Why is AC Unit Blowing Hot Air

There’s no greater frustration than an air conditioner blowing warm air, essentially defying its very purpose. Not only does it impact comfort, it also increases running costs. While a slight drop in air temperature can often be fixed through basic maintenance, an air conditioner blowing warm air is far more serious. 

Hot air is symptomatic of deeper AC issues, which could occur at various points of the system. It’s important to find and rectify such issues as quickly as possible to avoid further damage and expense.

Why is AC Unit Blowing Hot Air - hvac system 500x5001 1

How AC Unit should work

The AC unit is a marvel of design and engineering, which depends on a few key elements: compressor, refrigerant and coils. The refrigerant is compressed from gas to liquid, releasing heat onto the condenser coils, which is directed outward. 

This refrigerant is then passed through the expansion valve, where lower pressure causes it to become gaseous again, drawing heat from the indoor evaporator coils. The fan blows air over these cooled coils, reducing the temperature of the room. 

In order to blow cold air, all of these processes must take place properly. As AC unit ages, key components will wear and ultimately break, followed by a big drop in cooling performance.  

Why is AC Unit blowing Hot Air?

If you’re sure AC is set to COOL, the fan is on AUTO and you aren’t accidentally using lower-power modes, it may be time to check the system internals. Although air conditioning relies on basic thermal principles, there are complicated mechanisms at play. 

Here are some common reasons why AC is blowing hot air:

  1. Refrigerant Leak 

Air conditioners cannot exchange heat without a refrigerant (Freon). It’s a core principle of how an AC unit functions. Without sufficient refrigerant levels, the evaporator coil can’t cool the home. 

Less Freon equals less pressure, eventually causing the evaporator coil to freeze over, which in turn reduces the effectiveness of the system to lower temperature. AC is a closed loop, so low levels of refrigerant almost always means there’s a leak somewhere. 


While this issue is common, the fix isn’t easy. The majority of refrigerant leaks occur in the coils, but it is possible to find leaks in the refrigerant lines. In either case, you will have to replace these components, before refilling the Freon. 

It’s not generally advised to repair a refrigerant leak yourself. A HVAC professional will be able to find and repair the leak safely and quickly. As low refrigerant levels force the AC unit to work harder, fast response saves you from the higher electricity costs. 

  1. Defective AC Compressor 

The AC compressor has the sole function of compressing the refrigerant, which is then able to lower the temperature of the evaporator coils. When the compressor is running inefficiently, or outright broken, you’ll instantly notice AC blowing warm air. 

There is particular difficulty in repairing the compressor as there are many reasons why they become compromised. It could be an electrical failure, where contacts, fuses and wiring have been damaged. Alternatively, the condenser coil may have become partially blocked over time, which may result in the compressor overheating. 

Solution: Diagnosing a compressor issue can be challenging because it’s often the result of another problem. Repairing the compressor itself is notoriously difficult. When you call a HVAC professional, they will typically replace the entire component, as maintaining a worn-out AC compressor is inconvenient and expensive.

  1. Blocked Air Filters

An air conditioning unit blowing hot air may be suffering from blocked air filters, which seriously reduces airflow and puts extra strain on the other components. When air filters are dirty, you may notice that the room doesn’t feel as cold as usual. However, fully blocked air filters will result in an air conditioner blowing hot air. 

Solution: Cleaning and replacing air filters is part of regular AC maintenance. Blocked air filters simply need to be replaced. Clean filters will return optimal airflow and AC will run more efficiently.

  1. Electrical Issues

On a particularly hot day, older AC units struggle to keep up with the thermostat setting. The lower desired temperature, the more power the AC unit draws. An AC unit is protected from electrical surges by amp breakers and fuses. Sooner or later, an overloaded system leads to tripped breakers and blown fuses, which disables the circuit.

Solution: Check the circuit breaker for a tripped switch, before looking at the fuses located in the disconnect box outside. A blown fuse can be replaced easily. However, if the AC unit is repeatedly blowing fuses and tripping breakers, there is an internal issue. 

For the protection of yourself and your unit, any electrical work should be carried out by a HVAC professional.

Keeping your Cool

Regular maintenance reduces the likelihood of certain issues, yet sadly no air conditioning unit can run optimally forever. 

If the thought of AC repair gets you all hot and bothered, or you have trouble pinpointing the fault, keep your cool and rely on the experience of qualified HVAC specialists.

Jan 27, 2022

Contact Us