Tempstar 40VA Transformer for T8DNL125L20A1 — 115V to 24V | Ships Today

Overview

This is a genuine OEM 40VA control transformer (part number 1012722) for the Tempstar T8DNL125L20A1 gas furnace. The transformer steps down 115V line voltage to the 24V control circuit that powers your thermostat, gas valve, igniter circuit, and control board. Rated at 40VA, it supplies enough current to drive all low-voltage components simultaneously. When a control transformer fails, the entire low-voltage system goes dead — the thermostat screen goes blank, the furnace will not respond to a call for heat, and no 24V is present at the control board. A buzzing transformer, a tripped low-voltage fuse, or a burning smell at the control box are common precursors to total failure.

Specifications

Compatibility

Designed as a direct-fit replacement for the Tempstar T8DNL125L20A1 furnace and shared across many ICP-family platforms (Tempstar, Heil, Comfortmaker, Arcoaire) using the same 115V-to-24V 40VA control architecture. Always confirm the printed part number on your old transformer before installing.

Installation Guide

1. Safety First — Disconnect power at the furnace switch and breaker, then verify zero voltage with a multimeter.
2. Access — Remove the furnace control panel / blower compartment door to reach the control box.
3. Document — Photograph the primary (115V) and secondary (24V) wire connections before disconnecting anything.
4. Remove — Disconnect the four leads, remove the mounting screws or bracket, and lift out the failed transformer.
5. Install — Mount the new transformer, then reconnect the line and load leads exactly per your photo.
6. Test — Restore power, confirm 24V at the secondary terminals, and run a full heat cycle.

Signs You Need a Replacement

• Blank thermostat and no furnace response on a call for heat.
• No 24V reading at the secondary terminals or control board.
• Repeated blown low-voltage fuses on the control board.
• Audible buzzing or a burnt smell from the transformer.

Shipping & Returns

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