Transformers

A transformer is two coils of wire wrapped around the same iron core. AC voltage applied to the primary coil induces an AC voltage in the secondary coil, with the voltage ratio matching the turns ratio of the two windings. No moving parts, no electronics, nothing to wear out under normal operation — the transformer is one of the most reliable components in any HVAC system. When they fail, it’s usually because they were overloaded or struck by a lightning surge.

How it steps down

A residential 40 VA transformer has about 1200 turns on its 120V primary and 240 turns on its 24V secondary — a 5:1 ratio. 120V in produces 24V out. The iron core couples the two windings magnetically; the windings themselves aren’t electrically connected.

This magnetic coupling is useful beyond just stepping voltage. It provides galvanic isolation — the secondary circuit has no direct electrical connection to the primary, so a primary-side fault (lightning strike, line transient) doesn’t propagate directly into the 24V control wiring. The 24V side is electrically “floating” relative to line voltage, though it’s usually referenced to ground through the equipment chassis.

Sizing in practice

Pick your 40-VA-vs-75-VA choice by summing the connected 24V loads:

• Gas valve coil: ~7 VA
• Inducer relay: ~4 VA
• Blower relay: ~4 VA
• Ignition module: ~8 VA
• Thermostat (smart): ~3 VA
• Each zone valve motor: ~6 VA running, ~40 VA during startup

A single-zone gas furnace with a smart thermostat: 7 + 4 + 4 + 8 + 3 = 26 VA. A 40 VA transformer has comfortable margin.

A three-zone boiler with smart thermostats on each zone, all potentially calling simultaneously on a cold morning: 7 + 4 + 4 + (3 × 6) + (3 × 3) = 42 VA steady-state, with simultaneous zone-valve startup spikes pushing to 130+ VA briefly. A 40 VA transformer is undersized; 75 VA or 100 VA is needed.

Testing a transformer

Primary voltage. Measure 120 VAC across the two primary leads with the furnace powered on. 115–125 VAC is healthy. 0V means primary power hasn’t reached the transformer — check breaker, disconnect, door switch.

Secondary voltage (no load). Disconnect the 24V load. Measure across the secondary leads. Healthy: 24–28 VAC.

Secondary voltage (loaded). Reconnect load. Measure again. Healthy: 22–28 VAC. Significant drop between no-load and loaded = overloaded transformer or shorted load.

Primary resistance. Power off. Measure primary winding resistance. Typical: 50–200 Ω. OL = open winding, failed.

Secondary resistance. Power off. Measure secondary resistance. Typical: 1–10 Ω. OL = open winding, failed.

Common failures

Burned out secondary. Most common failure. Result of sustained overload or a shorted 24V load. Symptoms: 120V on primary, 0V on secondary. Often smells burnt. Fix: replace transformer. Before energizing the replacement, find and fix the cause of the original failure, or it’ll burn out the new one too.

Open primary. Less common. Usually from a lightning strike or a line-voltage surge. 0V on primary even with power available; or 120V at the terminals but 0V on secondary with no obvious secondary damage. Fix: replace.

Internal short (turn-to-turn). The primary or secondary develops partial shorts between adjacent turns, reducing effective voltage and drawing excess current. Secondary voltage is low even at no-load; primary draws more current than expected. Transformer runs hot. Fix: replace.

Lightning strike. Surge damages both windings. Primary is open, secondary is open, sometimes the transformer is visibly charred. Replace; consider adding a surge protector if the area sees frequent lightning damage.

Wiring

Residential transformers are typically mounted inside the furnace or boiler cabinet with four leads or terminals: two primary (120V in) and two secondary (24V out). Polarity on the primary doesn’t matter for the transformer itself (AC is symmetric), but polarity on the secondary is conventional and important — R is the “hot” side of the 24V secondary, C is the common return. Reversing these on a system where one side of the 24V is grounded will change which wire is “live” and cause control logic confusion.

Some transformers are factory-wired to the equipment; others are field-wired. Field wiring uses wire nuts or screw terminals. As always: land connections firmly, no stray strands, strain-relieve the cable so vibration doesn’t loosen the connection over time.

---

Check your understanding

0 / 3

01A 40 VA transformer's secondary reads 25 VAC no-load but sags to 18 VAC when the system is running. What's the most likely cause?

ATransformer is healthyBThe 24V loads total more than 40 VA, overloading the transformer; replace with a 75 VA transformerCPrimary voltage is lowDSecondary is shorted

02A transformer has burned out its secondary. You replace it with an identical-rated unit and the new one fails within a day. What did you skip?

AThe correct replacement procedureBFinding and fixing whatever caused the original failure — likely a shorted 24V load that's pulling excessive current through the transformerCGreasing the transformerDTightening the primary

03Why is galvanic isolation a useful property of a transformer?

AIt makes the transformer cheaperBThe secondary circuit isn't directly connected to the primary, so primary-side surges or faults don't propagate straight into the 24V control wiring — limits damage and shock riskCIt's required by NECDGalvanic isolation isn't useful

Before you close the chapter

You should now understand how transformers step voltage, how to size one correctly for a residential application, and how to diagnose primary vs. secondary failures. The next chapter starts the sequence-of-operation deep dive, beginning with hydronic systems.