Lennox LDT Series Information page 47

Sequence of Operation Gas Heat LDT156/240
Heating Type Determination - On a heating demand if
the outdoor air temperature is above Balance Set
Point, proceed to Heat Pump Heat. If outdoor
temperature is below the Balance Point Set Point,
proceed to First Stage Heat.
BLOWER OPERATION
Control Inverter A96 energizes B3 on a G demand
HEAT PUMP HEAT:
1- A55 Unit Controller receives W1 heating demand.
2- A55 energizes outdoor fans B4, B5 & B21 through
K10 & K68 respectively.
3- After A55 proves N.C. lower pressure switch S87
and N.C. high pressure switch S4, contactor K1 is
energized
4- N.O. K1 closes energizing compressor B1, and
N.C. K1-52 opens de-energizing HR1.
5- A55 energizes outdoor fans B22, B23 & B24
through K149 & K150 respectively.
6- After A55 proves N.C. lower pressure switch S88
and N.C. high pressure switch S7, contactor K2 is
energized.
7- N.O. K2 closes energizing compressor B2, and
N.C. K2-52 opens de-energizing HR2
FIRST STAGE HEAT:
1- Heating demand initiates at W1 in thermostat.
2- 24VAC is routed to the A55 Unit Controller. After
A55 proves N.C. primary limit S10, the
combustion air blower B6 is energized.
3- After the combustion air blower B6 has reached
full speed, the combustion air proving switch (S18)
contacts close. The A55 routes 24VAC through
N.C. burner 1 flame rollout switch S47 and the
closed contacts of the combustion air proving
switch (S18) to energize the ignition module A3.
After a 30 second delay A3 energizes the gas
valve GV1 on low fire.
4- As steps 2, 3 and 4 occur, A55 proves N.C.
primary gas heat limit S99 and the combustion air
blower B15 is energized.
5- After the combustion air blower B15 has reached
full speed, the combustion air proving switch (S45)
contacts close. The A55 routes 24VAC through
N.C. burner 2 flame rollout switch S69 and the
closed contacts of the combustion air proving
switch (S45) to energize the ignition module A12.
After a 30 second delay A12 energizes gas valve
GV3 on low fire.
SECOND STAGE HEAT:
6- With first stage heat operating, an additional
heating demand initiates W2 in the thermostat.
7- A second stage heating demand is received by
A55.
.
8- A55 will energize the corresponding gas valves
GV1 and GV3 on high fire.
OPTIONAL LOW AMBIENT KIT
(C.G.A. ‐50° C LOW AMBIENT KIT):
9- When heat section temperature drops below
-20°F, S59 opens and de-energized A3 and A12
ignition controls. At the same temperature, S60
closes and energizes K125. K125-1 contacts
close energizing HR6 Cold Weather Kit electric
heat.
10- When heat section temperature rises to 10°F, S59
closes allowing power to A3 and A12 ignition
controls. At the same temperature, S60 opens and
de-energizes K125. K125-1 contacts open
de-energizing HR6 Cold Weather Kit electric heat.
.
11- If heat section temperature rises above 50°F, S61
will open and de-energize K125. K125-1 contacts
will open and de-energize HR6 Cold Weather Kit
electric heat. If heat section temperature drops to
20°F, S61 will close and allow power to K125.
END OF SECOND STAGE HEAT:
12- Heating demand is satisfied. Terminal W2 is
de‐energized.
13- High fire on GV1 and GV3 are de‐energized by the
A55.
END OF FIRST STAGE HEAT:
14- Heating demand is satisfied. Terminal W1 is
de‐energized.
15- Ignition module A3 is de‐energized by A55 in turn
de‐energizing GV1. Combustion blower air blower
B6 is also de‐energized. At the same instant,
ignition module A12 is de‐energized by A55 in turn
de‐energizing GV3. B6 combustion air blower is
also de‐energized.
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