Reznor SHH (Indoor PreevA) Unit Installation Manual - Manual (Page 26)

Form I-PDH/SDH/PEH/SHH/PXH, Page 26
The pressure null switch used in Option GE15 is a Dwyer #1640-0 with a range of .01-
.20” w.c. It is shipped separately for field installation. Refer to the following paragraphs
and the manufacturer’s installation instructions included with the switch.
Description and Application - The pressure null switch is a diaphragm operated dif-
ferential pressure switch used in makeup air applications to control building pressure.
It maintains a selected positive or negative pressure setpoint by changing the amount
of outside air being introduced to the building through the modulating outside air damp-
ers. As more pressure is required in the building, the pressure null switch activates the
damper motor driving the outside air damper towards the full open position and the
recirculated air damper towards the closed position. Conversely, as less pressure is
required, the switch drives the dampers in the opposite direction.
Pressure Null Switch (Field installed to control Outside Air Dampers in
Option GE15) - PDH, SDH, SHH, PEH, PXH
Electric Box
Span
Adjust
Screw
Mounting Plate
Pressure Taps
Low High
Pressure
Adjust Screw
Increasing
Decreasing
Linkage to
Diaphragm
Low
Common
High
Span Adjustment
Span of
floating
contact
Switching action in the pressure null switch
FIGURE 19 - Pressure Null Switch (used with Inlet Air Option GE15)
IMPORTANT: To eliminate shipping
damage to the switch contacts, the
manufacturer reduced the span
adjustment to zero before shipping.
The span should be adjusted prior
to using the switch. (If the switch
has been installed, disconnect the
vent tube so that the null switch
is in a neutral position.) Remove
the electrical box cover and while
observing the contacts, turn the
span adjustment screw slowly in a
clockwise direction. Continue turn-
ing the adjustment screw until you
are able to see gaps between the
common and both the low and high
contacts. A minimum gap provides
the greatest sensitivity. The wider
the gap the lower the sensitivity.
Pressure Null
Switch, P/N 88052
Pressure Null Switch Installation Instructions
1. Select an indoor location free from excessive vibration where oil or water will not drip
onto the switch and where ambient temperature will be within a range of -30°F (dry
air) to 110°F.
2. Mount the switch securely with the diaphragm in a vertical plane to ensure proper
operation.
3. Connect the pressure taps on the top of the switch to sources of air pressure
differential. Metal tubing with 1/4” O.D. is recommended, but any tubing system
which will not unduly restrict the air flow may be used. To maintain a positive
building pressure, vent the low pressure tap to the outdoors and allow the
high pressure tap to monitor building pressure. To maintain a negative building
pressure, reverse the functions of the high and low pressure taps. In either case,
be sure that the outdoor vent is protected from the wind and screened from
insects.
4. Adjustment of the Switch - The “HIGH” actuation point of the null switch is
indicated on a calibrated scale secured to the transparent range screw enclosure.
Building pressure is set by turning the adjustment screw. The “Low” actuation point
is set by adjusting the span of the null by turning the span adjustment screw. The
span range is .01 to .03” w.c.
5. See the wiring diagram included with the furnace to make electrical connections.
6.4.4.3 Mixing Box Damper and Control Options (cont’d)
6.4.4 Optional Mixing
Box (factory installed)
(cont’d)
6.0 Mechanical
(cont’d)
6.4 Unit Inlet Air
(Supply Air)
(cont’d)