Tempstar 40VA Transformer for T8DNL125L20B1 — 115V to 24V | Ships Today

Overview

This OEM control transformer, Tempstar part number 1172810, steps the furnace's 115V line voltage down to the 24V control voltage used by the thermostat, gas valve, control board, and safety circuits. Rated at 40VA, it supplies enough current to drive the low-voltage controls on the T8DNL125L20B1 furnace. When a transformer fails — usually from a shorted thermostat wire, a failed gas valve coil, or simple age — the furnace goes completely dead: no thermostat display, no inducer, no response to a heat call. A buzzing or scorched transformer is a clear sign of overload.

Specifications

Compatibility

Direct-fit replacement for the Tempstar T8DNL125L20B1 furnace, and compatible with many related ICP-family models (Heil, Comfortmaker, Arcoaire) that use a 40VA 115V-to-24V transformer. Confirm your primary voltage is 115V and the VA rating matches before ordering.

Installation Guide

1. Safety First — Disconnect electrical power at the furnace switch and breaker. Verify zero voltage with a multimeter.
2. Access — Remove the furnace control-compartment service panel.
3. Document — Photograph every primary (line) and secondary (24V) wire connection before disconnecting.
4. Remove — Disconnect the primary and secondary leads, remove the mounting screws, and lift out the failed transformer.
5. Install — Mount the new transformer, then reconnect the 115V primary and 24V secondary leads exactly per your photo.
6. Test — Restore power, confirm 24V across the secondary, and run a heat cycle to verify the controls energize.

Signs You Need a Replacement

• Furnace is completely unresponsive — no thermostat display or 24V power.
• Transformer buzzes loudly, runs hot, or shows scorch marks.
• No measurable 24V across the secondary terminals.
• Repeated blown low-voltage fuse after a control short is cleared.

Shipping & Returns

Ships same day before 3 PM CST. 30-day returns. Backed by the National HVAC Parts fitment guarantee.