A technician finds that a 24V control transformer's internal fuse keeps blowing immediately after replacement. Voltage checks show 24V present at the transformer secondary before connecting the thermostat wires. What is the most likely cause?
- AA weak thermostat battery
- BAn oversized VA rating on the replacement transformer
- CA dirty air filter restricting airflow
- DA shorted low-voltage wire touching a grounded duct or metal cabinet
Show answer & explanationAnswer & explanation
Correct answer: D. A shorted low-voltage wire touching a grounded duct or metal cabinet
Since the transformer produces correct voltage before the low-voltage wires are connected, the fuse blowing on connection points to a short circuit in the field wiring, commonly caused by bare wire contacting a grounded metal surface. Filter restriction and battery issues don't cause immediate secondary short circuits, and an oversized VA transformer wouldn't blow immediately on connection unless there's an actual short.
Why the other options are wrong
- A. A weak battery affects thermostat display, not transformer fuses.
- B. Oversizing VA doesn't itself create a short; it would allow more current before failing.
- C. Airflow restriction affects blower performance, not the 24V circuit.
Low-Voltage Short Diagnosis
A repeatedly blown transformer fuse immediately upon reconnecting thermostat wiring usually indicates a short circuit in the low-voltage wiring or a shorted component.
- Test transformer output before connecting wires
- Bare wires touching grounded metal cause shorts
- Isolate wires one at a time to find the short
- Never bypass a fuse with foil or jumper
Memory trick: Fuse pops the instant wires connect? Somewhere there's a short circuit connect.