12.2 Transformers, Control Circuits & Low-Voltage Wiring

12.2 Transformers, Control Circuits & Low-Voltage Wiring

[!IMPORTANT] Nearly every no-heat and no-cool call is a control-circuit call. Line-voltage components fail loudly and rarely; 24 volt circuits fail quietly and constantly. The method in this section - measure to common at each successive terminal - resolves them without guesswork.

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How a Transformer Works

A transformer has no moving parts. Alternating current in the primary winding creates a changing magnetic field in a laminated iron core, and that changing field induces a voltage in the secondary winding. Direct current will not work, because a steady field induces nothing.

Turns Ratio

$\frac{E_{\text{primary}}}{E_{\text{secondary}}} = \frac{N_{\text{primary}}}{N_{\text{secondary}}} = \frac{I_{\text{secondary}}}{I_{\text{primary}}}$

A 240-volt to 24-volt transformer has a 10 to 1 turns ratio. Voltage steps down by 10; current steps up by 10. Power in equals power out, less core and winding losses.

VA Rating and Load Budgeting

Control transformers are rated in volt-amperes (VA), and the sum of the connected loads must not exceed the rating.

A 40 VA transformer carrying a contactor, a fan relay, a gas valve, and a humidifier is at or beyond capacity. Note that inrush current when a contactor coil first energizes is several times its sealed draw, which is why a marginal transformer will buzz, run hot, and eventually fail open.

Multi-Tap Primaries and Secondary Grounding

• Most HVAC transformers have multi-tap primaries - commonly 120, 208, and 240 volts. Landing the 240-volt supply on the 208-volt tap over-fluxes the core and burns the transformer; landing 208 on the 240 tap produces a weak 21-volt secondary that will not reliably pull in a contactor.
• One leg of the 24-volt secondary is normally bonded to the equipment chassis and becomes common (C). That is why touching R to the cabinet produces a short.
• Paralleling two transformers requires matched polarity. If the secondaries are out of phase they oppose each other and both transformers burn.

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Class 2 Control Circuits

NEC Article 725 classifies remote-control and signaling circuits. A Class 2 circuit is limited in voltage and power so that it presents minimal shock and fire-initiation hazard - which is why 24-volt thermostat wire may be run without raceway or cable armor, and why it may not be run in the same raceway or enclosure as line-voltage conductors unless a barrier or listed method separates them. The transformer or power supply itself must be a listed Class 2 source; that listing, not the wire, is what makes the circuit Class 2.

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The Thermostat Color Code and What Each Wire Does

[!WARNING] O versus B is a heat pump trap. Most manufacturers energize the reversing valve in cooling, which is the O terminal. A minority energize it in heating, which is the B terminal. Wire it to the wrong terminal and the system delivers heat when the thermostat calls for cooling. Confirm against the equipment wiring diagram, never against habit.

A Simple Cooling Sequence

1. The thermostat closes R to G, energizing the indoor fan relay.
2. The thermostat closes R to Y, energizing the compressor contactor coil.
3. The contactor pulls in, closing line-voltage contacts to the compressor and condenser fan.
4. On a heat pump in cooling, R to O energizes the reversing valve solenoid.

Every safety in the circuit - the low-pressure switch, the high-pressure switch, the float switch - is wired in series in that low-voltage path, so any one of them opening drops the contactor out.

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Relays, Contactors and Sequencers

• A relay uses a low-power coil to switch a separate circuit. Contacts are described as normally open (NO) or normally closed (NC), always with the coil de-energized.
• A contactor is a heavy-duty relay sized to switch motor loads, rated in amperes and by pole count. Pitted or welded contacts cause chattering, voltage drop across the contacts, and compressor overheating.
• A sequencer on electric strip heat closes its contacts in timed stages after its heater element warms, so the strips do not all inrush at once.
• A defrost board on a heat pump uses time and temperature to initiate and terminate defrost, energizing the reversing valve, stopping the outdoor fan, and bringing on auxiliary heat during the cycle.

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Low-Voltage Wire Sizing

Thermostat cable is normally 18 AWG solid conductor. On long runs the voltage drop becomes real: 18 AWG has roughly 6.4 ohms per 1,000 feet, so a 150-foot run carries about 300 feet of conductor and near 2 ohms of loop resistance. At a 0.5 ampere control load that is a 1-volt drop - tolerable. Double the load or the distance and the contactor may chatter or fail to pull in. On runs over about 100 feet with multiple loads, step up to 16 AWG.

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Troubleshooting a Dead Control Circuit

Work from the source outward, always measuring to common:

1. Confirm line voltage at the transformer primary.
2. Measure the secondary: 24 to 28 volts across R and C confirms the transformer.
3. If the secondary is dead, look for a shorted low-voltage conductor, a shorted valve or coil, or an open fuse. Many boards use a 3 or 5 ampere automotive-style blade fuse on the 24 volt circuit; a blown fuse means a short exists and must be found before the fuse is replaced.
4. With the secondary confirmed, measure R to C at each successive terminal and safety device with a call energized. The first place the 24 volts disappears is the open device.
5. Confirm the coil, not the contacts, when a contactor fails to pull in - measure 24 volts across the coil terminals and check coil resistance with power off.