Tempstar 40VA Transformer for T9MPV075F12C1 — 115V to 24V Control | Ships Today

Overview

This is the OEM control transformer (Part Number 1172810) for the Tempstar T9MPV075F12C1 gas furnace. It converts the 115-volt incoming line voltage to the 24-volt low-voltage power that runs the thermostat, ignition control board, two-stage gas valve, and relays. The 40VA rating provides the current headroom required for this variable-speed furnace's full control sequence. When the transformer fails, the entire low-voltage side goes dark — no thermostat display, no call for heat, no ignition. The most common cause is a shorted or overloaded secondary winding, which frequently blows the control board's low-voltage fuse the moment power is reapplied. Installing the correct OEM 40VA transformer restores stable 24V control power and protects the downstream board.

Specifications

Compatibility

Built for the Tempstar T9MPV075F12C1 furnace and shared across the ICP/Heil/Comfortmaker family of 9-series variable-speed gas furnaces that use the 1172810 transformer. Verify your existing unit is a 115V-to-24V, 40VA transformer before installing.

Installation Guide

1. Safety First — Disconnect power at the furnace switch and breaker; verify zero voltage at the transformer primary leads with a multimeter.
2. Access — Remove the furnace control/blower compartment door.
3. Document — Photograph all primary (line) and secondary (24V) wire connections before disconnecting.
4. Remove — Disconnect the line-voltage and 24V secondary leads, then unbolt the transformer from its bracket.
5. Install — Mount the new transformer, reconnect primary and secondary leads exactly per your photo, and tighten all terminals.
6. Test — Restore power, confirm 24V across the secondary, and run a full heat cycle to verify ignition and blower operation.

Signs You Need a Replacement

• No power to the thermostat or control board (dead 24V circuit).
• Repeatedly blown low-voltage fuse on the ignition control board.
• No measurable voltage across the transformer secondary with line power present.
• Burnt smell or visible scorching on the transformer housing.

Shipping & Returns

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