Tempstar 40VA Transformer for T8MPT125J20B1 — 115V to 24V Control Transformer | Ships Today

Overview

This genuine OEM control transformer (part number 1172810) steps line voltage down to low-voltage control power for the Tempstar T8MPT125J20B1 gas furnace. It converts 115V primary input to a 24V secondary output at 40VA, supplying the thermostat, gas valve, control board relays, and ignition circuit. A failed transformer is one of the most common causes of a completely dead control system: no thermostat display, no call for heat, and no blower response even though the furnace has line power. Transformers typically fail from age, a shorted secondary circuit, repeated overload, or a wiring fault that pops the internal winding. A burning smell, a tripped or open primary, or 0V at the secondary with line voltage present at the primary all point to transformer replacement.

Specifications

Compatibility

Built for the Tempstar T8MPT125J20B1 gas furnace and compatible ICP-family units (Tempstar, Heil, Comfortmaker) that use a 115V-to-24V 40VA control transformer. Confirm your original is rated 40VA before installing.

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-compartment service panel.
3. Document — Photograph all primary and secondary wire connections before disconnecting.
4. Remove — Label and disconnect the leads, remove the transformer mounting screws, and lift out the old transformer.
5. Install — Mount the new transformer, reconnect primary (115V) and secondary (24V) leads exactly per your photo.
6. Test — Restore power and verify 24V across the secondary terminals and a normal call for heat.

Signs You Need a Replacement

• No power to the thermostat or control board
• Furnace completely unresponsive despite line voltage present
• Burning smell or visible scorching on the transformer
• 0V at the secondary with proper voltage at the primary

Shipping & Returns

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