Lennox LCM Series - Page 52

Page 52
SEQUENCE OF OPERATION
EHA-15, 30, 45, 60, 90 - Y & G
The Y voltage diagram use elements configured in
a Wye. The G and J voltage diagram use elements
configured in a Delta. Both diagrams follow the fol
lowing sequence of operation:
NOTE:Two electric heat sections are used in all 15kW
through 90kW heaters. The heat sections are la
belled first electric heat section (left side) and
second electric heat section (right side). See fig
ure 14.
NOTE: In the case of EHA 15 and 30kW, the second
heat section (right side) is a slave (only has electric
heat elements 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 dis
connect closes. TB3 supplies line voltage to elec
tric heat elements HE1 through HE14. Each heat
ing 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 en
ergized.
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 en
ergized.
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 en
ergized.
10. Simultaneous with step eight, a N.O. contact in the
A55 Unit controller closes, allowing 24VAC to en
ergize 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 sec
tions one (left side) and two (right side) are de‐en
ergized.