HVAC Electrical Components

Most HVAC failures are electrical, and a small handful of parts cause most of them. A failed capacitor can make a healthy compressor hum and trip. Pitted contacts can kill a condensing unit on the hottest day of the year. If you can identify these parts and test them safely, you can solve many calls with a meter and a truck-stock part.

What You Will Be Able to Do

• Identify the main electrical parts in a condensing unit and an air handler.
• Handle capacitors safely after power is off.
• Test a capacitor, contactor and transformer.
• Compare motor amp draw to the nameplate.
• Find the common, start and run terminals on a compressor.

Safety Comes First

Condensing units usually run on 208 to 240 volts, and furnaces and air handlers on 120 volts, with a 24-volt control circuit. Line voltage can kill you. Testing live equipment is work for qualified people, and a helper should only do it under direct supervision.

1. Turn off the disconnect or breaker, and lock it out.
2. Test your meter on a known live source, test the equipment, then test the meter again.
3. Discharge any capacitor before you touch it.

Warning: A capacitor can hold a dangerous charge after the power is off. Never rely on a capacitor being empty. Discharge it with a rated discharge tool or a suitable resistor, and confirm zero volts with a meter before you touch the terminals. Do not short it across with a screwdriver. The sparks can burn you and damage parts.

Capacitors

A capacitor stores electrical charge and gives motors a boost.

• Run capacitor: stays in the circuit while the motor runs, which improves efficiency and helps it make torque.
• Dual run capacitor: one can serving both the compressor and the outdoor fan. It has three terminals, marked C (common), HERM (compressor) and FAN.
• Start capacitor: gives a brief strong boost at startup, then drops out. It is often part of a hard-start kit.

The label shows capacitance in microfarads (MFD or µF) and a voltage rating such as 370 or 440 volts. A replacement must match the microfarad rating. Its voltage rating must be the same or higher.

How to test a capacitor: with power off and the capacitor discharged and disconnected, set a meter to capacitance and read across the terminals. Compare with the label tolerance, which is commonly plus or minus 5 or 6%. A dual capacitor gets two tests, C to HERM and C to FAN. A swollen top or leaking oil means replace it.

Worked example: a capacitor labeled 45/5 MFD with a 6% tolerance. The compressor side should read about 42.3 to 47.7 MFD. A reading of 38 means it has failed.

Contactors and Relays

A contactor is a heavy-duty switch. A 24-volt coil pulls in contacts that switch line voltage to the compressor and outdoor fan. A relay does the same job for smaller loads, such as a blower.

• Pitted or burned contacts can drop voltage and make motors run hot. Replace the contactor.
• Ants and other insects can get crushed between contacts and keep the unit from running.
• If the coil is not pulling in, check for 24 volts at the coil. If you have 24 volts but no pull-in, the coil is likely bad.
• If you have no 24 volts, the problem is upstream, in the thermostat, wiring or a safety.

Transformers

A control transformer steps line voltage down to about 24 volts for the thermostat and controls. A reading of 24 to 28 volts on the secondary side is normal. A blown low-voltage fuse usually means a short, so find the cause before you replace it. Transformers are sized in VA, and adding accessories can overload a small one.

Motors

Measure amp draw with a clamp meter around a single wire. Compare with the nameplate:

• RLA is rated load amps on a compressor.
• FLA is full load amps on a fan or blower.
• LRA is locked rotor amps, the current a motor pulls if it cannot start.

Amps much higher than the nameplate point to high head pressure, a failing motor or low voltage. Amps much lower can point to low load, such as low charge.

Compressor Terminals

A single-phase compressor has three terminals: common (C), start (S) and run (R). With the power off, disconnected and discharged, measure resistance between each pair.

• The highest reading is between start and run.
• The middle reading is between common and start.
• The lowest reading is between common and run.
• Common-to-start plus common-to-run should equal start-to-run.

Worked example: C to R reads 1 ohm, C to S reads 3 ohms, S to R reads 4 ohms. The sum fits. An infinite reading means an open winding, or the internal overload may be open because the compressor is hot. Let it cool before you decide. Any reading from a terminal to the copper shell means the motor is grounded and the compressor has failed.

A Compressor That Hums and Trips

1. Check voltage at the contactor during start. Low voltage can stall a motor.
2. Check the capacitor.
3. Check amps, and compare with LRA on the nameplate. A hum at about LRA means the compressor cannot start.
4. Try a hard-start kit if the manufacturer allows it.
5. If it still will not start, suspect a locked compressor.

A failed capacitor is a much cheaper fix than a compressor, so always rule it out first.

Common Mistakes

Check Your Understanding

A condensing unit hums, then trips off. The contactor is pulled in and voltage is good. What do you check next? The run capacitor, after discharging it, then the amp draw at start.

A dual capacitor reads 35 MFD on HERM and is labeled 45. Is it good? No. It is far below the rating, so replace it with a matching one.

You get 24 volts across the contactor coil, and it will not pull in. What is the likely fault? A failed contactor coil.

A compressor reads open from C to R and C to S, but S to R shows a value. The shell is very hot. What should you do? Let it cool. The internal overload is likely open, not a dead winding.

Key Takeaways

• Lock out and verify zero volts before you touch parts, and discharge capacitors.
• Test capacitors against their label, and match the microfarads.
• A contactor needs 24 volts on its coil and clean contacts to do its job.
• Compare amp draw with the nameplate, using RLA, FLA and LRA.
• On a compressor, the highest resistance is start to run, and any reading to the shell means a grounded motor.