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AC Unit Smells Burnt? Here’s Why (+ How to Fix)

Generally, you should turn off your AC unit ASAP when it smells burnt. A major fault in the unit can trigger an electrical fire, so it’s safer to investigate the problem when the unit isn’t operating. 

Reasons why your AC unit smells burnt include a failing motor, bad capacitor, worn-out bearings, and damaged wiring. Also, your unit may have a short circuit, frying transformer, faulty breaker, worn-out belt, and blocked or insufficient airflow. 

Read on for more information on the reasons why an AC unit smells burnt and easy fixes. 

Why Your AC Unit Smells Burnt

When your AC unit smells burnt, the first step you should take is to locate its source. The odor may come from the indoor or outdoor unit, or — in some cases — the ducts or vents. If you can trace the source of the odor, you’ll have fewer AC unit components to check, and you’ll save more time. 

Also, the odor can offer clues as to what’s causing the issue. If it smells of melting plastic, the electrical wire casings are probably the culprit. On the other hand, the smell of burning rubber may indicate that the problem lies with the belt used in some HVAC units. 

Here are 8 reasons why your AC unit smells burnt.

1. The AC Unit Has a Short Circuit

An AC unit can develop a short circuit due to one or more of the following reasons:

  • The electric wires’ insulation is damaged. 
  • The electrical circuit is exposed.
  • The connections are loose or faulty. 
  • The electric wiring is wrong or damaged.
  • The AC unit has one or more faulty parts.
  • The circuit breaker is defective.
  • The AC unit’s circuit has a power surge. 

Essentially, an AC unit can have a short circuit if the current flow is altered, routed, or reversed from its normal course. For instance, a hot wire shouldn’t come into contact with neutral and ground wires. This can happen if the plastic insulation on these wires fails, cracks, or wears out.

Similarly, an exposed circuit, wrong electrical wiring, and loose or faulty connections can cause or lead to this problem. 

Operating an AC unit with a short circuit can melt the wires’ plastic insulation, causing the smell of burning plastic. A short circuit can also overheat certain parts of your AC unit, resulting in a burning smell. 

Sometimes, a faulty circuit breaker can fail to stop a power surge in the AC unit’s circuit. If your AC trips the circuit breaker and you detect a burning smell, you’re likely looking at a short circuit or overheating problem in the unit. 

2. The Motor’s Capacitor Is Defective

Your indoor AC unit has one capacitor connected to the motor. In turn, the outdoor condenser unit has a capacitor for the fan’s motor. Either or both of these capacitors could have turned bad, leading to the burning smell.

Here are the signs of a failing capacitor in your AC unit:

  • The motor might not start. 
  • The motor may start slowly.
  • The motor will vibrate unusually. 
  • The motor will make a noise. 
  • The motor can overheat.

These signs may appear in indoor and outdoor capacitors and motors. If your AC unit has start and run capacitors, you need to inspect both.

Start and Run Capacitors 

A start capacitor boosts the voltage for the motor when you start your AC. After the motor starts to spin, the run capacitor picks up from where the start capacitor left off. Therefore, either or both can be the cause of the problem depending on where the burnt smell comes from.

Suppose your AC unit’s motor struggles to start, makes a loud noise, or spins slower than normal. If you get the burnt smell at this stage, the start capacitor could be the problem. 

However, if the motor starts alright and you get the burnt smell sometime later, the run capacitor may be the issue. 

Take note that capacitors don’t always go completely bad. These devices store charge and sustain a specific voltage according to their specs and ratings. Therefore, a capacitor may gradually lose its ability to retain charge and sustain the voltage for the motor over time. 

Operating an AC unit with a failing capacitor will damage the motor, causing a more expensive problem. Even if the burning smell seems minimal, if the symptoms of a failing capacitor are present, you should get it tested ASAP and take the appropriate action. 

3. The Transformer Is Fried or Bad

All AC units have a transformer. If the transformer is bad or fried, a burnt smell may occur. That’s because a failing transformer or faulty circuit breaker causes the AC unit to experience a power surge, which can fry one or more components of your AC unit.  

Here’s how your AC unit transformer could go bad:

  • A faulty contactor can blow your AC unit’s transformer. 
  • A transformer with loose wiring can cause a short circuit since wiring problems can fry a transformer.
  • If the circuit breaker fails, it can cause the transformer to fail due to power surges.

A failing transformer may also vibrate, and you might hear a humming sound. 

Like the capacitors, a transformer may start to wear down over time. The AC unit components may experience unusual stress, and some parts may overheat. 

4. The Electrical Wiring Is Damaged

Damaged electrical wiring, especially cracked or worn-out insulation, can cause a burnt smell in an AC unit. These can cause your unit to short circuit, and — as I noted earlier — can lead to expensive repairs unless you address the problem ASAP. 

If you suspect that this is the issue, check all the wires and connections. Sometimes, the wires may be in good shape, but the connections are loose or damaged. Note that an AC unit is a high-voltage appliance that uses more electricity than smaller household appliances. Therefore, any damage to the connections can be dangerous.    

5. The Motor Is Failing or Broken

An old, failing, or broken motor can overheat. The worn-out parts inside a motor cause enormous friction, producing a burning smell. If there’s inadequate lubrication, the motor can overheat. Here are a few steps you can take when checking an AC unit’s motor:

  • Check if the motor makes weird noises, vibrates unusually, or doesn’t operate normally. 
  • Turn off the AC, wait for a few minutes, and touch the motor to check whether it’s unusually hot.
  • Check the fittings around the motor to see whether the fixtures are loose or broken.
  • Inspect the electrical connections and wires to ensure they’re okay.
  • Lubricate the motor for both indoor and outdoor units. 

Compared to capacitors, a motor is more expensive to replace. Therefore, you should make sure the motor is broken before replacing it. 

6. The AC Unit’s Belt Is Worn Out

Some AC, HVAC, and air handler units have a rubber belt. These belts can suffer wear and tear over the years. The rubber may give off a burnt smell due to the damage over time.

It’s also worth noting that a seriously degraded belt will not work optimally since it won’t provide enough tension to operate the pulleys. 

7. The Bearings Are Old or Broken

Bearings help the rotating parts of your AC unit function smoothly. Old or broken bearings can impair your AC unit’s efficiency, increase vibration, and cause overheating of some of its parts.

Therefore, a burnt smell may be caused by a motor’s worn-out bearings and the bushings of the housing. 

Check the indoor blower and the fan in the condenser unit outside. Also, inspect the fittings holding the electrical wires and connections in place. Any unusual vibration and friction can lead to frayed wiring and loose connections. 

8. The AC Unit Has Insufficient Airflow

Insufficient airflow through the return vents and inside the entire ductwork will stress an AC unit. That’s because the blower motor will work harder and overheat in the process. Also, inadequate airflow can affect the efficiency of the AC unit — specifically the cooling, evaporator, and condenser components. 

The common reasons for insufficient airflow are:

Lack of maintenance can cause major dust and debris buildup inside your AC, HVAC, or air handler unit.

Easy Fixes for When Your AC Unit Smells Burnt

As long as you’re able to isolate the root cause (or causes) of the problem, it should be easy to fix the problem of a burnt smell from your AC unit. Many of these fixes involve the replacement of the defective parts, though some require professional assistance altogether. Here’s what you can do when your AC unit smells burnt. 

Short Circuit: Ask for Professional Help

If you suspect the burnt smell is caused by a short circuit, don’t turn on the AC — or turn it off ASAP if it’s on. Call an HVAC technician to inspect the whole thing. Unless you’re a trained and certified professional, it’s better if you don’t try to DIY this problem. 

Defective Motor Capacitor: Replace the Capacitor

If a broken capacitor is causing the problem, replacement is usually the best option. Fortunately, AC capacitors are relatively inexpensive. Just make sure you obtain one that’s a match for your unit. If you’re not sure, check the manual or contact an expert. 

Here’s a YouTube video on how to replace an outdoor AC unit’s capacitor:

The process above also applies if the capacitor is for an indoor unit of your AC or HVAC. In this case, the capacitor should be mounted on or next to the motor of the indoor blower or fan. 

Fried or Bad Transformer: Replace the Transformer

When the transformer is fried, blown, or defective, you have to replace it. Similar to the capacitor, you should match your new transformer to the old one as closely as possible, and wire it accordingly. Otherwise, wrong wiring can lead to reverse polarity, causing other electrical problems. Ideally, you should get help from a certified technician when replacing the transformer.  

Damaged Electrical Wiring: Replace the Electrical Wires

If you spot damaged wires, you can replace the electrical wiring for that part of the circuit. All AC wires are color-coded, so you can easily match them with their respective connections or ports. Also, ensure the new wires are of the same gauge to support the required amperage (current). 

Defective Motor: Lubricate or Replace the Motor

Lubricating the motor may not solve your problem if the capacitor is bad. In that case, you should get a new capacitor. Still, you should lubricate the motor if it makes loud noises. If necessary, replace the worn-out wires and fix the loose or frayed connections. If the motor is burnt or broken, you have to replace it. 

Older motors with oiling holes are easier to lubricate. Watch this video to lubricate an electric HVAC fan motor with no ports:

Worn-Out Belt: Replace the Belt

If you smell burning rubber, chances are the belt is no longer in usable condition. Check the belt’s condition and tension to make sure this is the case. If it is, get a new belt and replace the old one. Below is a YouTube video on how you can change the belts in AC units.

Old or Broken Bearings: Replace the Bearings

Replace old or broken bearings and install them snugly. If some bearings are usable, lubricate these to reduce friction. Check all the fittings to ensure they’re functioning optimally. Inspect the other mechanical bearings as well to make sure you cover all bases. 

Insufficient Airflow: Clean or Replace Affected Components

Here are the steps to fix insufficient airflow:

  1. Check the air filters and clean them if they’re still usable. Otherwise, get new air filters. 
  2. Inspect the return vents and their filters. Clean or replace those filters if necessary. 
  3. Ensure the return vents don’t have furniture or other objects obstructing the airflow.
  4. Clean the air filter of the main return duct where it connects to the AC or HVAC unit.
  5. Inspect the supply ductwork and remove any significant blockage.  
  6. Clean the AC unit thoroughly and lubricate the mechanical parts. 

Final Thoughts

An AC unit can smell burnt due to one or more of the reasons I’ve discussed in this guide. You can fix the problem yourself most of the time. However, if you’re not sure what’s causing your AC unit to smell burnt, it’s best to contact an HVAC expert. 

Author

  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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