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D-Optional Electric Heat Components
Table 2 shows electric heat fuse ratings. See Options/Ac
cessories section (see table of contents) for LHT to EHO
match-ups. See Electrical/Electric Heat Data section (see
table of contents) of this manual for electrical ratings and
capacities.
All electric heat sections consist of electric heating ele
ments exposed directly to the air stream. See figure 10.
EHO parts arrangement is shown in figures 10 and 11. Mul
tiple‐stage elements are sequenced on and off in response
to thermostat demand.
1-Contactors K15, K16
Contactors K15 and K16 are three-pole double-break
contactors located on the electric heat vestibule. All
contactors are equipped with a 24VAC coil. The coils in
the K15 and K16 contactors are energized by a W2
thermostat demand, K9, and DL2. Contactor K15 ener
gizes the first stage heating elements, while K16 ener
gizes the second stage heating elements.
2-High Temperature Limits S15 (Primary)
S15 is a SPST N.C. auto‐reset thermostat located on
the back panel of the electric heat section below the
heating elements. S15 is the high temperature limit for
the electric heat section. When S15 opens, indicating a
problem in the system, contactor K15 is de‐energized.
When K15 is de‐energized, first stage and all subse
quent stages of heat are de‐energized. For
EHO102-150 units, the electric heat section thermo
stat is factory set to open at 170_F +
5_F (76_C +
2.8_C) on a temperature rise and automatically reset at
130_F +
6_F (54.4_C + 3.3_C) on a temperature fall.
For EHO100 units, the electric heat section thermostat
is factory set to open at 160_F +
5_F (71.0_C + 2.8_C)
on a temperature rise and automatically reset at 120_F
+
6_F (49.0_C + 3.3_C) on a temperature fall. The ther
mostat is not adjustable.
3-High Temperature Limit S20, S157, S158, S15, S160
& S161 (Secondary)
Limits are SPST N.C. manual‐reset thermostat . Like
the primary temperature limit, S20 is wired in series
with the first stage contactor coil (K15) and second
stage contactor coil (K16). When S20 opens, contac
tors (K15, K16) are de‐energized. When the contactors
are de‐energized, first stage and all subsequent
stages of heat are de‐energized. The thermostat is fac
tory set to open at 220_F +
6_F (104_C + 3.3_C) on a
temperature rise and can be manually reset when tem
perature falls below 160_F (71.0_C).
4-Terminal Strip TB2
Terminal strip TB2 is used for single point power
installations only. TB2 distributes L1, L2 and L3 power
to TB3. Units with multi-point power connections will
not use TB2.
5-Terminal Strip TB3
Electric heat line voltage connections are made to
terminal strip TB3 located in the upper left corner of the
electric heat vestibule. TB3 distributes power to the
electric heat components.
6-Heating Elements HE1 through HE6
Heating elements are composed of helix wound bare
nichrome wire exposed directly to the air stream. Three
elements are connected in a three‐phase arrange
ment. The elements in 208/230V units are connected
in a “Delta” arrangement. Elements in 460 and 575V
units are connected in “Wye” arrangement. Each stage
is energized independently by the corresponding
contactors located on the electric heat vestibule panel.
Once energized, heat transfer is instantaneous. High
temperature protection is provided by primary and re
dundant high temperature limits and overcurrent
protection is provided by fuses.
7-Fuse F3
Fuse F3 are housed in a fuse block which holds three
fuses. Each F3 fuse is connected in series with each
leg of electric heat. Figure 11 and table 2 show the
fuses used with each electric heat section. For simplic
ity, the service manual labels the fuses F3 - 1 through
F3 - 4.
8-Unit Fuse Block & Fuse F3 and F4
Three line voltage fuses F4 provide short circuit and
ground fault protection to all cooling components in the
LHT units with electric heat. The fuses are rated in
accordance with the amperage of the cooling compo
nents.
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