March 23, 2015

Throughout the years, we’ve seen and repaired a lot of AC problems. The more complex the system, the higher the potential for problems. Knowing the symptoms and what the problems are can potentially add a few years to the life of your system as you may be saving other parts in the process. Here is a list of the most common AC problems and their symptoms.

Broken Thermometer

In most systems, the thermometer is the key to proper functioning; without the thermometer, most systems don’t even know when to turn on. A broken thermometer can mean a system turns on and off for no reason, but it can also make your home warmer or cooler than you intended. The sensors on these thermometers do have a huge life expectancy, but sometimes they break earlier than they should. If you look at the thermometer and it says it’s 72º but it feels like 78º, then it’s possible that the thermometer’s sensor is broken.

As you may expect, the system will not keep the desired temperature. If the system is blowing cool air and does turn on and off properly (even if at the wrong times), but the temperature is not ideal, that’s a core sign the thermometer is broken. The easiest sign is if the thermometer doesn’t allow any input. A good test for you to try is to go up to the thermostat and set it to a high temperature and see if the system continues to cool. If so, the sensor is likely broken.

If your thermometer or the temperature sensor is broken, switch the system to manual mode and contact your local HVAC repair company.

Constantly Turning Off and On

As the temperatures change, the system will turn off and on accordingly. That part is natural, but doing this quickly and unnecessarily is not. More than likely, your system is doing this because it’s too big for your home. By the time the system finishes turning on and getting some cool air going, the room is cool and it doesn’t need to keep running because it will over-cool the room. If your system has always done this, or it does it after running for a little bit, this is likely your problem.

If your system has never done this before and does it all the time no matter what, turn it off for about 30 minutes and then turn it on. It should kick on and begin cooling properly. If it turns off in a few minutes and you do not think the home is at your optimal temperature, it may have bad wiring, loose wires, or a bad power source. It’s some form of an electrical problem and you’ll have to contact an HVAC repair company to come take a look at it.

Not Running at All

If the system does not run at all, it’s likely the same problem as #7: bad wiring, loose wires, or a bad power source. The only cause for an HVAC system to not turn on at all (including making no sounds) is a lack of power. Somewhere along the line, there’s a loss of power. We see this often when there are small electrical fires that burn the wiring inside the system. Although not a common occurrence, it’s a common reason for not being able to turn a system on. As always, you should contact an HVAC repair company to take a look at it.


Problems #5 through #1 are all very common problems that we see often. Leaks occur so often because water naturally occurs from condensation during the operation of any air conditioning system. The water accumulates around coils and is meant to drip down into a drainage pump. For window units, you’ll notice that the water drips constantly on the outside corners of the unit. This is because window units do not have a specified drainage location, so it just pools up inside and slowly leaks outside.

Central AC units work similarly, except they should not allow pooling. The most common cause for leaks in an AC unit is when the unit is improperly installed. A poor installation can cause unleveled condenser pans and even too much pressure built up in the unit.

Some other causes include:

  • Drainage Holes Could be Blocked
  • It’s Frozen (see #1)
  • Holes in the Condensation Pan (rare)

If the drainage holes are blocked, you can clean them manually or have an expert clean your system. If the system is frozen, it’s likely because it’s so hot outside. Bump the thermostat up a few degrees and gradually bring it down to prevent the air temperature from blowing at less than 32ºF. Also, be sure to leave the system off for a few hours and allow it to thaw before turning it on again.

If the problems continue, contact a professional.

Loud AC

Loud air conditioners are very common. In fact, air conditioners are supposed to make noise. There are quiet units available, but they are all supposed to be somewhat noisy. However, sometimes the systems are unusually loud, and that can indicate a problem. Unfortunately, there are a lot of reasons why a system would be running loud. The most common sounds are:

  • Rattling
  • Grinding
  • Popping
  • Hissing

Generally, when you hear one or more of these sounds it means you need to check and clean:

  • Ductwork (even if it doesn’t sound like it’s coming from there)
  • Fans
  • Exterior Vents

Dust, debris, and in some cases rodents can cause a clogged system somewhere along the line, causing strange noises. The most common complaint is the exterior unit is making a loud rattling noise as if something fell loose. Although it’s possible something is loose, most often there is debris flying around inside the system that fell in when the fans were not running. The fans cut on and the debris can’t escape. A quick cleaning and checkup on your system will get you going good as new (and quiet)!

Read more about loud AC noises and how to fix them in our blog post here.

Blowing Warm Air

One of the top complaints about an AC system is that it is blowing warm air. This is definitely one of the calls we see on a regular basis. Before we dive into the repairs of a system, keep in mind there is often a simple reason.

It’s always worth checking your system’s thermostat to make sure it is set properly. Make sure your thermostat is set to “Auto” and “Cool” with the temperature you want. If it says it’s on “Fan” or anything about recirculating only, that is the culprit. If your home is warm, it will recirculate that warm air throughout the home. The vents will be blowing warm air.

There are some other very common problems with air conditioners that cause the unit to blow warm air, such as:

  • Refrigerant Leaks
  • Frozen Units (see #1)
  • Blown Breaker
  • Broken Compressor

For a full description of each cause and how to fix it, check out our detailed post on it here.

Smells Bad

Unlike the loud noises, bad smells are not a natural part of the air conditioner’s cooling process. If you have a musty smell in your basement (or wherever you store the air conditioning unit), it’s likely the cause of a water leak. These leaks cause mold buildup, which is unhealthy to breathe and has a bad must smell to it.

If you smell rotten eggs, that means there is a natural gas leak, which is very dangerous. Shut off the unit and leave the house. You may also smell a burning smell. These burns are generally after-effects of the danger, but should not be ignored regardless. Burning smells only come from burns, and burns only come from fires.

Smells like this often come from leaks in the system, faulty wiring, or poor installation. Sometimes the unit was manufactured or transported poorly. Regular maintenance and system tune-ups can detect these problems before they become serious and expensive.

Take a look at our post on 5 HVAC Danger Signs You Should NOT Ignore for more on the danger signs, or for more on smells, take a look at our posts on bad smells here.

Frozen Units

Arguably the most common and strangest of all AC problems is a unit that is frozen solid. And yes, it can freeze solid with a coating of ice. It’s a noticeable problem if you take a look at the unit, but for most homeowners, the unit is out of sight.

The true most common problem among AC units is to blow warm air; however, the cause of that is often a frozen unit. Frozen units cause other problems too, which are often called in as individual problems before they notice the unit is not blowing cool air.

What’s actually freezing is the evaporator coil. The reason it freezes is actually fairly simple: condensation, cold temperatures, and no evaporation. Condensation is unavoidable, but if there are cold temperatures and the water does not evaporate, it will cause it to freeze. Generally, the water builds up on the evaporator coils through condensation naturally. Soon after, the water evaporates, if possible.

The question is: how cold is it in there? On very hot days, your AC is working hard to keep you cool and that means running at the max capacity and lowering temperatures. The AC will drop temperatures inside of the unit to around 30ºF or less. Anything under 32ºF is freezing and will cause ice buildups. Once the ice begins to build up, the temperatures become lower, and the amount of water evaporating drops significantly. There are sensors in place that are supposed to prevent this from happening, but sometimes they misread temperatures or the temperature drops so quickly that it doesn’t catch it until it’s already frozen.

The problem can be easily fixed by turning the system off, letting it thaw, removing the ice, and turning it back on in about an hour or 2. The system should warm up by then and then begin working properly. If it does it again, contact an HVAC professional to take a look at it.

Note: Frozen systems do not cause damage to the system if the system is turned off soon after freezing. Frozen systems left running could damage the compressor and evaporator coils.

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