Tempstar 40VA Transformer for T8MPL125J20B1 — 115V to 24V Class II | Ships Today

Overview

This OEM control transformer (part #1172810) converts 115-volt line power to the 24-volt control voltage your Tempstar T8MPL125J20B1 furnace needs to run its thermostat, gas valve, igniter, and control board. Its 40VA rating provides enough capacity for the complete low-voltage control circuit. A failed transformer kills the entire 24V system: the thermostat goes dark, the furnace ignores heat calls, and you may find a blown low-voltage fuse on the control board or detect a faint burnt-electrical smell. Buzzing or an overheating case indicates the windings are degrading and the transformer should be replaced before it fails completely.

Specifications

Compatibility

Direct OEM fit for the Tempstar T8MPL125J20B1 gas furnace and other ICP-family models (Heil, Comfortmaker, Arcoaire, Keeprite) that specify part 1172810. Verify the VA rating and voltage of your existing transformer before ordering.

Installation Guide

1. Safety First — Disconnect power at the furnace switch and breaker, then verify zero voltage at the transformer leads with a multimeter.
2. Access — Remove the furnace control/blower access panel.
3. Document — Photograph all primary (line) and secondary (24V) wire connections before disconnecting.
4. Remove — Disconnect the primary and secondary leads and unbolt the transformer from its bracket.
5. Install — Mount the new transformer, reconnect the line and 24V leads exactly per your photo, and confirm the ground.
6. Test — Restore power and verify 24 VAC across the secondary terminals and normal furnace start-up.

Signs You Need a Replacement

• Blank thermostat or no response to a heat call
• Repeatedly blown low-voltage control-board fuse
• Audible buzzing or hot/burnt smell from the transformer
• No measurable 24 VAC on the secondary side

Shipping & Returns

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