Tempstar 40VA Transformer for T8MPL125J20A1 — 115V to 24V | Ships Today

Overview

This is the genuine OEM 40VA control transformer (part number 1012722) for the Tempstar T8MPL125J20A1 gas furnace. It steps the incoming 115V line voltage down to the 24V secondary that powers the furnace control board, gas valve, thermostat, and other low-voltage components. When this transformer fails, the entire 24V control circuit goes dead — the thermostat screen may go blank, the furnace won't respond to a call for heat, and no relays will energize. A 40VA rating provides ample capacity for the furnace's control loads. A blown control fuse, a dead thermostat, or a furnace that does nothing on a heat call are classic symptoms of a failed transformer.

Specifications

Compatibility

Built for the Tempstar T8MPL125J20A1 furnace and other ICP / Heil / Comfortmaker / Tempstar models calling for a 115V-to-24V 40VA transformer. Verify your primary and secondary voltages and VA rating before installing.

Installation Guide

1. Safety First — Disconnect power at the switch and breaker, then verify zero voltage with a multimeter.
2. Access — Remove the furnace control-compartment service panel.
3. Document — Photo all primary and secondary wire connections before disconnecting.
4. Remove — Disconnect the line-voltage and 24V leads, then unbolt the old transformer from its bracket.
5. Install — Mount the new transformer, reconnect the 115V primary and 24V secondary leads exactly per your photo.
6. Test — Restore power and confirm 24V at the secondary and normal furnace operation on a heat call.

Signs You Need a Replacement

• Blank or dead thermostat with good batteries
• Furnace completely unresponsive to a call for heat
• No 24V reading across the secondary terminals
• Repeatedly blown low-voltage control fuse

Shipping & Returns

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