1. A-90 Pressure Switches 2013 TSA Meeting
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2. Single Stage Pressure Switches
Single stage furnaces have 2 pressure switches
Housing Pressure Switch (HPS)
Tapped into Inducer Housing
Connects electrically to the CPU of the furnace control board
Collector Box Pressure Switch (LPS)
Tapped into Collector Box
Connects electrically to GVR via furnace control board
The HPS has a lower set point than the LPS
HPS functions as a blocked vent switch
LPS functions as a blocked drain switch
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3. Single Stage Wiring Diagram
Refer to the legend
HPS
Inducer Housing Pressure Switch, SPST (N.O.)
LPS
Collector Box Pressure Switch, SPST (N.O.)
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4. Single Stage Pressure Switches
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5. Single Stage & Two Pressure Switches
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6. Two Stage Pressure Switches
Two stage furnaces have 2 pressure switches
Each switch has its own reference point
Not connected by pressure internally
High Heat Pressure Switch (HPS)
Tapped into Inducer Housing
Connects electrically to high heat stage of gas valve
Low Heat Pressure Switch (LPS)
Tapped into Collector Box
Connects back to GVR contact via furnace control board
The HPS has a lower set point than the LPS
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7. Two Stage Wiring Diagram
Refer to the legend
HPS
High-Heat Pressure Switch, SPST (N.O.)
LPS
Low Heat Pressure Switch, SPST (N.O.)
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8. Two Stage Wiring Diagram
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9. Modulating Furnace Pressure Switches
Modulating Furnace uses 3 pressure switches
Low
Tapped to Medium & High pressure switch
Medium
Tapped to Collector Box & Low pressure switch
High
Tapped to Low pressure switch
Set points are progressive higher
Low to High
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10. Modulating Furnace Pressure Switches
Modulating Furnace uses 3 pressure switches
Low
Tapped to Medium & High pressure switch
Medium
Tapped to Collector Box & High pressure switch
High
Tapped to Low pressure switch
Set points are progressive higher
Low to High
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11. Modulating Furnace Wiring Diagram
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12. Modulating Furnace Wiring Diagram
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13. Modulating Furnace Pressure Switch Labels
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14. Modulating Furnace Pressure Switches
Low
High
Medium
To Collector Box
Switches are linked so one reference hose pulls on the diaphragm of each switch
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15. A-90 Pressure Switch Settings
Single and Two Stage Pressure Switches
Part Number
Collector Box Pressure Switch
Housing Pressure Switch
Max Make-IWC
Nom Break-IWC
 0.83
0.68 +/- 0.05
0.49
0.34 +/- 0.05
 0.90
0.75 +/- 0.05
0.45
0.30 +/- 0.05
0.93
0.78 +/- 0.05
 0.93
0.56
0.41 +/- 0.05
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16. A-90 Pressure Switch Settings
Modulating Pressure Switch Settings
Maximum Heat Pressure Switch
Intermediate Heat Pressure Switch
Minimum Heat Pressure Switch
Input
Max Make-IWC
Nom Break-IWC
Nom Break- IWC
60,000
17” casing
1.21
1.05 +/- 0.06
0.85
0.70 +/- 0.05
0.62
0.47 +/-0.05
21” casing
1.46
1.29 +/- 0.07
1.00
0.85 +/- 0.05
0.64
0.49 +/- 0.05
80,000
1.58
1.40 +/- 0.08
0.75
0.60 +/-0.05
100,000
1.08
0.93 +/- 0.05
0.87
0.72 +/- 0.50
120,000
1.05
0.90 +/- 0.05
0.80
0.65 +/-0.05
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17. A little demo time