284809-YIM-B-0707
Unitary Products Group
49
RESET
Remove the call for cooling, by raising thermostat set-
ting higher than the conditioned space temperature.
This resets any pressure or freezestat flash codes.
ELECTRIC HEATING SEQUENCE OF OPERATIONS
The following sequence describes the operation of the
electric heat section.
Single-stage heating: (applies only to 18 KW heater, all
other heaters MUST use a two-stage thermostat:).
a. Upon a call for heat by the thermostat, the heater
contactor (6M) will be energized. After completing
the specified fan on delay for heating, the UCB will
energize the blower motor.
b. The thermostat will cycle the electric heat to satisfy
the heating requirements of the conditioned space.
Two-stage heating: (applies to all heaters except 18
KW):
a. Upon a call for first-stage heat by the thermostat, the
heater contactor (6M) (6M & 7M on 72 KW, 240V)
will be energized. After completing the specified fan
on delay for heating, the UCB will energize the
blower motor.
If the second stage of heat is required, heater con-
tactor (7M) will be energized. Note that on the 54
KW, 240V heater, heater contactors (7M & 8M) will
be energized and on the 72 KW, 240V heater,
heater contactors (8M & 9M) will be energized. After
completing the specified fan on delay for heating,
the UCB will energize the blower motor.
b. The thermostat will cycle the electric heat to satisfy
the heating requirements of the conditioned space.
NOTE:
All 240 & 480V heaters are provided with man-
ual reset backup protection limits. These will
de-energize the heaters should the primary
limit fail to open or the contactors fail to open in
a failure mode.
HEATING OPERATION ERRORS
TEMPERATURE LIMIT
If the UCB senses zero volts from the high temperature
limit, the indoor blower motor is immediately energized.
This limit is monitored regardless of unit operation sta-
tus, i.e. the limit is monitored at all times.
If the temperature limit opens three times within one
hour, it will lock-on the indoor blower motor and a flash
code is initiated (See Table 27).
SAFETY CONTROLS
The unit control board monitors the temperature limit
switch of electric heat units.
The control circuit includes the following safety con-
trols:
1. Temperature Limit Switch (TLS 1, 2).
This control is located inside the heater compart-
ment and is set to open at the temperature indi-
cated in the Limit Control Setting Table 22. It resets
automatically. The limit switch operates when a
high temperature condition, caused by inadequate
supply air flow occurs, thus shutting down the
heater and energizing the blower.
2. Temperature Limit Switch (TLS 3, 4, 5 and 6).
This control is located inside the heater compart-
ment and is set to open at the temperature indi-
cated in the Limit Control Setting Table 22. It is a
manual reset limit. These limit switches will de-
energize the heaters should the primary limit fail to
open or the contactors fail to open in a failure
mode.
For units with VFD and electric heat, the speed
of the indoor blower motor continues to be con-
trolled by duct static pressure via the VAV con-
trol board.
If there are VAV boxes present in the duct sys-
tem, the boxes must be driven to the full-open
position using a customer-supplied power
source to assure adequate airflow across the
heating elements.
