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Why Do My Lights Dim When the AC Turns On?

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If the lights dim when the AC turns on in your home, it may not be cause for alarm. Your home’s electrical wiring is a limited circuit, so when one appliance draws a lot of power, it’s taking it away from anything else that you have plugged in. It’s also possible, that the dimming lights are a sign of a problem.

Of everything in your home that uses electricity, your heating and cooling system use by far the most. The AC needs a lot of power when it first turns on. In some homes, especially older ones with dated electrical wiring, the lights dim slightly when the AC turns on because a lot of power is being drawn momentarily. In this case, the lights dimming briefly is normal and doesn’t signal a problem.

However, in other homes, the dimming lights do signal a problem. It could be within the AC system or within the electrical wiring in your home. Electrical issues are potentially very dangerous. If they aren’t resolved, they can cause house fires, electrical shorts that burn out appliances and more. This is why it’s so important to contact a professional for AC repair if your lights dim when the AC turns on—especially if it only began happening recently. If it’s a new occurrence, it could mean something has changed or broken within the AC system.

So how can you tell if this is a potential problem in your home? Take a look at the symptoms. Do the lights dim just a little bit, and only for a few seconds, right when the AC turns on? If so, and if they then return to their normal brightness, it’s probably normal.

Signs of a Problem

Here are some signs that the dimming lights are not normal. If any of these is happening in your home, there may be a problem for a professional:

  • The lights dim quite a bit, not just slightly, when the AC turns on.
  • Rather than dimming once and then brightening again, the lights keep flickering repeatedly.
  • The lights get very bright for a split second, and then start to flicker.

Any of these symptoms could mean there is a problem either within your AC system or within the home’s electrical wiring. This could be the result of several different situations:

  • Loose electrical wiring, or damaged wiring. Flickering lights when the AC turns on could mean that there is loose or damaged wiring somewhere within your home’s network of electrical wires. This can cause lights to flicker when the AC turns on because of the combination of the damaged wiring plus the extra draw on the power from the AC unit. If you experience this, you should contact a professional as soon as possible.
  • An overloaded electrical circuit. If the lights blink out completely for a moment when the AC turns on, it could mean that you have an overloaded electrical circuit. Another sign of this is hearing a crackling sound when the lights dim. This is more common in older homes with dated wiring, but it can happen in any home if there are too many appliances and other items plugged in.
    The AC system should also be on its own electrical circuit. If there are other appliances plugged into that same circuit, it will get overloaded when the AC turns on. This could be a critical situation, so unplug anything that is on the same circuit as the AC and call a licensed professional to assess the situation as soon as possible.
  • A weak or damaged AC capacitor. When the AC turns on for a new cycle, the capacitor provides extra power to the compressor when the motor starts running. If the lights dim significantly when the AC kicks on, it’s a sign that you have a damaged capacitor or it’s not working at full capacity. An HVAC professional can take a look at your capacitor and determine if this is the issue with your AC.

Damaged electrical wiring and circuit problems can be very dangerous, bringing the risk of fires and electrical shock. If your lights dim when the AC turns on, it’s worth reaching out to a professional to diagnose any AC problems.

Bad AC Capacitor Symptoms

A central AC is a complex system made up of several different interconnected parts. There are two different units—one inside the house, one outdoors. Each one has several components, including a capacitor. When this is damaged, there are several bad AC capacitor symptoms that show it’s time to reach out to an AC specialist to replace the part and preserve your heating and cooling system.

The capacitor is one of the parts within the outdoor AC unit, called the compressor. A compressor needs a lot of power to turn on at the start of an AC cycle. Capacitators stores power, so that when the AC turns on, it can give a quick power boost to the compressor. This enables the compressor’s motor to start up. Once the system is running, the capacitor powers down somewhat, but it keeps running throughout the cycle to assist with the AC system’s high voltage needs.

Speaking of voltage, it’s important to know that the capacitor is a particularly high-voltage component of your AC system. If you think your AC is showing bad capacitor symptoms, don’t try to figure it out or replace the part yourself. The capacitor can cause severe electrical shock if it isn’t handled correctly. This is why only trained professionals should handle AC capacitor repairs.

Symptoms of a Weak or Damaged Capacitor

When your AC’s capacitor is weak or damaged, there will be certain specific symptoms that you might notice. Here are the most common symptoms of a bad capacitor:

  • The AC won’t turn on, or it doesn’t start right away.
  • Your AC turns on but doesn’t blow cold air.
  • Smoke or a burning smell comes from the AC unit while it’s running.
  • The AC makes unusual noises when it starts up.
  • Your electricity bills are getting higher month after month.
  • The AC randomly turns off sometimes, for no clear reason.

If your HVAC system is older, it’s not unusual for the capacitor to get weak or go bad over time. Aging AC systems naturally start to fail due to normal wear and tear, and parts start needing repair or replacement. When you live in an area with very high summer temperatures, the intense heat can also cause extra wear on the capacitor, making it overheat.

If your AC is showing any of the above symptoms, it’s time to call in an AC professional to inspect the system. When inspecting your system for a failing capacitor, the AC professional will use a tool called a multimeter to test its current resistance and voltage. They will also inspect the capacitor, looking for issues like cracks, bulging or liquid coming out of it. These will tell the AC pro whether the capacitor is functioning at full capacity or if it needs replacing.

Since the AC includes two units, many parts and many feet of wiring and ductwork, it’s a good idea to have your entire heating and cooling system checked annually by a licensed AC specialist. Air conditioning preventative maintenance keeps your AC running at peak efficiency, which keeps your monthly utility bills lower. Maintenance also heads off potential problems before they turn into big, costly repairs. An AC pro can identify problems like a bad AC capacitor and fix the issue quickly.

How to Fix an Overloaded Circuit

Since AC units consume a lot of power, they should run on their own electrical circuit. When other appliances use the same circuit as the AC, the circuit is overloaded. This can cause problems—even dangerous ones. Knowing how to fix an overloaded circuit will help you avoid a potentially serious situation.

A circuit overloads when too many appliances are trying to use more energy than the circuit is able to handle or provide. One sign of an overloaded circuit is when you turn on a light or an appliance, only to have it shut itself off again. This means the electrical circuit has overloaded, triggering the circuit breaker to shut off to avoid problems.

It’s annoying when the circuit breaker trips, but it’s actually a good thing. This is a fail-safe mechanism designed to prevent major issues like a house fire or electrocution. There are other signs that you have an overloaded circuit, such as flickering or dimming lights, or buzzing sounds coming from an outlet or a switch. Other signs include black burn marks or burning smells coming from the little holes on an outlet. The outlet or switch cover might also be warm to the touch.

If you have an overloaded circuit, you’ll need to plug things into a different circuit that you use less. You may also need to make sure you aren’t running too many appliances or other electrical items at the same time. If overloaded circuits are an ongoing problem, it’s a good idea to have your home’s electrical wiring professionally inspected. It’s possible that you need to upgrade your wiring, outlets or light switches to accommodate your home’s electrical needs.

ABC Can Diagnose and Fix AC Problems

If you’re concerned about your AC unit, contact ABC Home & Commercial Services. Our licensed professionals can provide any needed AC repair and are available all day and night. We also offer ongoing AC maintenance to help extend the life of your system.

Tom Riggs

Tom Riggs is the Division Manager for Mechanical Services, overseeing sales and operations for HVAC, Plumbing, Electrical, Appliance Repair and Water Quality for all ABC Austin branches. He joined ABC in 2014. Before ABC, he was an HVAC Service Technician, HVAC Comfort Advisor/Sales and Operations Manager. Tom attended Universal Technical Institute. He's an avid outdoorsman and enjoys country living with his wife and two sons.

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