Lennox Merit ML296UH110XV60C - Unit Adjustments and Operation Modes

Page 51
Other Unit Adjustments
Primary Limit.
The primary limit is located on the heating compartment
vestibule panel. This limit is factory set and requires no
adjustment.
Flame Rollout Switches (Two)
These manually reset switches are located on the front of
the burner box.
Pressure Switch
The pressure switch is located in the heating compartment
on the cold end header box. This switch checks for proper
combustion air inducer operation before allowing ignition
trial. The switch is factory-set and must not be adjusted.
Temperature Rise
Place the unit into operation with a second-stage heating
demand. After supply and return air temperatures have
stabilized, check the temperature rise. If necessary, adjust
the heating blower speed to maintain the temperature rise
within the range shown on the unit nameplate. See “Al-
lowable Heating Speeds” table on page 44. Increase the
blower speed to decrease the temperature rise. Decrease
the blower speed to increase the temperature rise. Failure
to properly adjust the temperature rise may cause erratic
limit operation.
Electrical
1 - Check all wiring for loose connections.
2 - Check for the correct voltage at the furnace (furnace
operating). Correct voltage is 120VAC + 10%
3 - Check amp-draw on the blower motor with blower
access panel in place.
Unit Nameplate__________Actual__________
Exhaust and Air Intake Pipe
1 - Check exhaust and air intake connections for
tightness and to make sure there is no blockage.
2 - Is pressure switch closed? Obstructed exhaust pipe
will cause unit to shut o󰀨 at pressure switch. Check
termination for blockages.
3 - Obstructed pipe or termination may cause rollout
switches to open. Reset manual ame rollout
switches on burner box assembly if necessary.
Heating Sequence of Operation
On a call for heat from the room thermostat, the control
board performs a 1 second self check. Upon conrma-
tion that the pressure switch contacts are in an open posi-
tion, the control energizes the combustion blower on high
speed. The control then checks for adequate combustion
air by making sure the low-re pressure switch contacts
are closed.
The igniter energizes and is allowed to warm up for 20
seconds before the gas valve energizes on 1st stage and
burners ignite. 45 seconds after the control conrms igni-
tion has occurred, the control drops the combustion blow-
er to low speed.
The circulating blower ramps up to 50% of 1st stage heat
speed and operates at that speed for one minute (includ-
ing ramp up time), then at 75% of 1st stage heat speed for
an additional minute. After that, the circulating blower op-
erates at full 1st stage heat speed until either the heat call
is satised or the thermostat initiates a call for 2nd stage
heat. On a call for 2nd stage heat, the control energizes
the circulating air blower on full CFM 2nd stage heat.
If the automatic heat staging option is being used the fur-
nace does not switch to 2nd stage heat in response to a
call from the thermostat but instead operates at 1st stage
heat for the duration of the selected time before automati-
cally switching to 2nd stage heat.
When the call for heat is satised, the gas valve and com-
bustion air blower shut down. The control board shuts o󰀨
the gas valve and runs the combustion blower for an ad-
ditional 15 seconds. The circulating air blower continues
to run for 2 minutes at 82% of the selected heating speed
(low re or high re) before ramping down.
In the event the unit loses ignition, the control will attempt
to recycle up to ve times before it goes into a 1 hour lock-
out. Lockout may be manually reset by removing power
from the control for more than 1 second or removing the
thermostat call for heat for more than 3 seconds.
If during a heating cycle the limit control senses an ab-
normally high temperature and opens, the control board
de-energizes the gas valve and the combustion blower
while the circulating blower ramps up to 2nd stage heat
speed. The circulating blower remains energized until the
limits are closed.
Fan On
When the thermostat is set for continuous fan operation
and there is no demand for heating or cooling, a call for
fan closes the R to G circuit and the circulating blower mo-
tor runs at 38% of the selected cooling CFM until switched
o󰀨. When the call for fan is turned o󰀨, the control de-ener-
gizes the circulating blower.
Cooling
The unit is set up at the factory for single stage cooling.
For two stage cooling operation, clip the jumper wire lo-
cated between the Y to Y2 terminals on the integrated ig-
nition/ blower control board. If the active dehumidication
feature is enabled, the circulating blower runs at 70% of
the selected cooling speed as long as there is a call for
dehumidication.
WARNING
The system must not be in either the passive or active
dehumidication mode when charging a cooling system.