Tempstar 40VA Transformer for T8MPT100J20A1 — 115V to 24V Control Power | Ships Today

Overview

This OEM control transformer (Part #1012722) supplies the low-voltage power that runs your Tempstar T8MPT100J20A1 furnace's control circuit. It steps incoming 115-volt line power down to 24 volts at 40VA, energizing the control board, thermostat, gas valve, igniter circuit, and blower relays. Without a healthy transformer the entire low-voltage side of the furnace goes dead — no thermostat response, no call for heat, and no fan operation. A transformer typically fails from a shorted secondary winding, an overloaded 24V circuit, a nearby lightning surge, or simple age-related insulation breakdown. Common symptoms include a completely unresponsive thermostat, a blank control board, repeatedly blown low-voltage fuses, or a faint burning-electrical smell at the furnace. This is a direct OEM replacement rated to the exact 40VA load of your equipment.

Specifications

Compatibility

Designed for the Tempstar T8MPT100J20A1 gas furnace. This same 115V-to-24V 40VA transformer is shared across many ICP-family furnaces (Tempstar, Heil, Comfortmaker, Arcoaire) of the same vintage. Always confirm the original part number and the primary/secondary voltage stamped on your existing transformer before installing.

Installation Guide

1. Safety First — Disconnect power at the furnace switch and breaker, then verify zero voltage with a multimeter at the transformer terminals.
2. Access — Remove the furnace control/blower compartment service panel.
3. Document — Photograph every wire on the primary (line) and secondary (24V) terminals before disconnecting.
4. Remove — Disconnect the primary and secondary leads, then unbolt the transformer from its mounting bracket.
5. Install — Mount the new transformer, reconnect primary leads to line voltage and secondary leads to the 24V circuit exactly per your photo.
6. Test — Restore power, confirm 24V across the secondary, and verify the thermostat calls for heat and the board powers up.

Signs You Need a Replacement

• Thermostat is completely dead or display is blank.
• Control board shows no power and the furnace won't start a heat cycle.
• Low-voltage fuse blows immediately or repeatedly.
• Burning smell or discoloration at the transformer body.

Shipping & Returns

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