Reznor HRPD (Outdoor Duct Furnaces) Unit Installation Manual - Manual (Page 17)

Form I-RP/HRPD, P/N 132210 R14, Page 17
8.2 Limit Control
8.3 Combustion Air
Proving Switch
The combustion air proving switch, which ensures that proper combustion airflow is
available is a pressure switch. The switch is a single-pole, double-throw switch, which
senses pressure caused by the flow of combustion air from the venter. To prevent the
switch from responding to sudden temporary pressure fluctuations and to provide a
prepurge, a small diameter orifice is installed in the outlet fitting of the pressure switch.
The electrical circuit of this heater is designed to check for proper switch position
before each complete heat cycle. Only after checking the state of the pressure switch,
and proving that combustion air is present, will the gas ignition sequence begin.
DANGER
Safe operation requires proper venting flow. Never bypass the
combustion air proving switch or attempt to operate the unit
without the venter running and proper flow in the vent system.
Hazardous condition could result. See Hazard Levels, page 2.
8.4 Gas Controls
8.4.1. Operating Valve
WARNING
The operating valve is the prime safety shutoff. All gas supply lines
must be free of dirt or scale before connecting the unit. See Hazard
Levels, page 2.
All furnaces are standardly equipped with a 24-volt combination valve which 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 gas valve
allows for single-stage control from a single-stage, 24-volt thermostat.
8.4.2 Optional Two-
Stage Operation -
Heating Only
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. First stage (low fire) is factory set (not field adjustable). Both high and low
stages are controlled by a Servo regulator, maintaining constant gas input under wide
variations in gas supply pressure. See instructions packed with the unit for specific gas
valve specifications, wiring, and operating instructions.
8.4.3 Optional Two-
Stage Operation -
Makeup Air
Two-stage makeup air units are equipped with a two-stage gas valve, but instead of
control from a two-stage room thermostat, the outlet air temperature is monitored and
controlled by a two-stage ductstat. When the discharge air temperature drops to the
setpoint, low fire is energized. If low fire cannot satisfy the ductstat setting, high fire is
energized.
Makeup air applications are usually adjusted to discharge an outlet air temperature
between 65°F and 75°F. In all applications, the allowable temperature rise of the fur-
nace in the installation dictates the limits of the ductstat temperature setting.
Depending on the option selection, the sensor is either connected by capillary tubing
to the unit-mounted ductstat ( FIGURES 12A and 12B) or electrically connected to a
remote electronic temperature selector ( FIGURE 13). See Paragraph 6.3.5 for instruc-
tions on locating the sensor in the ductwork.
There are two methods of achieving multiple-stage makeup air operation. In addition,
for each of these methods, there are two types of control mechanisms. Consult the
wiring diagram on the furnace to identify the optional control system (Option AG 3, 15,
4, or 17).
The first method, identified by Options AG3 or AG15 is applicable to both Model
RP and HRPD and is comparable to the two-stage heating units. Instead of control
from a two-stage room thermostat, the discharge air temperature is monitored and
the two-stage gas valve is controlled by a two-stage ductstat. When the discharge air
temperature drops to the setpoint, low fire is energized. If low fire cannot satisfy the
The heater is equipped with a non-adjustable high limit switch which 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. See Paragraph 9.3 for limit control check.