Lennox ML193UH070P36B

Questions and Answers

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michellecookAug 6, 2025
What to do if Lennox ML193UH070P36B fails to fire in the heating mode, combustion air inducer does not energize?
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  nancyfullerAug 6, 2025
  If your Lennox furnace fails to fire and the combustion air inducer isn't energizing, check for correct wiring and secure connections to the combustion air inducer. Correct any wiring issues or replace loose connections. Also, verify that the pressure switch is open when the combustion air inducer is not operating; replace the switch if it's stuck closed.
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Nicholas MillerAug 11, 2025
What to do if my Lennox ML193UH070P36B unit fails to operate in cooling, heating, or continuous fan mode?
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  Gary StokesAug 11, 2025
  If your Lennox furnace fails to operate, start by checking the 120V main voltage supply and address any power failures. Inspect the furnace wiring and connections, ensuring they are correctly wired to the power make-up box and transformer, and that the 24V wiring to the control is correct. Replace any blown fuses, and if they continue to blow, check for shorts. Verify that the door switch is activated when closed, check the wire connections, and test the switch's continuity; replace if defective. Confirm the transformer output is 24V, replacing it if faulty. If these steps don't resolve the issue, the control might be defective and require replacement.
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Walter FarrellAug 16, 2025
Why Lennox Furnace indoor blower fails to operate in cooling, heating, or continuous fan mode?
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  Patrick PerryAug 16, 2025
  If the indoor blower in your Lennox furnace isn't working, first check the wiring and connections at the control and blower motor, correcting any miswiring or loose connections. Then, verify that 120V is supplied to the blower by individually energizing 'Y', 'G', and 'W' on the low voltage terminal strip. Remember that when 'W' is energized, the blower starts after a 45-second delay. If there's 120V to each motor tap but the blower remains inoperative, replace the motor. If there's no 120V when 'Y', 'G', or 'W' is energized, the control is likely defective and needs replacement. Finally, check and replace the run capacitor if it's defective.
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Scott NicholsAug 21, 2025
What to do if Lennox ML193UH070P36B Furnace burners light but unit shuts down prematurely?
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  nathan14Aug 21, 2025
  If your Lennox furnace burners light but the unit shuts down too soon, check for a restricted intake or exhaust vent and remove any blockages. Ensure the correct concentric vent kit is installed. Also, inspect the condensate drain line for proper slope and any blockages, ensuring condensate flows freely during operation; repair or replace any improperly installed lines.
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Joshua JohnsonAug 27, 2025
What to do if Lennox ML193UH070P36B fails to fire in heating mode, combustion air blower energizes, ignitor is energized?
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  Samantha WilliamsAug 27, 2025
  If your Lennox furnace isn't firing, but the combustion air blower and ignitor are working, check that gas is being supplied to the unit. Verify the line pressure at the gas valve, ensuring it doesn't exceed 13" WC for both natural and propane, and reads a minimum of 4.5" WC for natural and 11.0" WC for propane. Inspect the gas valve wiring and connections at multi-pin control amp plugs or the valve for correctness and tightness. If 24V is supplied to the gas valve approximately 35 seconds after heat demand but the valve doesn't open, replace the valve. If 24V is not supplied, replace the control.
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james19Dec 1, 2025
What to do if Lennox Furnace fails to fire in the heating mode, combustion air inducer energizes, ignitor is not energized?
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  Philip GrahamDec 1, 2025
  If your Lennox furnace isn't firing, the combustion air inducer is working, but the ignitor isn't starting, ensure the pressure switch lines are correctly routed. Remove any obstructions from the pressure lines and/or taps. Check the pressure switch lines for leaks and replace any broken lines. Also, check for condensate in the pressure switch line and remove it. Verify the differential pressure across the pressure switch exceeds the set point listed on the switch, and check for a restricted inlet vent, removing any blockage and ensuring proper vent sizing and run length. Confirm that the proper pressure switch is installed; replace if necessary. Check the wiring and connections at the pressure switch for correctness and tightness. If all these checks pass, the pressure switch may be fault...

Summary

Unit Information

Series Units Overview

Provides an overview of the ML193UH series furnaces, their capabilities, and features.

Safety Warnings

Highlights critical safety precautions, including electrical shock and sharp edges.

Specifications

Optional Accessories and Gas Heat Accessories

Installation Clearances

Details required clearances for installation around the unit.

Blower Performance Data

Parts Identification

Component Locations

Identifies major components of the ML193UH furnace via a diagram.

Unit Components

Control Box Components

Details the transformer, door interlock switch, and ignition control within the control box.

Electronic Ignition System

Explains the integrated control, ignitor, and flame sensor functionality.

Fan and Control Terminals

Fan Control Settings

Describes how to adjust the fan off delay settings for comfort.

Integrated Control Terminal Designations

Lists and explains the terminal designations on the integrated control board.

Diagnostic Codes

Integrated Ignition Control Details

Ignition Control Terminal Designations

Provides terminal designations for 4-pin and 12-pin connectors.

Troubleshooting Codes

Lists diagnostic codes and their corresponding LED status for troubleshooting.

Fan Time Control and Twinning

Fan Time Adjustments

Explains how to adjust heating fan on/off times and cooling fan settings.

Furnace Twinning Configuration

Details the process and requirements for twinning two furnaces.

Twinning Field Wiring

Heating Components Identification

Ignitor, Flame Sensor, and Rollout Switches

Identifies and describes the ignitor, flame sensor, and rollout switches.

Ignitor Checks

Key Component Details

Primary Limit, Heat Exchanger, and Burners

Describes the function and characteristics of the primary limit, heat exchanger, and burners.

Gas Valve Function

Explains the operation and replacement of the gas valve.

Combustion Air System Components

Combustion Air Inducer and Header Box

Details the combustion air inducer, its housing, and associated components.

Combustion Air Pressure Switch

Explains the function and operation of the combustion air pressure switch.

Pressure Measurement and Blower Compartment

Measuring Pressure Differential

Guides on how to measure pressure differential for system checks.

Blower Motor and Capacitor

Describes the blower motor and capacitor within the blower compartment.

Placement and Installation: Pipe Specifications

Piping and Fittings Specifications

Lists approved piping and fitting materials and specifications for venting.

Outdoor Termination Usage and Cementing

Outdoor Termination Usage Table

Details outdoor termination options based on input size and vent style.

Joint Cementing Procedure

Provides step-by-step instructions for cementing pipe joints.

Venting Practices and Guidelines

General Venting Practices

Covers pipe suspension, wall thickness, and general venting considerations.

Confined Space Equipment

Shows equipment setup for confined spaces (attic/crawlspace).

Vent Piping Guidelines and Sizing

Vent Piping and Termination Guidelines

Explains vent pipe options and installation rules.

Piping Size Process

A step-by-step guide to sizing vent pipe diameter based on various factors.

Maximum Vent Length Tables

Exhaust Pipe Connections

Upflow;Non-Direct Vent Exhaust

Illustrates typical exhaust pipe connections for upflow direct or non-direct vent.

Horizontal Exhaust Pipe Connections

Shows typical exhaust pipe connections for horizontal direct or non-direct vent.

Air Intake Pipe Connections

Upflow Direct Vent Intake

Illustrates typical air intake pipe connections for upflow direct vent applications.

Horizontal Direct Vent Intake

Shows typical air intake pipe connections for horizontal direct vent applications.

Intake Piping Details

Direct Vent Intake Piping

Instructions for routing intake piping in direct vent applications.

Non-Direct Vent Intake Piping

Guidelines for intake piping in non-direct vent applications.

Vent Termination Guidelines

General Vent Termination Rules

Covers general rules for vent terminations, including location and clearances.

Maximum Uninsulated Vent Lengths

Vent Termination Clearances (Non-Direct Vent)

Vent Termination Clearances (Direct Vent)

Direct Vent Piping Terminations

Exhaust Pipe Termination Size Reduction

Table specifying exhaust pipe size reductions for different models.

Field Fabricated and Alternate Terminations

Field Fabricated Wall Termination

Instructions and diagrams for field-fabricated wall vent terminations.

Alternate Elbow Terminations

Shows alternative termination methods using tees and elbows.

Non-Direct Vent Exhaust Terminations

Condensate Piping Installation

Explains the routing and installation of condensate piping.

Trap;Drain Assembly (3;4 Connection)

Trap;Drain Assembly (1;2 Connection)

Start-Up Procedures

Preliminary and Seasonal Checks

Covers initial checks before starting the furnace, including wiring and gas.

Failure To Operate Diagnosis

Lists common issues and checks when the unit fails to operate.

Heating System Service Checks

Gas Piping and Testing

Details gas piping requirements, leak testing, and pressure checks.

Manifold Pressure and Combustion

Covers manifold pressure measurement and proper combustion checks.

High Altitude Adjustments

Operating Characteristics and Testing

Flame Signal Measurement

Explains how to measure the flame signal using a transducer.

Blower and Temperature Adjustments

Covers blower operation, adjustment, and temperature rise measurement.

External Static Pressure Testing

Details the procedure for measuring external static pressure.

Maintenance Procedures

Filter Inspection and Replacement

Guidelines for inspecting and replacing air filters for optimal performance.

Winterizing and Condensate Trap Care

Instructions for preparing the unit for winter and maintaining condensate traps.

Condensate Hose Screen Cleaning

Steps for cleaning condensate hose screens to prevent blockages.

Cleaning the Heat Exchanger

Cleaning the Burner Assembly

Wiring Diagram and Heating Sequence

ML193 UH Wiring Diagram (Integrated Control 100973)

Provides a wiring diagram for units with integrated control 100973.

Heating Sequence of Operation

Explains the step-by-step process for furnace heating operation.

Cooling Sequence of Operation

Continuous Fan Sequence

Wiring Diagram (Integrated Control 103085)