What happens
An upstream interruption prevents the transformer from receiving its input; the transformer itself may be healthy.
Typical symptoms
Blank thermostat and dead relays; no correct voltage across the primary input.
Checks that distinguish this cause
Trace the wiring diagram back through the supply, disconnect, door interlock and neutral or second leg. A qualified technician measures across the intended primary conductors, not just one conductor to ground.
What can look similar
An open winding still has the correct supply across its input. A floating conductor can produce misleading voltage readings.
Repair direction
Restore the failed upstream circuit or connection after identifying why it opened.
Verify the repair
Confirm specified primary voltage and stable secondary voltage under a normal control load.
Equipment-specific limits
Read the primary taps, secondary rating, VA capacity and protection arrangement. A normal unloaded voltage can differ from the rated loaded voltage. There is no universal winding-resistance value.
Practice this diagnosis
This cause is a reference topic; it does not yet have a dedicated simulated call. Browse the practice board.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Resideo: Universal transformer installation instructions (69-0251)Model-specific primary taps, output ratings and checkout; use your own transformer label.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
Editorial review: 2026-09-29. Verify the current manual for the installed model.