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Differential Air Pressure Switch Check
Differential air pressure switch is safety device which
prevents boiler from firing if there is air intake, boiler heat
exchanger or vent blockage.
Following steps and diagram indicate location of connection
points required to measure offset pressure using inclined
Manometer or Differential pressure gauge. Only one device
is required to measure offset pressure.
Turn off service switch, or lower thermostat setting.
1.
Remove vinyl caps from Tee.
2.
Install testing lines as shown to inclined manometer or
3.
differential pressure gauge with ability to read 0.01" to
at least 6.0" w.c.
Turn on service switch and set thermostat to call for
4.
heat.
If manometer readings do not correspond to Table 11
5.
check for possible causes:
• Blockage or moisture in suction lines to switch.
• Blockage or moisture in orifice installed in suction
lines to switch.
• Blockage in air intake or vent pipes.
• Undersized air intake or vent pipes.
• Loose blower wheel on motor shaft.
• Incorrect pressure switch or pressure switch set-
point.
• Gas valve out of adjustment.
When pressure reading corresponds to Table 11
6.
and pressure switch is operating properly, remove
testing lines and reinstall vinyl caps to tee and 3-way
connector.
15 - TROUBLESHOOTING
Figure 22 - Locations Of Connection Points
Table 11 - Correct Manometer Readings
Boiler Status
Not Running
Running
Without
Blockage
(Approximate
minimum vent
lengths)
Running With
Blockage
47
Required To Check Differential Air
Pressure
Differential Pressure Switch
0"
1.00" for Model 125
1.50" for Model 150
1.50" for Model 175
2.00" for Model 200
Greater Than or Equal
To Setpoints
Contacts
3.5"
Closed
0.5"
Open
3.5"
Closed
0.5"
Closed
3.5"
Open
0.5"
Closed
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