Why is my AC not cooling or blowing cold air? HVAC experts reveal the 8 most common problems

When the mercury rises outside, nailing down why your AC is not cooling becomes a top priority.

There are eight common reasons why this happens, including dirty filters, and some of which you can fix yourself. Others will need an HVAC pro to fix it, our experts say.

Learning how to clean an air conditioner is a great start, but if that isn't the issue, here’s how to get your AC to blow cold air again.

Why is my AC not cooling my home as expected?

'If your air conditioner is blowing warm air, it is important to diagnose the problem so that it can be fixed as soon as possible. Otherwise, you may find yourself dealing with an uncomfortably warm home all summer long,' say the experts at Thomas HVAC Company in Alabama.

'Each of the problems has different solutions, so diagnosing the issue is the first step before you can even consider fixing the issue.'

1. Wrong temperature

Your thermostat has a huge influence over your air conditioning unit. Therefore, if your AC is not working properly, the solution could be as simple as adjusting the temperature to not provide cool air.

Brendan McCarthy, CEO and Owner of SALT Service Company, says, ‘I’ve seen plenty of cases where the thermostat was set wrong.

Richie Drew, VP of Operations at One Hour Heating & Air Conditioning, who has 27 years of experience in the industry, explains that sometimes it’s as simple as adjusting the thermostat setting or reprogramming your smart thermostat to run your AC at different times of the day.

Top tip: Check your thermostat setting and make sure it's set to cool, as sometimes someone in the household may have changed the setting by accident.

2. Dirty air filters

Brendan McCarthy says, ‘When someone tells me their AC is running but the air isn’t cold, I always start with the basics. Most of the time, it’s a dirty air filter and replacing that is a simple fix that can make a bigger difference than people realize. Also, some AC units will have two air filters, so be sure to replace both.’

How often you'll need to clean your air filters or replace them will vary depending on whether you have pets or not.

Richie Drew, says, ‘You should replace it every one to two months, but check to see what the manufacturer’s manual says. If you have pets, you might need to replace the filter more frequently.

‘Dirty air filters reduce airflow, which limits the way your air is evenly distributed throughout your home.’

Similarly, look to see if the AC unit has become dirty. Richie advises checking the base of your AC unit, as ‘dirt can settle underneath and make the entire unit unlevel. Gone unchecked, this can cause performance issues and lead to major repairs down the road.’

As long as filters are kept clean, you can run your AC even when the air quality is bad.

3. Frozen evaporator coil or low refrigerant

Brendan says, ‘A frozen evaporator coil or low refrigerant usually points to a leak. Don’t run the unit if the evaporator coil is frozen or the system is making strange noises, since this can lead to further problems.'

He warns that homeowners should never try to add refrigerant themselves, as 'it’s not safe or legal without proper certification.’

That’s one to certainly call the pros for, rather than attempting to fix yourself.

Richie explains, ‘This chemical compound is designed to convert from a gas to a liquid and back as it absorbs heat from inside and releases it outside. This can cause your AC unit to run longer than normal and lead to increased utility bills, as well as the AC unit blowing warm air.’

This problem is commonly the result of a leak within the air conditioning system that could stem from a faulty valve or evaporator coil.

It is often hard to diagnose this problem, as any leaks typically evaporate as they leak out of the AC. However, your unit may make a hissing or bubbling noise if this is the case.

4. Malfunctioning compressor 

If your outdoor compressor is malfunctioning, it may cause your entire AC system to operate inefficiently as it won't be able to transfer the air properly.

You can often diagnose a malfunctioning compressor from unconventional noises (such as rattling and clattering) coming from your unit.

In these cases, it's best to turn off your unit entirely before fixing the issue, if you know how, or calling in an expert to run diagnostics and make the repairs needed.

5. Duct work leak

Brendan says, ‘Sometimes the cold air is doing its job, but it’s just leaking out through the ductwork before it ever reaches the desired rooms. Cooling problems are frustrating for homeowners, but a little troubleshooting goes a long way before calling in a licensed HVAC technician.’

This is not typically one you can resolve alone, but identifying the problem will help you figure out your next steps and understand how much it might cost to repair.

6. Mold or mildew

Richie explains that cleaning your evaporator coil is essential during the hot season because there is typically an increase in humidity, which can cause mold and mildew.

Your AC system is responsible for pulling excess moisture out of the air and when your air conditioner is not performing at its best, your whole house can get damp.

‘This can cause dry goods to spoil, mildew to appear on the walls, ceilings, or even on your clothes, and mold to spread throughout your house.’

To prevent this, drain the pan below the condenser and clear all lines to allow excess water pulled from the air to drain out.

If the spores have already taken hold, learn how to clean mold from walls.

7. Odd sounds and smells

If your AC is not cooling, and produces odd noises and strange smells, our pros reckon you should be on alert.

'Your HVAC system is designed to help keep your home properly ventilated,' Richie explains. 'With this in mind, it is often time for an air conditioning replacement when you begin to notice more dust in your home or unusual sounds and smells.

‘Strange smells are often associated with your HVAC unit burning off excessive dust. However, in some cases, you could have mold or melted wires within your system. Odd sounds are often indicative of failing components.’

These components can be expensive to fix, which is why you should call professional technicians to better determine if installation of a new unit will cost less than major AC repairs.

Learn how much it costs to install central air and the different types of AC.

8. Obstructed condenser unit

Richie puts this issue on his list of things to always check when troubleshooting why your AC is not cooling.

He says, ‘The condenser unit is located outside your home and the condenser coil runs around the unit's periphery, usually protected by a removable cover or panel. It’s usually lined with thin, shiny metal fins.

‘If these fins are blocked by dirt or grass, or covered by a tarp for protection, they can't draw in enough air, which can lead to warm air blowing into your home.’

To wrap up, Brendan says, ‘There are a few things I always tell homeowners to avoid when their AC isn’t working right. Don’t keep lowering the thermostat, thinking it’ll cool faster, that causes the system to work harder and does not fix the real problem.’

This can be a thermostat mistake, and shorten your AC system’s lifespan by putting unnecessary pressure, plus extra wear and tear from making your system work harder than necessary.

‘If you’ve replaced the filter, checked the thermostat, and the system still isn’t cooling, or if you notice warm air, ice buildup, hear odd sounds, or the outdoor unit isn’t turning on, it’s time to call a licensed HVAC technician. Catching the issue early can save you from a much more expensive repair down the line.’