.
HEATING SEqUENCE OF OPERATION
(follow steps below or see Figure 18 for more detail)
1. When thermostat calls for heat, combustion
air blower starts.
2. Combustion air pressure switch proves blower operation.
Switch is factory-set and requires no adjustment.
3. After a 15-second prepurge, the hot surface
ignitor energises.
4. After a 20-second ignitor warm-up period,
the gas valve solenoid opens. A 4-second trial
for ignition period begins.
5. Gas is ignited, flame sensor proves the flame,
and the combustion process continues.
6. If flame is not detected after first ignition trial, the
ignition control will repeat steps 3 and 4 four more
times before locking out the gas valve ("WATCHGUARD"
flame failure mode). The ignition control will then
automatically repeat steps 1 through 6 after 60 minutes.
7. To interrupt the 60-minute "WATCHGUARD" period,
move thermostat from "Heat" to "OFF" then back to
"Heat." Heating sequence then restarts at step 1.

GAS PRESSURE ADJUSTMENT

GAS FLOW
To check for proper flow to combustion chamber, determine
Btu input from appliance rating plate. Divide this input rating
by the Btu per cubic foot of available gas. Result is the
required number of cubic ft. per hour. Determine the flow of
gas through the gas meter for two minutes and multiply by
30 to get the hourly flow of gas.
SUPPLY PRESSURE MEASUREMENT
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 barb fitting and connect a
manometer to measure supply pressure. See table 4 for
proper line pressure. Replace the threaded plug after
measurements have been taken.
MANIFOLD PRESSURE MEASUREMENT
1. Remove the threaded plug from the outlet side of the gas
value and install a field-provided barbed fitting. Connect
to manometer to measure manifold pressure.
2. Start unit and allow 5 minutes for unit to reach steady state.
3. While waiting for the unit to stabilise, observe the flame.
Flame should be stable and should not lift from burner.
Natural gas should burn blue.
4. After allowing unit stabilise for 5 minutes, record manifold
pressure and compare to value given in table 4.
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.
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