🗓 Updated July 17, 2026 · EcoBack editorial team
The good news first: a portable AC that suddenly stops cooling is rarely broken. It is usually being sabotaged by something small — a full tank, a blocked filter, a squashed hose, or the most overlooked cause of all: your own hot exhaust air leaking straight back into the room. Work through the seven checks below, from most common to least.
1. Full water tank — the compressor has quietly shut off
Portable ACs pull moisture out of the air while they cool. Most re-evaporate that water through the exhaust hose, but in humid weather the internal tank can still fill up — and nearly every model then shuts the compressor down to protect itself. The fan keeps blowing, but the air coming out is room temperature.
Look for a blinking light or a code like FT, FL, P1 or E5 on the display, drain the tank (usually a plug low on the back), and the compressor should kick back in within a few minutes. If water ends up on the floor instead of in the tank, that is a different fault: see portable AC leaking water.
2. Clogged air filter — the slow, invisible killer
The filter behind the intake grille collects dust every hour the unit runs. As it clogs, less air passes over the cold coils, the unit cools less, and eventually the coils can ice over — which makes cooling collapse entirely.
Pull the filter out (no tools needed on most models), vacuum it or rinse it lukewarm, let it dry completely, and put it back — every one to two weeks during daily use. A dirty filter is also the usual reason a unit starts to smell — if yours does, see portable AC smells musty.
3. Kinked, squashed or over-extended exhaust hose
Everything the machine achieves depends on that ribbed hose pushing hot air outside. A kink, a tight bend behind the sofa or a hose squashed by the window sash chokes the airflow: heat backs up and cooling drops sharply. Run the hose as short and straight as possible, with gentle curves instead of sharp bends.
Be careful with extensions, too — every extra metre adds resistance and radiates heat back into your room. If the standard hose genuinely will not reach a window, do it properly: insulated, correct diameter, no improvised taped-on tubes. Our guide to extending a portable AC hose covers what is safe and what kills performance.
4. Leaky window seal — the biggest hidden cause
This one wastes more cooling than everything else on this list combined. Your portable AC blows hot exhaust air out of the window — but it also creates a slight vacuum in the room. If the window is simply propped open around the hose, that vacuum pulls outside air (including your own hot exhaust) straight back in through the gap. The unit ends up cooling air that it just heated: it runs non-stop and the room barely changes.
The test is simple: hold your hand around the window gap while the unit runs — warm air streaming in means you have found your problem. The fix is a proper window seal kit — a fabric or board seal that closes the entire opening except the hose outlet. It is the single highest-impact upgrade for any portable AC. European tilt-and-turn windows are the trickiest case, and we cover the working solutions in portable AC with tilt-and-turn windows.
5. Wrong mode or setpoint — check before you blame the machine
Trivial but genuinely common: the unit is in fan or dehumidify mode instead of cooling. Look for the snowflake symbol — only that mode runs the compressor. Then check the setpoint: if the target is 25°C and the room is already 25°C, the compressor has nothing to do. Set it a few degrees below room temperature: within a minute the compressor should start with a noticeable hum and the outlet air turn distinctly cold. Also check that a mis-set timer isn't switching cooling off.
6. Undersized unit — a BTU mismatch no setting can fix
If the outlet air is cold, the window is sealed and the room still will not cool down, the unit may simply be too small. Cooling capacity is measured in BTU, and the rule of thumb for European rooms is roughly 340 BTU per square metre — more with large windows, sun exposure or a roof directly above. A 7,000 BTU unit asked to cool a 35 m² living room will run flat out forever and lose. Our calculator-backed guide how many BTU do I need gives the exact number for your room.
If it is undersized, no cleaning or sealing will close the gap — the honest fix is a machine that matches the room:
Small rooms up to ~20 m²: the Comfee MPPH-09CRN7 (9,000 BTU) is a frequent price-performance pick in public German tests.
Large rooms of 30 m² and more: the Midea PortaSplit — a portable split unit that public reviews consistently rate far quieter and more efficient than classic monoblocs.
Comfee MPPH-09CRN7 → Midea PortaSplit →Note: we haven't tested these units ourselves — we summarise public test results and user feedback.
7. Refrigerant loss — the rare case that isn't DIY
Last and least likely: the refrigerant circuit. A portable AC is a hermetically sealed system — the refrigerant is not consumed and never needs "topping up" in normal life. But a fall during transport, a hard knock or a manufacturing fault can cause a slow leak. The signature: airflow is strong, filter, hose and seal all check out, the compressor audibly runs — yet the outlet air is barely cooler than the room.
Do not buy a "recharge kit" online: handling refrigerant is regulated work for certified technicians, and DIY attempts usually destroy the compressor. Under warranty, claim it — refrigerant loss is the manufacturer's problem. Out of warranty, get a repair quote first; on budget monoblocs the repair often costs more than the machine is worth.
Frequently asked questions
Why is my portable air conditioner running but not cooling?
In most cases the cause is one of four things you can fix yourself in minutes: a full internal water tank that has stopped the compressor, a dust-clogged air filter, a kinked or over-extended exhaust hose, or hot exhaust air leaking back in through a badly sealed window. Only if all of those check out is an undersized unit or a refrigerant fault the likely reason.
Why does my portable AC blow cold air but the room stays hot?
Usually because heat is coming back in as fast as the unit removes it. The most common culprit is a leaky window seal: the hose pushes hot exhaust air outside, and the gap around it sucks that same hot air straight back into the room. A properly sealed window kit typically fixes it. If the seal is fine, the unit is probably too small for the room.
How long should a portable AC take to cool a room?
A correctly sized and sealed portable AC should make a room noticeably cooler within 20 to 30 minutes and reach a comfortable temperature in roughly one to two hours, depending on room size, sunlight and outside temperature. If nothing has changed after an hour, work through the checklist on this page instead of just letting it run.
Can I run a portable air conditioner without the exhaust hose?
Not if you want cooling. The hose carries the heat the machine extracts out of the room; without it, all that heat is dumped straight back inside and the unit works as an expensive fan at best. In dehumidifier mode some units can run hoseless, but in cooling mode the hose must always vent outside.
Does a portable AC need to be refilled with refrigerant?
No — a portable AC is a sealed system and should never lose refrigerant in normal use. If the airflow is strong but no longer cold, and the filter, hose and window seal all check out, the refrigerant circuit may have a leak. That is not a DIY refill job: it is a warranty or professional repair case, and on cheap older units often not worth the cost.