Lennox LDT Series Information page 35

TABLE 12
Manifold Pressure "W.C.
Natural LP/Propane
Low High Low
1.6+ 0.2 3.7+0.3 5.5+ 0.3
Combustion gases
Flue products must be analyzed and compared to the unit
specifications. Problems detected during the inspection
may make it necessary to temporarily shut down the
furnace until the items can be repaired or replaced.
5-Proper Gas Flow
To check for proper gas flow to burners, determine Btuh
input from unit rating plate or the gas heating capacity in the
SPECIFICATIONS tables. Divide this input rating by the
Btuh per cubic foot of available gas. Result is the number of
cubic feet per hour required. Determine the flow of gas
through gas meter for two minutes and multiply by 30 to get
hourly flow of gas to the burners.
NOTE - To obtain accurate reading, shut off all other gas
appliances connected to meter.
6-Inshot Burner
Burners are factory set for maximum air and cannot be
adjusted. Always operate unit with access panel in place. A
peep hole is furnished in the heating access panel for flame
viewing. Natural gas should burn basically blue with some
clear streaks. L.P. gas should burn mostly blue with some
clear yellow streaks.
Figure 29 shows how to remove burner assembly.
1- Turn off power to unit and shut off gas supply.
2- Remove screws holding the burner support cap.
3- Slide each burner off its orifice.
4- Clean and reassemble (reverse steps 1-3).
5- Be sure to secure all wires and check plumbing.
6- Turn on power to unit. Follow lighting instructions
attached to unit and operate unit in heating mode.
Check burner flames. They should be blue with
yellow streaks.
7-Spark Electrode Gap
The spark electrode assembly can be removed for
inspection by removing two screws securing the electrode
assembly and sliding it out of unit.
For proper unit operation, electrodes must be positioned
and gapped correctly.
Spark gap may be checked with appropriately sized twist
drills or feeler gauges. Disconnect power to the unit and
remove electrode assembly. The gap should be between
0.125" + 0.015" (3.2 mm + .4 mm). See figure 25.
Supply Pressure
"W.C.
LP/
Natural
Propane
High
4.7 -
10.8 -13.5
10.5+
10.5
0.5
Page 35
TYPICAL GAS BURNER ASSEMBLY
BURNERS
BURNER
SUPPORT
ORIFICE
FIGURE 29
8-Heat Exchanger
To Access or Remove Heat Exchanger From Unit:
1- Turn off gas and electric power.
2- Remove access panel(s) and unit center mullion.
3- Remove gas valve, manifold assembly and
burners.
4- Remove combustion air inducer and flue box. Pay
careful attention to the order in which gaskets and
orifice are removed.
5- Support heat exchanger (to prevent it from falling
when final screws are removed.)
6- Remove screws supporting heat exchanger.
7- To install heat exchanger, reverse procedure. Be
sure to secure all wires and check plumbing and
burner plate for airtight seal. Screws must be
torqued to 35 in.‐lbs. to ensure proper operation.
9-Flame Sensing
Flame current is an electrical current which passes from the
ignition control through the sensor electrode during unit
operation. The current passes from the sensor through the
flame to the ground electrode (located on the flame
electrode) to complete a safety circuit. The electrodes
should be located so the tips are at least 1/2" (12.7 mm)
inside the flame envelope. Do not bend electrodes. To
measure flame current, follow the procedure on the
following page:
NOTE-Electrodes are not field adjustable. Any
alterations to the electrode may create a hazardous
condition that can cause property or personal injury.
1- Disconnect power to unit.
2- Remove lead from sensing electrode and install a
0-50DC microamp meter in series between the
sensing electrode and the sensing lead.
3- Reconnect power and adjust thermostat for heating
demand.
4- When flame is established, compare reading to
table 13. Do not bend electrodes.
5- Disconnect power to unit before disconnecting
meter. Make sure sensor wire is securely
reconnected before reconnecting power to unit.
BURNER
SUPPORT CAP
SENSOR