Why is my portable air conditioner not cooling?

In most cases it is something simple: the unit is in fan mode instead of cooling, the exhaust hose is kinked or not properly routed out the window, the filter is clogged, or the condensate tank is full. Check these in that order. If the unit runs but the room stays hot, it may lack BTU for the area.

How many BTU do I need for my room?

Roughly 7,000 to 9,000 BTU up to 15 m2, 9,000 to 12,000 BTU for 15 to 25 m2, 12,000 to 14,000 BTU for 25 to 35 m2 and 14,000 to 18,000 BTU for 35 to 45 m2. These are guidelines for a standard room; step up a tier if the room is very sunny, top-floor or has high ceilings.

Could the exhaust hose be the cause of the problem?

Yes, it is the most common cause. If the hose is kinked, too long, or does not really route hot air out the window, that air returns to the room and cancels the cooling. Keep the hose short, straight and pointed outside, and seal the window well around it.

How often should I clean the filter?

During peak use, clean the filter every two to three weeks. A clogged filter reduces airflow and therefore cooling capacity. Wash it with lukewarm water and mild soap or vacuum the dust, then let it dry completely before refitting to avoid mould and odours.

When should I call a technician instead of fixing it myself?

Call a certified technician if you suspect a refrigerant gas leak (cooling that drops progressively), or in case of a burning smell, sparks, abnormal noises or unresolved error codes. Never try to recharge the gas yourself: it is pressurised and its handling is regulated. If the unit is under warranty, do not open it.

Portable Air Conditioner Not Cooling: Causes and Fixes 2026

My portable air conditioner is not cooling: where to start

It is one of summer's most frustrating issues: you plug in the unit, set it to 18 degrees, leave it running for hours and the room stays just as hot. The good news is that in the vast majority of cases, when a portable air conditioner is not cooling, the problem is not a serious breakdown but an installation, maintenance or sizing issue you can fix yourself in minutes.

Before calling a repair service or buying a new unit, run a methodical diagnosis. In this guide we go through the real causes, from the most common to the most serious, with concrete step-by-step fixes. If you are still deciding which system suits you, see our portable AC vs fan comparison.

Keep one idea in mind: a single-hose portable AC does not work like a fixed wall split. It is far more sensitive to installation, room size and outdoor temperature. Often, when the unit is not cooling enough, it is not broken at all: it is fighting unfavourable conditions that are easy to improve. Let us look at them one by one, starting with the one-minute checks and leaving the faults that need a professional for the end.

Quick diagnostic table

Symptom	Likely cause	Fix
Air comes out but neither cold nor hot	Fan mode instead of cooling	Switch to Cool mode (snowflake)
Cools little and the hose is hot	Exhaust hose poorly placed or kinked	Straighten the hose and route it out the window
Very weak airflow	Clogged filter	Clean or replace the filter
Stops by itself after a few minutes	Condensate tank full	Empty the water tank
Runs but the room does not get cooler	Insufficient BTU for the area	Reduce the area or increase power
Cools much less than before	Possible refrigerant gas leak	Call a certified technician

Cause 1: the mode is set wrong

It sounds trivial, but it is the number one cause of false faults. Many units default to fan mode (Fan) or dehumidify mode (Dry), which move air or remove humidity without really lowering the temperature. If the compressor never starts, the unit will never truly cool.

Press the Mode button until you see the snowflake or the word Cool.
Set the target temperature several degrees below the current room temperature.
Check that you hear the compressor start (a low hum) after one or two minutes.
Disable the timer and Eco mode during the test to rule out any throttling.

Cause 2: the exhaust hose is poorly installed

By far the most common cause. A portable AC extracts heat from the room and expels it outside through the exhaust hose. If that hot air does not really leave the room, it comes back in and cancels the cooling effect. It is the most widespread mistake and, at the same time, the easiest to fix without spending a penny.

A detail many people miss: the longer and more twisted the hose, the more its own surface heats up and radiates heat back into the room. The hose temperature is a good clue: if its final section feels very hot after a while, the route is probably too long or has bends.

How to check and fix the hose

The hose must run straight to a window or outside vent, never loop inside the room.
Keep it as short and straight as possible: every bend and extra metre cuts performance.
Avoid kinks, crushing or U-shapes: hot air builds up and flows back inside.
Make sure the hose is firmly attached to the rear outlet and has not come loose.
Do not extend the hose with homemade parts or swap it for a longer one: its length is calculated for the unit's airflow.

Cause 3: poor window sealing

There is no point pushing hot air out if outside air comes back in around the hose. Most units ship with a window sealing kit precisely for this.

Fit the supplied sealing kit; if it is missing, cheap universal panels exist.
Seal gaps around the hose with tape or foam strip.
Close doors and windows and lower blinds during the hottest hours.
On a tilt-and-turn window, look for a specific kit: the triangular gaps they leave let in a huge amount of heat.

A simple test: with the unit running, hold your hand near the window frame and the hose seal. If you feel a stream of warm air coming in, you have found a leak that is ruining the unit's work.

Cause 4: the filter is clogged

The filter traps dust. When clogged, the unit draws in less air, airflow drops and cooling capacity collapses. It is basic maintenance that is often forgotten.

Locate the filter (usually behind a grille at the back or side) and remove it.
Clean it with lukewarm water and mild soap, or vacuum the dust gently.
Let it dry completely before refitting: a damp filter encourages mould.
Repeat every two to three weeks during heavy use.

Cause 5: the condensate tank is full

While cooling, the unit produces water through condensation. Some models evaporate it, others collect it in an internal tank. When full, a safety sensor stops the compressor: the fan still blows but no longer cools, sometimes showing an "FL" or "Full" code.

Empty the water tank as per the manual.
If supported, connect a continuous drain hose to avoid manual emptying.
Check the drain plug is properly closed after emptying.

Cause 6: the room is too big (insufficient BTU)

If the unit runs fine but the room does not cool enough, it likely lacks power. Cooling capacity is measured in BTU per hour. A small unit in a large living room can run flat out without ever reaching the set point, like a small car loaded to the roof struggling up a hill.

This is a key point when buying and a common cause of disappointment: many people pick the model on price and end up with a unit that is too small for their living room. Sizing the BTU correctly is what separates a unit that really cools from one that just makes noise.

How many BTU by area

Room area	Recommended BTU
Up to 15 m2	7,000 - 9,000 BTU
15 - 25 m2	9,000 - 12,000 BTU
25 - 35 m2	12,000 - 14,000 BTU
35 - 45 m2	14,000 - 18,000 BTU

These are guideline values for a standard room. Strong sun, top-floor rooms, high ceilings or many appliances mean stepping up a tier. An undersized unit will never reach the set point and will consume more. To compare power and consumption, see our comparison by room size.

Cause 7: extreme outside temperature

In a heatwave, with 40 degrees outside, even a well-sized unit loses performance: it must dump heat into already very hot air. This is not a fault, but a physical limit.

Close blinds and curtains during the hottest hours.
Start the unit before the room becomes an oven.
Do not chase 18 degrees: a realistic 5 to 8 degree gap from outside already brings great comfort.

Cause 8: possible refrigerant gas leak

If everything else is ruled out and the unit cooled well before but barely does now, there may be a refrigerant gas leak. Without the correct gas charge, the compressor runs but no longer transfers heat.

Be careful: refrigerant gas is pressurised and its handling is regulated. Never try to recharge it or open the circuit yourself. This must always be done by a technician certified in fluorinated gases. Handling a refrigerant circuit without training is not only illegal, it can cause cold burns and irreversibly damage the compressor.

Remember that a leak is not normal on a new, well-used unit. If yours is a few months old and already seems to be losing gas, claim the warranty through the seller rather than opening anything yourself.

When to call the repair service

You have checked the mode, hose, sealing, filter and tank, the unit is correctly sized and it still does not cool. Then contact the service, especially if you notice:

Burning smell, sparks or tripping the breaker.
Abnormal metallic compressor noises.
Error codes not resolved by the manual.
Suspected gas leak (progressive performance loss).

If the unit is under warranty, do not open it. For year-round comfort, also see our connected dehumidifier guide.

Good habits for better cooling every summer

Beyond the one-off diagnosis, a few simple habits help your portable AC perform at its best and last longer. The idea is to help it not start from an already overheated room and to breathe without obstruction.

Switch it on ahead of time on days when heat is forecast, rather than waiting for the room to become an oven.
Keep the air intake and outlet grilles clear: do not push the unit against the wall or cover it.
Clean the filter regularly and, at the end of the season, store it clean and dry to avoid odours next summer.
Pair the AC with good heat protection: awnings, lowered blinds and night ventilation when it cools down.
If needed, combine it with a fan to spread the cool air around the room without raising consumption.

With these habits the unit works less for the same comfort, which shows in both the temperature and the bill.

Conclusion

A portable air conditioner not cooling is rarely beyond saving. In order, check the mode, exhaust hose, sealing, filter and condensate tank, and make sure the BTU match the area. These steps fix the vast majority of cases. Only after that do a gas leak and a certified technician become relevant.