Transformers in
HVAC Systems

How Transformers Work

Transformers transfer electrical energy between two circuits through electromagnetic induction - no physical connection between primary and secondary windings. They only work with AC current, which is why AC is used for control circuits (and why DC-powered systems need different voltage regulation methods).

Control Transformer Basics

The standard HVAC control transformer steps line voltage down to 24VAC:

VA Ratings and Circuit Loading

The VA (volt-amp) rating determines how much load the transformer can supply:

Common control circuit loads and their approximate current draw:

Adding too many components can overload the transformer. Signs of an overloaded transformer: transformer runs hot, output voltage drops below 24V under load, transformer hum is louder than normal, or the transformer fails completely.

Testing a Control Transformer

1. With power on - Measure voltage at primary terminals. Should read line voltage (120V or 240V). If not: check breaker, disconnect, and wiring upstream of transformer.
2. With power on - Measure voltage at secondary terminals. Should read 22-26VAC. If primary has voltage but secondary does not: transformer has failed.
3. With power off - Measure resistance of primary winding. Open circuit (infinite resistance) = failed winding. Same for secondary.
4. Under load - Measure secondary voltage while the system is running. Voltage should stay within ~10% of 24V. Large drops indicate transformer is undersized or a control component is drawing too much current (like a shorted coil).

Common Transformer Failures

• Overloaded and burned out - Control circuit is drawing too much current. Find and fix the cause before installing a new transformer, or it will fail again. Check for shorted contactor coil or other short circuit in control wiring.
• Winding open - No output voltage. Often from a voltage surge or momentary short circuit.
• Shorted windings - Transformer runs extremely hot, output voltage is wrong, hum is very loud. Replace immediately.