Lennox 15D57 Defrost Board

The Lennox 15D57 Defrost Board is a high-performance OEM defrost control kit specifically engineered for direct replacement in Lennox heat pump systems. As a veteran-grade HVAC replacement part, this kit integrates a demand defrost control logic that actively monitors outdoor coil temperatures via the included 28W71 ambient sensor harness. This prevents excessive frost accumulation and optimizes system efficiency during low-ambient operation. The 15D57 is the primary heat pump control board replacement for several legacy Lennox, Allied, and Armstrong part numbers, including 15D5701, 42W04, 81W99, and 100135-03. Its plug-and-play architecture ensures seamless HVAC repair and compatibility with 1/4 inch quick-connect terminals, high-pressure switch interlocks, and auxiliary heat (W1) outputs. Using a Lennox 15D57 ensures your system maintains a precise defrost cycle, preventing cold-blow and reducing metabolic strain on the compressor during winter months.

Key Benefits:

• Broad Compatibility: Serves as a direct OEM upgrade for legacy parts including 13U60, 56M85, 40W00, and 10M89.
• Enhanced System Protection: Integrated anti-short cycle delay and high-pressure switch monitoring protect the compressor from rapid cycling and over-pressurization.
• Optimized Efficiency: Intelligent demand defrost control initiates cycles only when necessary, improving SEER/HSPF performance compared to time-temperature boards.
• Complete Kit: Includes the 81W99 control board, 28W71 sensor harness, and updated wiring instructions for rapid field installation.

Common Questions:

How do I verify if the 15D57 is compatible with my heat pump?
This board replaces the 100135 series and is used across many Lennox XP and HPX series units. Check your existing board for part numbers like 100135-03 or 42W04; the 15D57 is the direct technical successor for these models.

Why is my heat pump outdoor fan staying on during a defrost call?
If the fan does not de-energize during defrost, the board relays or the reversing valve signal may be failing. Use a multimeter to check for 24V between the O and Common terminals to confirm the board is correctly commanding the shift.

Does this board require the sensor harness for operation?
Yes. To function in demand defrost mode, the coil sensor must be installed. Without it, the board may default to a fail-safe mode or fail to initiate a defrost cycle, leading to outdoor coil icing.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.