Ao Smith 100074412 High Limit Switch

Product Description

The AO SMITH 100074412 is a replacement high limit switch designed for use in tankless water heater systems. A high limit switch is a safety control component that monitors heat levels within the unit and interrupts operation when temperatures exceed a safe threshold, protecting the heat exchanger and internal components from thermal damage. This switch is a direct OEM-style replacement and is installed by licensed plumbers and HVAC technicians during routine maintenance or after a fault condition triggers a no-heat or lockout event on a compatible tankless unit.

This high limit switch is a specified replacement part for a wide range of AO SMITH tankless water heater models covering both natural gas and liquid propane configurations. Compatible units include indoor, outdoor, and direct-vent installations across residential and light commercial applications. Refer to the compatibility list below for confirmed model fitment before ordering.

Key Features

• Direct OEM-style replacement for AO SMITH tankless water heater high limit switch
• Interrupts heater operation when internal temperatures exceed safe limits, preventing heat exchanger damage
• Compatible across multiple AO SMITH tankless model families in both NG and LP fuel types
• Supports indoor, outdoor, and direct-vent installation configurations
• Restores proper thermal safety control after fault or lockout events

Compatibility

Confirmed compatible with the following AO SMITH tankless water heater models: T-KJR2U-IN (NG), T-M199 (LP), T-H3S-DV (NG), TK-310U-I (NG/LP), TK-310U-E (NG/LP), T-KJR2-IN (NG), T-KD20 (NG), T-KD20 (LP), T-H3M-DV (LP), T-H3M-OS (LP), T-H3S-OS (NG), T-KJR2-OS (NG), T-M199 (NG), T-H3J-OS (LP), T-K4-OS (NG), T-K4-OS (LP), T-K4-IN (NG), T-KJR2-OS (LP), T-H3J-DV (NG), TK-310C-NI (NG), TK-110C-NI (NG), T-H3J-OS (NG), T-M1 (NG), T-H3S-DV (LP), T-K4U-OS (NG), T-KJR2U-OS (NG), T-KJR2-IN (LP), T-H3M-OS (NG), T-K4U-IN (NG), TK-110U-I (NG/LP), T-H3J-DV (LP), T-H3S-OS (LP), T-K2 (LP), T-M1 (LP), T-K4-IN (LP), TK-110U-E (NG/LP), T-H3M-DV (NG), T-K2 (NG).

Installation Notes

Installation should be performed by a licensed plumber or qualified HVAC technician. Before servicing, shut off the gas supply and disconnect electrical power to the unit to eliminate shock and ignition hazards. Verify the replacement switch matches the original component location and wiring configuration before reconnecting. After installation, restore power and gas, then test the unit through a full heating cycle to confirm normal operation and proper safety cutoff function.

Technical Specifications

• Application: Replacement Part
• Compatible Fuel Types: Natural Gas (NG), Liquid Propane (LP)

Standard Information

• Brand: AO SMITH
• Manufacturer Part Number: 100074412