It is the middle of July in Aiken. The temperature is 97 degrees. Your air conditioner stops cooling. You call an HVAC company, wait hours for a technician, and then hear: "Your system is fine mechanically. You have an electrical problem." Now you need an electrician, and you have lost a day in the heat.
This scenario plays out constantly during Aiken's long, brutal summers. A significant percentage of HVAC failures are electrical in nature, not mechanical. Understanding the electrical side of your HVAC system can save you time, money, and a lot of discomfort. Here are the most common electrical problems that cause HVAC systems to fail.
Disconnect Box Failures
Every outdoor air conditioning or heat pump unit has a disconnect box mounted on the wall nearby. This is a small metal or plastic enclosure that contains either a pull-out fuse block or a circuit breaker. Its purpose is to provide a local means of disconnecting power to the unit for service, as required by the NEC.
Disconnect boxes are exposed to the elements year-round, and in Aiken's climate, they take a beating. The most common failures include:
Corroded or burned fuse contacts. The pull-out fuse block has metal blade contacts that can corrode over time, especially when exposed to moisture. Corroded contacts create high resistance, which generates heat. You may see melted plastic, blackened contact points, or a burning smell around the disconnect box. In severe cases, the fuse block welds itself into the contacts and cannot be removed without breaking the box.
Blown fuses that look intact. The cartridge fuses inside a pull-out disconnect can blow without any visible sign. The fuse looks fine from the outside, but the internal element has opened. Your HVAC technician tests the unit and finds no power reaching it. The fuses test open, but since they looked normal, no one checked them first. This is an easy fix once identified, but a common source of confusion.
Water intrusion. Older disconnect boxes with cracked covers or missing knockouts allow rain and irrigation water inside. Water on live electrical connections causes corrosion, arcing, and eventual failure. If your disconnect box cover does not close tightly or you see water stains inside, it needs attention before it causes a failure on the hottest day of the year.
Capacitor Issues
Capacitors are electrical components inside your outdoor HVAC unit that store and release energy to start the compressor and fan motors. There are two types: the start capacitor, which provides the initial burst of energy to get the compressor spinning, and the run capacitor, which provides continuous energy to keep the motors running efficiently.
Capacitors are one of the most frequent failure points in HVAC systems, and the failures are almost always accelerated by heat. In Aiken, where outdoor units regularly operate in ambient temperatures above 95 degrees for months at a time, capacitors are under extreme stress.
Signs of a failing capacitor include: the outdoor unit hums but does not start, the fan runs but the compressor does not engage, the system starts but shuts off after a few minutes, or you hear a clicking sound from the outdoor unit followed by nothing. A swollen or leaking capacitor is visually obvious, but many failed capacitors look perfectly normal from the outside. Testing with a multimeter is the only reliable way to confirm a capacitor failure.
While HVAC technicians commonly replace capacitors, the underlying cause is sometimes electrical. If capacitors are failing repeatedly, every season or more often, the problem may be voltage irregularities on the circuit feeding the unit. Low voltage from an undersized circuit, a loose connection in the panel, or a shared circuit with other large loads can cause the compressor to draw excessive current, which overheats and destroys capacitors prematurely.
Contactor Problems
The contactor is an electrically operated switch inside the outdoor unit that controls power to the compressor and fan motor. When your thermostat calls for cooling, it sends a low-voltage signal (24 volts) to the contactor, which closes the high-voltage circuit (240 volts) and starts the unit.
Contactors wear out over time because they physically open and close thousands of times per cooling season. In Aiken, where air conditioners may run 12 to 16 hours per day from May through October, contactors accumulate wear much faster than in milder climates.
Pitted or welded contacts are the most common contactor failure. Each time the contactor closes, a small arc occurs at the contact points. Over thousands of cycles, the contacts become pitted and rough, which increases resistance and heat. Eventually, the contacts may weld shut, meaning the unit runs continuously and will not shut off even when the thermostat is satisfied. This wastes energy, overworks the compressor, and can cause the system to freeze up.
A contactor that chatters (rapidly opens and closes) indicates a problem with the low-voltage control circuit, often a thermostat wiring issue rather than a contactor failure. Replacing the contactor will not fix a chattering problem if the real issue is in the thermostat wiring.
Thermostat Wiring Issues
The thermostat in your home communicates with the HVAC system through a bundle of low-voltage wires, typically 18-gauge, color-coded conductors running from the thermostat to the air handler and outdoor unit. These thin wires carry 24-volt control signals that tell the system when to heat, cool, and run the fan.
Thermostat wiring problems cause some of the most confusing HVAC symptoms because the system appears to have power but does not respond correctly to thermostat commands.
Corroded or loose wire connections at the thermostat or air handler. The screw terminals where thermostat wires connect are small and easily loosened by vibration or thermal expansion. A wire that is making intermittent contact will cause the system to cycle erratically, short-cycling on and off or failing to start at all.
Damaged wire in the wall or attic. Thermostat wire runs through walls and attics where it can be damaged by rodents, staples driven during other work, or heat degradation in attic spaces that regularly exceed 140 degrees in Aiken summers. A broken conductor in the middle of a wire run is invisible from either end and requires tracing the wire to find the damage.
Insufficient conductors for new thermostats. If you are upgrading to a smart thermostat like a Nest, Ecobee, or Honeywell Home, you may need more conductors than your existing thermostat wire provides. Older systems often have 4-conductor wire, but modern smart thermostats typically require 5 conductors (including a dedicated C-wire for continuous power). Running new thermostat wire is a straightforward job for an electrician, and it is far better than using adapter kits that work around the missing wire.
Dedicated Circuit Requirements
Every air conditioning system, heat pump, and furnace requires its own dedicated circuit from the electrical panel. This means a breaker that serves only the HVAC equipment, with no other loads sharing that circuit. The NEC requires this for safety and performance reasons.
In older Aiken homes, especially those built before central air conditioning was standard, HVAC systems were sometimes added after construction and connected to circuits that were not properly sized or dedicated. Common problems include:
Undersized circuit breaker or wire. A 3-ton air conditioner typically requires a 30-amp, 240-volt dedicated circuit with 10-gauge wire. If the system was installed on a 20-amp circuit with 12-gauge wire, the breaker will trip under full load, especially during the hottest days when the system runs hardest. Some installers have been known to simply upsize the breaker without upgrading the wire, which is a serious fire hazard because the wire overheats before the breaker trips.
Shared circuits. If your HVAC circuit shares a breaker with other loads, such as outdoor outlets, a pool pump, or workshop equipment, the combined draw can exceed the circuit's capacity. The breaker trips, your AC shuts off, and the cause is not obvious unless you know the circuit is shared.
Long wire runs with voltage drop. In large homes or when the panel is far from the outdoor unit, the wire run can be long enough to cause significant voltage drop. A unit designed to operate at 240 volts that is receiving only 220 volts due to wire length will draw more current to compensate, stressing components and shortening their lifespan. This is especially common in homes where the HVAC unit is on the opposite side of the house from the panel.
When to Call an Electrician Instead of an HVAC Tech
Call an electrician first if: the breaker for your HVAC trips repeatedly, you have no power at the outdoor disconnect box, you see burn marks or melting at the disconnect or panel, your HVAC was recently installed and is not performing correctly, you need a new dedicated circuit for a replacement system, or you are upgrading your thermostat and need new wiring. An electrician can diagnose and fix the electrical infrastructure problems that HVAC technicians are not equipped or licensed to address.