J
Check.Use a Magnehelic gauge to
check the operation of the negative
pressure switch. The test must be
made with the BURNER COMPART-
MENT DOOR IN PLACE.
1. Place the Magnehelic gauge
outside the furnace.
2. Connect two (2) hoses to the
Magnehelic gauge ports.
3. Run the hoses through an
unused entry port into the
vestibule and seal the opening
through which the hoses pass.
4. Place a tee in the pressure
switch hose.
5. Connect one Magnehelic gauge
hose to the stem of the tee.
Allow the other hose to just
"dangle" near the pressure
switch.
6. Run the induced draft blower
only to check each switch.
7. Note the negative pressure on
the Magnehelic gauge. The
negative pressures must
correspond to the settings in
Table 22 OR BE MORE
NEGATIVE to close the switch
contacts.
8. Use a voltmeter to measure the
voltage across the switch
contacts.
9. Run the voltmeter leads to the
switch through a sealed port like
the Magnehelic hoses. A zero
volt reading confirms a closed
switch. 24 volts indicates an
open switch.
There are several reasons for the
pressure switch not to close.
1. An inoperative induced draft
blower.
2.
A loose or leaky pressure switch
hose.
3.
A blockage or restriction in the
vent system or combustion air
intake.
4.
A leaky gasket at the induced
draft blower.
5. A blocked drain line, or trap.
6. A trap that hasn't been filled with
water before operation.
The pressure switch contacts
must open before the unit can go
through another heating cycle.
FIGURE 104
MAGNEHELIC GAUGE CONNECTION
HIGH LIMIT CONTROL
Operation.All furnace models use a
high limit control. It detects
abnormally high air temperatures in
the heat exchanger area and
prevents the air leaving the unit from
exceeding the maximum outlet air
temperature. When the limit opens it
communicates with the integrated
furnace control to stop the burners. It
resets automatically after the air
temperature drops. The high limit is
set at the factory and cannot be field
adjusted. It is a 24 volt control.
If the limit opens, the main blower
and induced draft blower operate
to cool the limit switch. Allow the
unit time to reset the limit.
The
blower must run through its off delay
before the IFC allows the furnace to
operate again.
89
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