CONTROLS
This furnace uses several safety
controls, some of which are unique
to this model. However, many are
common with the Low Profile
Furnace. These controls monitor the
operation of the furnace and shut it
off if a malfunction occurs.
NEGATIVE PRESSURE SWITCHES
There are two negative pressure
switches used with the 90 Plus
upflow furnace. One monitors the
negative pressure in the induced
draft blower housing, the other
connects to the outlet of the
secondary heat exchanger and
checks for a blocked drain.
Downflow/Horizontal models
use only the one connected to the
induced draft blower.
INDUCED DRAFT BLOWER
NEGATIVE PRESSURE SWITCH
Operation.
The induced draft blower
(IDB) causes a negative pressure in
the inducer housing which closes the
pressure switch contacts. The IDB
must generate a negative pressure
at least 0.1" beyond its contact
opening setting to close the switch.
The pressure switch contacts route
power to the integrated furnace
control board to start the ignition
sequence.
If there is a problem in the air intake
system, or vent system, or with the
induced draft blower, the pressure
switch does not close its contacts.
This does not allow the unit to start
or shuts the unit down. The unit then
tries to restart or
locksout.
DRAIN PRESSURE SWITCH
Operation.
The drain negative
pressure switch connects to the
drain in the secondary heat
exchanger collector box. The
induced draft blower causes a
negative pressure in the heat
exchanger. If there is blockage in the
drain line, trap, or secondary heat
exchanger, the switch contacts
open. This does not allow the unit
to start or shuts the unit down. It
then tries to ignite again or locks out.
This switch wires in series with the
gas valve.
TABLE25
NEGATIVE PRESSURE
SWITCHES
InducedDraft Blower NegativePressureSwitch
FurnaceInput Set Point Max. Make Point
45.000 BTUH ·1.30' -1.48"
60.000 BTUH
-1.50'
-1.63'
75,000 BTUH ·1.30"
-1.48"
90,000 BTUH
-150"
• 1.63"
105,000 BTUH
-1.11"
-1.29'
120,000 BTUH
-0.80" -0.95"
Drain Negative PressureSwitch
MaximumSwitchMake Setting
Set Point Max. Make Point
ALL SIZES
ALL MODELS
·040" -0.55"
High Altitude InducedDratt Blower
NegalivePressureSwitch
FurnaceInput Sel Point Max. Make Poinl
45,000 BTUH -1.11" • 1.29"
60,000 BTUH
-1.11'
·1.29"
75.000 BTUH
-0.95"
·1.10"
90,000 BTUH
-1.11' ·1.29"
105,000 BTUH ·0.70
-085"
120,000 BTUH ·0.60' -0.75"
HIGH ALTITUDE
FIGURE 102
PRESSURE SWITCH
PRESSURE SWITCHES
Units installed above 5000 feet must
use the high altitude pressure switch
that comes with the RXGY -F04,
-F05, -F06, and ·F07 high altitude
kits. They replace the pressure
switch attached to the IDB.
FIGURE 103
MAGNEHELIC GAUGE
,
•
88
To Next Page
Loading...