Trane IntelliPak WEHE Series Installation, Operation And Maintenance Manual page 72

System Start-up
Electric Heat Sequence of Operation
A typical electrical schematic for a 3-circuit, 70 kW electric
furnace control circuit appears in
review it and the sequence of operation described below,
remember these points:
Whenever there is a call for heat, 1U50-K1 energizes. This
energizes HEAT 1 contactors 4K34 and 4K35 which, in turn,
energize two of the six 4HR3 heating elements.
Important: Electric heater 4HR3 will only energize if
both of the heat section's high limit safety
controls—4S27 and 4S33—are closed.
1. High limit Switch 4S27 will trip if exposed to a
temperature of 133 + 5°F, and reset automatically once
the temperature falls to 110 + 5°F. It is mounted on the
control-box-side of the electric heat element assembly.
2. Linear high limit 4S33 is encased in a capillary that
extends across the unit's supply air opening, and is
anchored near the bottom of the heat section control
box. It is designed to trip if the temperature across any
6" span of the capillary exceeds 185 + 10°F (Refer to
Table 34, p. 99).
The HEAT 2 (4K36, 4K37) and HEAT 3 (4K38, 4K39)
contactors are not energized unless the 1st stage bank of
heating elements already operating are not satisfying the
heating load.
Wet Heat Sequence of Operation
Electrical circuitry for units with steam or hot water heat is
limited to the connections associated with the modulating
valve actuator (4U15) and the freezestat (4S12).
Like the furnaces described earlier, SL/SH control systems
are wired to ensure that simultaneous heating and cooling
cannot occur. The supply fan will cycle "On" and "Off" with
each call for heat during both an occupied and unoccupied
period.
Whenever there is a call for heat, 1U50-K3 energizes. This
allows a modulated voltage signal to be sent to the "Wet"
heat actuator 4U15. Depending on the value of this signal,
4U15 regulates the flow of steam or hot water through the
coil by positioning the valve stem at some point between
fully closed (6 VDC) and fully open (8.5 VDC).
72
Figure 41, p. 73. As you
Freeze Protection
A freezestat (4S12) is mounted inside the heat section of
WLHE and WSHE units to prevent the "wet" heat coil from
freezing during the "Off" cycle. If the temperature of the air
leaving the heating coils falls to 40°F, the freezestat's
normally-open contacts close, completing the heat fail
circuit on the UCM. When this occurs:
a. The supply fan is turned "Off".
b. "Wet" heat actuator 4U15 fully opens to allow hot
water or steam to pass through the heating coil and
prevent freeze-up.
c. A "Heat Fail" diagnostic is displayed on the Human
Interface LCD screen.
For heating control settings and time delay specifications,
refer to Table 34.
RT-SVX35H-EN