Check transformer phasing on unmarked transformers

What is going on

Some electronic boards that use flame rectification need the low voltage side to be in phase with the line side. If a transformer has no phasing marks, you have to find out yourself. Wrong phasing can stop the board working at all.

Likely causes

• Unmarked transformer wired with the secondary reversed
• Transformer replaced without checking phasing
• Board with flame rectification that will not accept out-of-phase power
• Flame sense faults appearing right after a transformer swap

Check in this order

1. With power on, measure from the low voltage common/ground to the hot line leg
2. About 96 V means primary and secondary are in phase
3. About 144 V means they are out of phase, so reverse the low voltage wires
4. The readings are approximate, so a clear gap between the two numbers is what matters
5. Wire the furnace to match the phasing. Some transformers have phasing symbols to help identify the correct orientation
6. Power off before changing wires, then retest and confirm the board starts normally

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)