Electric Heat; Thermostat - Lennox LGA Series Unit Information

Diagrams A7 and A6 are the EHA electric heat sec-
tions used in the LHA and LCA units. The Y voltage
diagram (A7) use elements configured in a Wye.
The G and J voltage diagram (A6) use elements
configured in a Delta. Both diagrams A7 and A6 fol-
low the following sequence of operation:
NOTE:Two electric heat sections are used in all 30kW
through 120kW heaters. The heat sections are la-
belled first electric heat section (left side) and sec-
ond electric heat section (right side). See figure 28.
NOTE: In the case of EHA 30kW, the second heat
section (right side) is a slave (only has electric
heat elements and a limit). In this case the A60
module, T2 transformer, and K9 heat relay are not
used. 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) and
all control is through the A55 module.
HEATING ELEMENTS:
1 − Terminal strip TB3 is energized when the unit discon-
nect 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 through TB34 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), and redundant electric heat limit S63,
the electric heat contactor K15 and heat relay K9 are
energized. Indoor blower is energized with no time
delay.
4 − N.O. contact K15-1 closes allowing the first bank of
elements in heat section one (left side) to be ener-
gized.
5 − At the same time, line voltage is routed through
transformer T2, which provides 24VAC to the
electric heat control module A60. A60 is ener-
gized when N.O. contacts K9-1 close. A N.O.
contact in A60 closes,
energizing electric heat relay K17.
SEQUENCE OF OPERATION
A7 DIAGRAM − EHA−30, 45, 60, 90, 120 − Y
A6 DIAGRAM − EHA−30, 45, 60, 90, 120 − G, J
Page 72
6 − N.O. contacts K17-1 close allowing the first set of ele-
ments 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 mod-
ule 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 electric heat control module A60 closes, al-
lowing 24VAC to energize electric heat contac-
tor 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 ther-
mostat 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 ther-
mostat 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.