Lennox SL280UHVK Furnace Wiring Diagram Addendum

Document Number: 508811-01

Date: 08/2025

This document serves as an addendum to the installation instructions for the Lennox SL280UHVK furnace, featuring Wiring Diagram 538484-02. It supplements Installation Instruction 508553-01 and provides critical wiring details for furnace installation.

Wiring Diagram 538484-02 Description

The diagram illustrates the electrical connections for the Lennox SL280UHVK gas furnace. It details the integration of various components, safety controls, and the main control board (A92 Integrated Control).

Power and Control Input

The system receives power via Line (L1) and Neutral (N) connections, along with an Equipment Ground connection. A circuit breaker (CB8) protects the 120V power supply. The transformer (T1) steps down the 120V AC line voltage to 24V AC for control circuits, providing 24VAC COM and 24VAC (RO(IN)).

Integrated Control (A92)

The A92 Integrated Control board is the central processing unit. It receives inputs from various sensors and safety devices and controls the furnace's operation. Key connections include:

Thermostat Connections: Terminals for R, W1, W2 (heating stages), Y1, Y2 (cooling stages), G (fan), C (common), O (reversing valve for heat pumps), DS (defrost sensor), DH (dehumidification), L (optional).
Safety Inputs: Connections for the Primary Limit (S10), Secondary Limits (S21), Rollout Switches (S47), High Heat Pressure Switch (S102), Low Heat Pressure Switch (S128), and Door Interlock (S51).
Ignition System: Connections for the Ignitor (R33) and Flame Sensor.
Gas Valve (GV1): Controlled by the A92 board.
Blower Motor (B6): Connections for motor speed control and power.
Inducer Motor (B3): Connections for power and control.
Sensors: Connections for Outdoor Air Sensor and Discharge Air Sensor (LGWP).
Optional Components: Terminals for Humidifier and Zone Alarm.

Safety Devices and Interlocks

The diagram shows critical safety interlocks that ensure proper operation:

Primary Limit (S10): Opens circuit if furnace temperature exceeds a safe limit.
Secondary Limits (S21): Additional temperature limit controls.
Rollout Switches (S47): Detect flame rollout from the combustion chamber.
Pressure Switches (S102, S128): Monitor inducer motor pressure for proper venting.
Door Interlock (S51): Interrupts power if the access panel is not securely in place.

Ignition and Combustion

The system uses an ignitor (R33) to ignite gas supplied through the Gas Valve (GV1). A flame sensor monitors flame presence, and the control board shuts off gas if no flame is detected.

Blower Operation

The Blower Motor (B6) circulates air, controlled by the A92 board. Speed settings are configured via DIP switches or jumpers, as detailed in the Blower Speed Chart.

Notes and Warnings

WARNING: Electric Shock Hazard. Can cause injury or death. Unit must be grounded in accordance with national and local codes.

NOTE: If any wire is replaced, it must be with wire of like size, rating, insulation thickness, and termination.

NOTE: Refer to installation instructions for blower speed settings based on cooling tonnage and heating temperature rise.

NOTE: Diagram shows a typical 2 Heat/2 Cool system with a conventional thermostat. Refer to installation instructions for other equipment and accessories.

NOTE: Discharge and Outdoor Air terminals are for icomfort use only.

NOTE: Thermostat connections are for non-communicating systems only. Refer to icomfort installation instructions for communicating systems.

NOTE: DH and L terminals are only used with icomfort thermostat systems.

NOTE: Jumper W915 (Y1 to Y2) is factory installed for single-stage cooling. Cut jumper for two-stage cooling.

NOTE: Cut W914 jumper ("DEHUM OR HARMONY") from DS to R on A92 board for Comfortsense 7000 thermostat use.

NOTE: Cut W951 jumper (O to R) labeled "HEAT PUMP" on A92 board for dual fuel applications.

NOTE: L13 used on 1 HP motors only.

NOTE: DIP SWITCH factory default is for a two-stage thermostat.

NOTE: Use copper conductors only.

Blower Speed Chart

This chart details blower speed configurations for various Lennox SL280UHVK models.

Furnace Model	Program Unit Capacity/Size	Circulator Motor Size	Character on 7-Segment LED (w dot ON)	FACTORY DEFAULT	HIGH FACTORY DEFAULT
SL280UH070V36A / SL280UH070XV36AK	070(X)V36A	1/2 HP	A	DEFAULT
SL280UH090V36B / SL280UH090V36BK / SL280UH090XV36AK	090V36B	1/2 HP	b	DEFAULT
SL280UH090V48B / SL280UH090V48BK / SL280UH090XV48BK	090(X)V48B	1 HP	C		HIGH FACTORY DEFAULT
SL280UH090V60C / SL280UH090V60CK / SL280UH090XV60CK	090(X)V60C	1 HP	d		HIGH FACTORY DEFAULT
SL280UH110V60C / SL280UH110V60CK / SL280UH110XV60CK	110(X)V60C	1 HP	E		HIGH FACTORY DEFAULT
SL280UH135V60D / SL280UH135V60DK	135V60D	1 HP	F		HIGH FACTORY DEFAULT

Component Identification Summary

The wiring diagram labels numerous components. Key components and their typical connections are summarized below:

Lennox Logo: Branding at the top.
ADDENDUM: Title indicating the document's purpose.
Model Numbers: SL280UHVK and specific variants are listed.
Wiring Diagram 538484-02: Identifier for the diagram.
CB8 (Circuit Breaker): Protects 120V power.
T1 (Transformer): Converts 120V to 24VAC.
A92 (Integrated Control): Main control board managing furnace operations.
S10 (Primary Limit): High-temperature safety switch.
S102 (High Heat Pressure Switch): Safety switch for high heat operation.
S128 (Low Heat Pressure Switch): Safety switch for low heat operation.
S47 (Flame Rollout Switch): Safety switch detecting flame rollout.
S51 (Door Interlock): Safety switch for access panel security.
R33 (Ignitor): Component that ignites the gas.
GV1 (Gas Valve): Controls gas flow to burners.
B3 (Inducer Motor): Vents combustion gases.
B6 (Blower Motor): Circulates air for heating and cooling.
S1 (Room Thermostat): User interface for temperature control.
LGWP Sensors: Outdoor Air Sensor and Discharge Air Sensor for advanced control.
Humidifier Terminals: For connecting an optional humidifier.
Zone Alarm Terminals: For connecting an optional zone alarm.

Wiring Connections Summary

The diagram details specific wire routing and connections. Key connections include:

Line voltage (120V) supplies the transformer (T1) and inducer motor.
24VAC from T1 powers the A92 Integrated Control and is distributed to thermostat connections and other low-voltage components.
Safety switches (limits, pressure switches, rollout switches, door interlock) are wired in series, interrupting power to the gas valve or blower if a fault is detected.
Thermostat wires signal heating, cooling, and fan demands to the A92 board.
Jumper settings on the A92 board (e.g., W914, W915, W951) are crucial for configuring the furnace for different system types (heat pump, dual fuel) and thermostat capabilities (single-stage vs. two-stage cooling).
Sensors like Outdoor Air and Discharge Air connect to specific A92 board terminals for advanced control algorithms.

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