What happens
An unintended low-resistance path between control supply and return draws excessive current. Pinched thermostat cable or a failed coil may be responsible.
Typical symptoms
Control fuse opens, transformer protection trips or secondary voltage collapses when a particular branch is called.
Checks that distinguish this cause
With power isolated, label and separate downstream branches. Inspect damaged insulation and isolate suspected coils before checking resistance. A normal coil is a load, so a continuity beep alone does not prove a short.
What can look similar
An overloaded but intact set of accessories can also cause voltage sag. An open circuit usually stops current instead of increasing it.
Repair direction
Repair the proven cable or load fault, then fit only the specified protective device. Do not repeatedly replace fuses to hunt the fault.
Verify the repair
Confirm stable control voltage and acceptable load current through each operating mode.
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.