Lennox LHT Series Service Manual page 43

SEQUENCE OF OPERATION
HEAT PUMP
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.
EHA-15, 30, 45, 60, 90 - Y & G
NOTE:Two electric heat sections are used in all 15kW
through 90kW heaters. The heat sections are labeled first
electric heat section (left side) and second electric heat
section (right side). See figure 16.
NOTE: In the case of EHA 15 and 30kW, the second heat
section (right side) is a slave (only has electric heat ele-
ments and a limit). Line voltage is supplied to elements in
both heat section one (left side) and two (right side) by the
contactors in heat section one (left side).
HEATING ELEMENTS:
1 - Terminal strip TB3 is energized when the unit
disconnect closes. TB3 supplies line voltage to
electric heat elements HE1 through HE14. Each
heating element is protected by fuse F3.
FIRST STAGE HEAT:
2 - Heating demand initiates at W1 in thermostat.
3 - 24VAC is routed to the main control module A55.
After A55 proves N.C. primary limits S15 (heat
section one, left side), S107 (heat section two, right
side), the electric heat contactor K15 and heat relay
K9 are energized.
4 - N.O. contact K15-1 closes allowing the first bank
of elements in heat section one (left side) to be
energized.
5 - At the same time, N.O. contacts K9-1 close. A N.O.
contact in A55 closes, energizing electric heat relay
K17.
6 - N.O. contacts K17-1 close allowing the first set
of elements in heat section two (right side) to be
energized.
SECOND STAGE HEAT:
7 - With the first stage heat operating, an additional
heating demand initiates at W2 in the thermostat.
8 - 24VAC is routed through the main control module
A55, which in turn energizes the electric heat
contactor K16.
9 - N.O. contacts K16-1 close allowing the second set
of elements in heat section one (left side) to be
energized.
10 - Simultaneous with step eight, a N.O. contact in
the A55 Unit controller closes, allowing 24VAC to
energize electric heat contactor K18.
11 - N.O. contacts K18-1 close allowing the second set
of elements in heat section two (right side) to be
energized.
END OF SECOND STAGE HEAT:
12 - Heating demand is satisfied. Terminal W2 in the
thermostat is de-energized.
13 - Electric heat contactors K16 and K18 are de-
energized.
14 - The second set of electric heat elements in heat
sections one (left side) and two (right side) are de-
energized.
END OF FIRST STAGE HEAT:
15 - Heating demand is satisfied. Terminal W1 in the
thermostat is de-energized.
16 - Electric heat contactors K15 and K17 are de-
energized.
17 - The first set of electric heat elements in heat
sections one (left side) and two (right side) are de-
energized.
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