Aprilaire 1150 - Manual (Page 23)

Description:
The airflow proving switch is a safety device that will prevent
the humidifier from producing steam when there is no airflow
present in the HVAC system. The switch is a pressure differential
measuring device that determines if there is airflow present
based on the pressure differential between the HVAC system and
atmosphere. The switch contains a diaphragm, calibration spring,
and a snap acting SPDT switch. See
Figure 20 for details.
Operation:
When the HVAC equipment fan is off, the airflow proving
switch is open. In the event of a demand “call” from the
humidistat in this state, the airflow proving switch would prevent
the humidifier from starting or producing steam.
When the airflow proving switch senses that the HVAC
equipment fan is on, the switch closes. This position will allow
the humidifier to come on and produce steam.
Installation:
The airflow proving switch is mounted on any surface free from
vibration. The airflow proving switch must be installed with the
diaphragm in a vertical plane. (See
Figure 20 for acceptable and
unacceptable mounting configurations.) Avoid mounting the switch with the inlet line connections facing up.
The air sampling probe (included) must be located a minimum of 1.5 duct diameters downstream from the discharge of the HVAC
equipment blower. It is connected to the airflow proving switch by a 1/4” field supplied plastic tubing. The hose is connected to
inlet H on the switch housing, while inlet L remains open to the atmosphere. (See
Figure 20)
Using the ‘Normally Open’ and ‘Common’ terminals inside the terminal enclosure, wire the airflow proving switch to the field
wiring terminal block in the humidifier according to the wiring diagram on page 21 depending on the humidifier model number.
See
Figures 16-19 on page 21.
The airflow proving switch is field adjustable from .05” w.c. to 12” w.c.. To adjust the set point, turn the adjusting screw counter
clockwise (CCW), until motion has stopped. Next, turn the adjusting screw 4 complete turns in a clockwise direction to engage the
spring. From this point, the next 10 turns will be used for calibration. The optimum setting is one that will only close the switch
when the fan is in operation. Each full turn of the adjustment screw represents approximately 1.2” w.c.
To properly calibrate an airflow proving switch, a digital manometer or other pressure measuring device should be used to confirm
the actual set point.
SUPPLY WATER AND DRAIN PIPING
The humidifier is designed to operate with hot or cold, hard or soft water. Be sure to set slide switches on the internal controller
board to guidelines on page 11. Supply water piping may be of any code-approved material (copper, steel, or plastic). The fill
valve connection size is a ¼” (DN8) compression fitting. In cases where water hammer may be a possibility, consider installing a
shock arrestor. Water pressure must be between 25 psi and 80 psi (175 kPa and 550 kPa).
Drain piping may be of any code-approved material (copper, steel, or plastic rated for 140°F [60°C] minimum). If drainage by
gravity is not possible, use a reservoir pump rated for 140°F (60°C) water.
AIRFLOW PROVING SWITCH INSTALLATION INSTRUCTIONS
90-889
FIGURE 20
Diaphragm
Mounting
Bracket
Terminal
Enclosure
Inlet L: Low Inlet H: High
Do not mount in this
manner. Diaphragm
must be mounted in
a vertical plane
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