Page 23
IV − Electric Heat Components
See electric heat tables (table of contents) for electric heat
matchups. EHA units consists of electric heating elements
exposed to the air stream. Multiple−stage elements are se-
quenced on and off by time delays in response to thermo-
stat demand.
1 − Heating Elements HE1, HE2, HE3 & HE4
Heating elements are composed of helix wound bare
nichrome exposed directly to the air stream. Heating
elements are energized directly by contactors. Once
energized, heat transfer is instantaneous. Over tem-
perature protection is provided by primary and secon-
dary high temperature limits. Overcurrent protection is
provided by fuses. Each stage of electric heat consists
of three elements connected in a three-phase arrange-
ment. 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 en-
ergized independently by a three-pole double-break con-
tactor and is protected by safety limits.
2 − Contactors K15 & K16
Contactors K15 and K16 are three−pole double break re-
lays with a 24 volt coil that energize their respective heating
elements on thermostat demand. K15 energizes first stage
heat elements and K16 energizes second stage elements.
3 − Electric Heat Sequencer Relays K32
Relay K32 is a N.O. sequencer relay with a resistive ele-
ment for a coil and a bi-metal disk which actuates the
contacts. The relays are located on the electric heat vestibule
panel and are energized by a 24V heating demand (W1, W2).
When energized, the internal resistance heats the bi-metal
disk causing the contacts to close. When the relay is de-ener-
gized, the disk cools and the contacts open. The relay
energizes different stages of heat.
4 − Relays K9 and K19
Relays K9 and K19 are used to electrically isolate the TAA
24 volts components from the T3EHA 24 volt components.
The coil on the relays are connected to first stage and sec-
ond stage heat. On a first stage heat demand K9 is ener-
gized. K9−1 closes energizing first stage heat contactor
K15. On a second stage heat call K19 is energized. When
K19−1 closes contactor K16 is energized which energizes
relay K32.
5 − Fuse F3
Heating elements in all T3EHA units are protected by fuse
F3. The fuse is connected in series with each leg of electric
heat.
6 − Fuse F4
F4 serves the same purpose as F3 but is in line with line volt-
age and protects the indoor blower B3.
7 − Transformer T2
T2 is line voltage to 24VAC which provides 24VAC to power
to all T3EHA contactor coils, relays and timers.
8 − High Temperature Limit S15 (Primary)
S15 is the primary high temperature limit. It is located in
the electric heat unit immediately downstream from the
heating elements. S15 is a single-pole single-throw nor-
mally closed thermostat wired in series with the first stage
contactor coil.
When S15 opens, indicating a problem in the system, contac-
tor K15 is de-energized. When K15 is de-energized, first stage
and all subsequent stages of heat are de-energized. Since the
indoor blower is controlled by demand (K9 remains ener-
gized), the indoor blower continues operating.
9 − High Temperature Limit S20 (Secondary)
Each heating element assembly is electrically connected
to two high temperature limits S20 (refer to wiring diagrams
in back of this manual). Each limit is connected in series
with one leg of the three-phase element assembly. The
third leg of each assembly is not equipped with a limit.
Three-phase operating characteristics allow one of the
other two limits to protect the third leg.
