1088917-YIM-B-1014
Johnson Controls Unitary Products 51
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 contactor
(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 manual 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.
Electric Heat 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 status, 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.
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Temperature Limit Switch (TLs)
1. Temperature Limit Switch (TLS 1, 2).
This control is located inside the heater compartment and
is set to open at the temperature indicated in the Limit
Control Setting Table 21. 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 compartment and
is set to open at the temperature indicated in the Limit
Control Setting Table 21. 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.
Reset
Remove the call for heating by lowering the thermostat setting
lower than the conditioned space temperature.
Electric Heat Anticipator Setpoints
It is important that the anticipator setpoint be correct. Too high
of a setting will result in longer heat cycles and a greater
temperature swing in the conditioned space. Reducing the
value below the correct setpoint will give shorter “ON” cycles
and may result in the lowering of the temperature within the
conditioned space. Refer to Table 22 for the required electric
heat anticipator setting.
For units with VFD and electric heat, the speed of the
indoor blower motor continues to be controlled by duct
static pressure via the VAV control board.
If there are VAV boxes present in the duct system, the
boxes must be driven to the full-open position using a
customer-supplied power source to assure adequate
airflow across the heating elements.
Table 21: Limit Control Setting
Unit
(Tons)
Voltage
Heater
Kw
Temperature,
Limit Switch
1, 2
Opens,
°F
Temperature,
Limit Switch
3, 4, 5, 6
Opens,
°F
15 240
18 120 170
36 120 170
54 120 170
72 120 170
17.5, 20
and 25
240
18 140 200
36 140 200
54 140 200
72 140 200
15, 17.5,
20 and 25
460
18 120 170
36 120 170
54 120 170
72 120 170
15, 17.5,
20 and 25
600
18 120 -
36 120 -
54 120 -
72 120 -
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