104 RT-SVX36G-EN
Unit Start Up
Electric Heat Sequence of Operation
The control system for the rooftop units are wired to ensure that the heating and cooling do not
occur simultaneously. Refer to the electric heat wiring diagrams that shipped with the unit while
reviewing the following sequence of operation. As you review the sequence of operations,
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.
Note: Electric heater 4HR3 will only energize if both of the heat section’s high limit safety
controls—4S27 and 4S33—are closed.
• 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.
• 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 59, p. 174 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 do not 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).
Freeze Protection
A freezestat (4S12) is mounted inside the heat section of SLH_ and SSH_ 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 59, p. 174.
Use the checklist provided below in conjunction with the “General Unit Requirement” checklist to
ensure that the unit is properly installed and ready for operation. Be sure to complete all of the
procedures described in this section before starting the unit for the first time.
• Turn the field supplied disconnect switch, located upstream of the rooftop unit, to the “Off”
position.
To Next Page
Loading...
Need help?
General
