50
SSCBL-RPBL-IOM (10-22) 149159-C
CONTROLS—CONTINUED
High Temperature Limit Control
Each furnace is equipped with a non-adjustable high temperature limit switch that shuts off the gas in the event
of motor failure, lack of air due to dirty filters, or restrictions at the inlet or outlet of the unit . The furnace farthest
downstream in each system is also equipped with a linear-type limit control . The switch is mounted on a bracket
on the bottom of the junction box with the capillary sensor extending across the discharge opening of the furnace .
Reverse Flow Limit Control (Model RPBL)
The furnace is factory-equipped with an automatic-reset reverse flow limit control . This control is located in the blower
compartment, is mounted in the blower junction box adjacent to the blower inlet opening, and is wired in series
with the main limit control mounted on the heat exchanger duct side . In case of belt breakage or motor failure, the
limit control will be opened by the high temperatures caused by reverse flow from the heat exchanger to the blower
compartment, thus breaking the circuit to the automatic electric gas valve and preventing burner operation .
High Ambient Limit Control (Model RPBL)
The optional high ambient limit control functions to shut off the burner when the entering outside air reaches a set
temperature . The temperature setting is field-adjustable from 0°F to 100°F .
Airflow Proving Switch (Makeup Air with Option BW1)
The optional airflow proving switch ensures that the circulating air blower provides an adequate amount of airflow
prior to the unit being fired . The switch is a single-pole/normally-open device that closes when an increase in pressure
above the setpoint is sensed in the circulating air blower . The switch is located in the blower junction box . Contacts
are set to close at 0 .26 IN WC (maximum) .
Variable Frequency Drive
•
When an optional variable frequency drive is ordered, the motor operates on two speeds as determined by the
electrical frequency . High speed is used for cooling and low speed for heating . 60 Hertz is the maximum high
speed . Maximum speed for low speed heating is the frequency that will provide the maximum temperature rise of
the heater . Speeds must be within the temperature rise range of 30–90°F .
•
Follow the variable frequency controller manufacturer’s instructions that are packaged with the heater (in the
owner’s envelope) to program the variable frequency drive settings . The formula for motor speed is
N
=
120
×
f/p
where
N
= speed,
f
= frequency,
p
= number of poles, a 3,600-rpm motor has two poles, and an 1800-rpm motor
has four poles . For example, for an 1,800-rpm motor at 60Hz,
N
= 120 × 60/4 = 1800
(1,800 = synchronous speed,
assume 2% slip) .
•
The motor will run between 1,750 and 1,790 rpm at full load depending on design . For the same motor run at 45Hz,
120 × 45/4 = 1350
(1,350 rpm less 2% slip = about 1,300 rpm) .
Combination Gas Valve
⚠ WARNING ⚠
The combination gas valve is the prime safety shutoff. To ensure positive closure, all gas supply
lines must be free of dirt or scale before connecting them to the unit.
All furnaces are equipped with a 24V combination gas valve that includes the automatic electric ON/OFF valve
controlled by the room thermostat, the pressure regulator, the safety pilot valve, and the manual shutoff valve . The
standard combination gas valve allows for single-stage control from a single-stage, 24V thermostat .
Optional Two-Stage Control (Heating Only Application)
The standard combination control valve is replaced with a two-stage combination gas control valve providing for
low fire or high fire operation controlled by a two-stage thermostat . The first stage (low fire) is factory-set (not
field-adjustable) . Both low fire and high fire stages are controlled by a Servo regulator that maintains constant gas
input under wide variations in gas supply pressure . Refer to the instructions provided with the unit for gas valve
specifications, wiring, and operating instructions .
