Dunkirk Q90-125, Installation, Operation & Maintenance Manual, Page: 41 / 44

41
The differential air pressure switch is safety device which
will prevent boiler from fi ring if there is air intake, boiler
heat exchanger or vent blockage.
Following steps and diagram indicate the location of the
connection points required to measure the offset pressure
using an inclined Manometer or a Differential pressure
gauge. Only one device is required to measure the 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
3.
or differential pressure gauge with the 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 the chart
5.
at right, check for possible causes:
Blockage or moisture in suction lines to switch. •
Blockage or moisture in orifi ce 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 the pressure reading corresponds to the chart
6.
at right and the pressure switch is operating properly,
remove the testing lines and reinstall the vinyl caps to
the Tee and 3-way connector.
Boiler Status Differential Pressure Switch Contacts
Not Running 0”
3.5” Closed
0.5” Open
Running With-
out Blockage
(Approximate
minimum vent
lengths)
1.00” for Model 125
3.5”
0.5”
Closed
Closed
1.50” for Model 150
1.50” for Model 175
2.00” for Model 200
Running With
Blockage
Greater Than or Equal
To Setpoints
3.5” Open
0.5” Closed
Locations Of The Connection Points Required To Check
The Diff erential Air Pressure.
Differential Air Pressure Switch Check
TROUBLESHOOTING