What happens
A broken primary winding or an internal non-resettable protective device interrupts the input path.
Typical symptoms
Correct primary voltage, absent secondary output and no functional controls.
Checks that distinguish this cause
After safe isolation, disconnect enough leads to exclude parallel circuits and compare winding continuity with the manufacturer's procedure. Do not diagnose from a generic ohm target.
What can look similar
A downstream short can pull output down without an open primary. Missing primary power produces the same blank thermostat.
Repair direction
Replace with an approved transformer and investigate overheating or a downstream fault before restoring power.
Verify the repair
Check rated output while the expected relays and accessories are operating.
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
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
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.