Lennox 17W56 Transformer

The Lennox 17W56 transformer, also identified as model LB-111888A, is a heavy-duty Class 2 transformer engineered for precision voltage step-down in residential and commercial HVAC systems. This 40VA control transformer features a primary input of 120V AC at 60Hz and a stabilized 24V AC secondary output, making it the standard HVAC 24V power supply for critical components such as furnace control boards, 24-volt contactors, and digital thermostats. Built with high-grade laminated steel plates to minimize audible humming, it includes a foot-mount chassis and 1/4-inch quick connect terminals for rapid field replacement. This unit is the direct OEM replacement transformer for widespread Lennox, Armstrong, and Ducane furnace series including the G1D, G1N, and CG series. It provides the necessary current to pull in contactors and energize ignition systems without overloading the low-voltage circuit.

Key Benefits for Technicians:

• Broad Compatibility: Serves as a direct replacement for Lennox part numbers 29M71, 100972-01, and multiple Allied Air brands.
• System Reliability: The 40VA rating provides sufficient overhead to power multiple accessories, including humidifiers and electronic air cleaners, without the risk of voltage drop.
• Seamless Installation: Features a standard foot-mount design with spade terminal connections for a secure, vibration-resistant fit in tight control cabinets.
• Safety Standard Compliance: Engineered as a Class 2 power source to prevent fire and shock hazards by limiting power output during a short circuit.

Common Questions:

What are the signs of a failing Lennox 17W56 transformer?
Common indicators include a furnace that refuses to initiate its ignition sequence, an unresponsive thermostat, or a loud vibrating noise coming from the control compartment. If you measure zero voltage at the 24V secondary terminals while 120V is present at the primary, the internal thermal fuse has likely blown due to a low-voltage short.

Can I use this 40VA transformer to replace a 30VA model?
Yes, you can safely upgrade from a lower VA rating to a 40VA unit. The higher VA rating ensures the transformer can handle a greater electrical load without overheating, which is often necessary when adding smart thermostats or high-draw relays to an existing system.

Will this transformer fix a furnace that cycles but blows cold air?
It can. If the transformer is failing, it may provide enough power to run the blower motor but insufficient current to keep the gas valve or ignitor energized. Verify secondary voltage under load to ensure it maintains at least 21.5V AC to properly hold in the contactors.

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.