Why is My Portable AC Not Cooling? [Troubleshooting Guide]

Written by: Paul Cathro

Updated on: December 23, 2022

portable ac vented out of a window

It's just typical that a portable air conditioner experiences cooling problems when you need it most, but the fix isn't usually complicated.

Most cooling issues are related to incorrect thermostat settings – too low a setting can freeze the coils, and too high a setting can be insufficient to drop the room temperature.

Sometimes, portable air conditioners have a leaky exhaust hose, or we expect them to cool too large a space. 

External factors like room leaks and humid emissions from showers and cooking appliances can also impact a portable AC's cooling capability. 

This troubleshooting guide covers the most common reasons for portable AC cooling issues to help you get your unit working again

Why is My Portable AC Not Cooling My Room?

Here's a list of quick checks you can make and hopefully simple fixes to the problem.

10. Check the AC Mode Setting

The first one is a simple one that's surprisingly common to hear about.

Most portable air conditioners have 3 settings.

They work as fans, dehumidifiers, and air conditioners.

You need to select the right setting for those features to work. 

In AC mode, a portable air conditioner vents warm air outside and circulates cool air inside, lowering the room's temperature over time. 

If you only use fan mode, warm air will circulate in your room, and in dehumidifier mode, the unit will strip moisture from the air, but it won't cool it. 

So, just double check you're using the AC (cool) mode.

9. Check the Thermostat Temperature

If the thermostat is near room temperature, you won't feel much of a cooling effect because the air will only be drier.

Portable air conditioners can achieve a room temperature drop of around 20F, so use this as a benchmark for optimal comfort. 

However, beware that low-temperature settings can freeze the evaporator coil, rendering your portable AC unusable until the ice build-up melts. 

The system needs heat exchange to balance cooling and stop the evaporator coil from freezing.

Any temperature below 62 degrees Fahrenheit puts your portable AC at risk of freezing, so set it above this to be safe. 

For most people, a temperature of 65 to 70 degrees F is optimal for comfort. 

8. Check for Room Leaks

When there are openings in your room, cool air escapes through them. This reduces the cooling effect and places strain on the portable AC. By making it work harder than it's meant to and increasing wear on internal components, its lifespan will be reduced.

Leaks can be external or internal. External leaks include open windows, seals and draught proofing gaps, and cracks in walls.

Internal leaks include open internal doors, insufficient insulation, and gaps between floorboards. 

Most leaks are identifiable with a wet finger or hand – run this along windows and doors to see if cold air rushes past them. 

While no room or house has a perfect seal, you can significantly improve cooling capacity by eliminating leaky windows and doors. 

7. Check the Exhaust Hose for Leaks

A leaky exhaust hose will let loads of hot air back into your home. The exhaust hose needs a tight seal and an uninterrupted path to the vent so that warm air is exhausted correctly out of your home with no opportunity to re-enter it. 

If your exhaust hose is leaky, you can feel it when you run your hands across it – check the seal on the unit and vent wall. You have a leak if you feel rushing warm air (not radiant heat, which is typical for the hose). 

It's also worthwhile inspecting the vent for blockages because a blocked vent can send hot air back into the system, forcing it out. 

6. Your AC isn't Sized Correctly for the Room

There's no replacement for displacement!

Portable air conditioners are only as powerful as their cooling capacity. We measure cooling capacity in BTU (British Thermal Units), and you need around 20 BTUs for every square foot of living space for good cooling performance. 

For example, to cool 500 square feet, you need around 10,000 BTUs. 

If your air conditioner isn't powerful enough, it will never bring your room down to the temperature you set on the thermostat. The poor thing will work at maximum capacity and never cycle off, which is terrible for longevity! 

Some experts recommend as many as 30 BTUs for every square foot. However, it's important not to over-specify – while this will cool the room faster, it also increases your energy consumption, and the unit will cycle off too quickly (cycle length is really important!). 

Simply put, you must match the unit's cooling power accurately to the size of the room - aim for 20 BTUs for every square foot. 

5. Your AC Needs Cleaning 

Portable air conditioners require periodic cleaning to function at their best. 

A typical cooling issue is a clogged air filter which reduces airflow into the system, meaning less cool air circulates into the room. 

Evaporator coils can also catch dirt and develop grime, reducing the unit's ability to cool air and creating a moldy, damp smell. 

Here's what we recommend you do:

  • Check the air filter for dirt and grime. Submerge it in warm, soapy water and scrub it with a soft-bristled brush to clean it. 
  • Open the portable AC cover and inspect the coils – if they have dirt and grime, clean them with a non-rinse degreasing spray and a brush.  
  • Clean the condensate tank and drain line with distilled vinegar or an antibacterial detergent to sterilize the system.  

Periodic cleaning will keep your portable air conditioner running at its best and discount any cooling issues related to dirt build-up. 

4. Your Unit Needs Replacing

If your portable AC is over three years old and has suddenly stopped cooling, it's probably time to replace it with a newer model. 

Portable air conditioners generally last three to five years, depending on frequency and intensity of use. 

Running a portable air conditioner at extremely low temperatures, maximum fan speeds, or continuously with short cycles causes premature wear. 

Additionally, portable air conditioners work harder in humid climates, so high humidity can reduce the unit's lifespan.

There's nothing you can do about living in a humid climate, but you can at least specify a suitable air conditioner.  

3. Check the Blower Fan 

Your portable air conditioner's cooling performance depends on the blower fan operating correctly.

Even a slight misalignment can significantly change the airflow, causing problems with cooling capacity and air circulation. 

Your AC might be cooling just fine but if the blower is impeded then it can't circulate that lovely cold air.

Try turning your portable AC on to full fan mode and see what it does – does it blow air straight and true, or is it shaky and turbulent? 

Although not all blower fan problems are apparent, a squeaky noise or a shaky, wobbly fan are clear signs that something is wrong. 

The trouble is that blower fans are easily damaged when the unit is moved around harshly, or they can break due to a loose screw or mount failure. 

In addition to the housing and fan blades, the fan motor can also cause malfunction. 

2. There's a Problem with the Coils

Evaporator coils do not usually break – malfunctions are usually due to other component failures like switch relays and control boards.

However, coils can freeze if the thermostat is set too low or if the refrigerant leaks (see below). 

An exception applies if the portable AC is impacted or punctured, which can irreparably damage the coils. Excessive wear and tear can also wear down coils, causing debris build-up in the coils from corrosion. 

If you hear unusual sounds like clinks and hissing from the coils, they might be the reason behind your cooling problems.

Over time, the coils can lose cohesion, making it harder for them to maintain a consistent temperature.

1. There's a Refrigerant Leak

Refrigerant leaks are uncommon in portable air conditioners because they happen when the evaporator coils have a crack or a hole or the refrigerant port is compromised. Since a casing protects the coils, an impact or wear and tear is the most likely cause. 

The problem is that most portable air conditioners are sealed units, so they cannot be re-gassed, i.e., the refrigerant is not serviceable. If this is the case, your only option is to replace the unit with another model. 

If you are lucky enough to have a portable AC that you can re-gas, you need to figure out why the refrigerant is leaking. Is there a hole or crack in the coils? Does the refrigerant port have a dodgy seal?

An air con engineer will help you determine if your unit is fixable. In any case, it is usually cheaper and easier to buy a new portable AC.

We've got guides to the smallest and quietest models on the market here at Tiny House, Huge Ideas.

About the Author Paul Cathro

Paul is an ex-HVAC engineer with 5 years 'in the trade'.

He acquired tinyhousehugeideas.com in 2022 and aims to make it the internet's most comprehensive HVAC resource for small homes in the next few years.

You can learn more about Paul's story here.

Browse his published work on the website here.

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