Tempstar 40VA Transformer for T9MPV125L20A1 — 115V to 24V | Ships Today

Overview

This OEM 40VA control transformer (part number 1012722, "TRANS 115>24 40VA") supplies the low-voltage control power for the Tempstar T9MPV125L20A1 gas furnace. It steps the 115V line voltage down to the 24V signal that runs the thermostat circuit, gas valve, control board, and the rest of the furnace's low-voltage components. When the transformer fails — often from a shorted thermostat wire or a stuck contactor drawing too much current — the entire control system goes dead: no thermostat response, no ignition, and no blower. Replacing a failed transformer restores 24V power and brings the system back to life.

Specifications

Compatibility

Direct-fit replacement for the Tempstar T9MPV125L20A1 furnace. The 40VA 115V-to-24V transformer is a common control component shared across many ICP, Heil, Comfortmaker, and Tempstar furnace models. Confirm the existing part number is 1012722 and that your primary line voltage is 115V before installing.

Installation Guide

1. Safety First — Disconnect furnace power at the breaker and verify zero voltage at the transformer leads with a multimeter.
2. Access — Remove the control-compartment service panel to reach the transformer.
3. Document — Photograph every primary and secondary wire connection before disconnecting.
4. Remove — Label and disconnect the leads, then unbolt the transformer from its mounting bracket.
5. Install — Mount the new transformer, reconnect the 115V primary and 24V secondary leads exactly as photographed, and confirm no loose terminals.
6. Test — Restore power, measure ~24V across the secondary, and confirm the thermostat, ignition, and blower operate normally.

Signs You Need a Replacement

• Furnace is completely dead — no response from the thermostat.
• No 24V reading across the transformer secondary terminals.
• Blown low-voltage fuse on the control board that keeps re-blowing.
• Burnt smell or visibly scorched/melted transformer housing.

Shipping & Returns

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