Lennox LHT Series Service Manual page 27

D-OPTIONAL ELECTRIC HEAT
See ELECTRICAL / ELECTRIC HEAT DATA and ELEC-
TRIC HEAT CAPACITIES (table of contents) for LCH to
EHA match-ups and electrical ratings.
EHA parts arrangement is shown in figures 17 and 18.
All electric heat sections consist of electric heating el-
ements exposed directly to the air stream. Two electric
heat sections (first section and second section) are used
in all 15kW through 90kW heaters. See figure 16. Multi-
ple-stage elements are sequenced on and off in response
to thermostat demand.
HEAT SECTION PLACEMENT
FIGURE 16
1-Main Control Box Components A55, K9
The main control box (see figure 3) houses the A55 Unit
Controller and the K9 electric heat relay.
2-Contactors K15, K16, K17 and K18
Contactors K15, K16, K17 and K18 are all three-pole dou-
ble-break contactors located on the electric heat vestibule.
K15 and K16 are located on the first electric heat section,
while K17 and K18 are located on the second electric heat
section. However, in the 15 and 30kW heaters, the first
section houses all contactors and fuses. All contactors are
equipped with a 24VAC coil. The coils in the K15, K16,
K17 and K18 contactors are energized by the main pan-
el A55. Contactors K15 and K17 energize the first stage
heating elements, while K16 and K18 energize the second
stage heating elements.
COMPRESSOR
COMPARTMENT
SECOND ELECTRIC
HEAT SECTION
(RIGHT SIDE)
FIRST ELECTRIC
HEAT SECTION
(LEFT SIDE)
Page 27
3-High Temperature Limits S15 and S107 (Primary)
S15 and S107 are SPST N.C. auto-reset thermostats lo-
cated on the back panel of the electric heat section below
the heating elements. S15 is the high temperature limit
for the first electric heat section, while S107 is the high
temperature limit for the second electric heat section. Both
thermostats are identical and are wired to the A55 Unit
Controller. When either S15 or S107 opens, indicating a
problem in the system, contactor K15 is de-energized.
When K15 is de-energized, first stage and all subsequent
stages of heat are de-energized. The thermostats used
on EHA360-45-1 Y/G/J are factory set to open at 200 F +
5 F on a temperature rise and automatically reset at 160 F
+ 6 F on a temperature fall. All other electric heat section
thermostats are factory set to open at 170 F + 5 F on a
temperature rise and automatically reset at 130 F + 6 F on
a temperature fall. The thermostats are not adjustable.
4-Terminal Strip TB3
Electric heat line voltage connections are made to termi-
nal strip TB3 (or a fuse block on some models) located in
the upper left corner of the electric heat vestibule.
5-Heating Elements HE1 through HE14
Heating elements are composed of helix wound bare ni-
chrome wire exposed directly to the air stream. Three ele-
ments are connected in a three-phase arrangement. The
elements in 208/230V units are connected in a "Delta" ar-
rangement. Elements in 460 and 575V units are connect-
ed in "Wye" arrangement.
Each stage is energized independently by the correspond-
ing contactors located on the electric heat vestibule panel.
Once energized, heat transfer is instantaneous. High tem-
perature protection is provided by primary and redundant
high temperature limits and overcurrent protection is pro-
vided by fuses.
6-Fuse F3
Fuse F3 are housed in a fuse block which holds three fus-
es. Each F3 fuse is connected in series with each leg of
electric heat. Figures 17 and 18 and table 5 shows the
fuses used with each electric heat section. For simplicity,
the service manual labels the fuses F3 - 1 through F3 - 8