The short answer
Start with the simple causes: the mode and setting on the remote, timers, dirty filters, open windows and a blocked outdoor unit. If those are fine and it still struggles, it may have a fault or leak that needs an engineer.
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A system that seems weak often has nothing wrong with it. A setting, a dirty filter or a normal pause in operation can all look like a fault. Work through the checks below before booking an engineer, and note what you find, because it helps if you do need one.
Check the settings
- Mode. Choose cool or heat, rather than fan, which only moves air, or dry, which is aimed at removing moisture. In auto mode the unit decides for itself, so picking cool or heat directly makes it easier to see what's happening. The modes explained covers each one.
- Temperature. For cooling, set it below the room temperature, and for heating, above. Once the unit judges that the room has reached the setting, it slows right down and may only move air.
- Timers, schedules and sleep mode. A timer or app schedule may be switching the unit off or changing the setting. Sleep mode typically lets the temperature drift by a degree or two over the night.
- Eco and quiet modes. These can limit how hard the unit works.
- Fan speed and louvres. The lowest fan speed, with the air pointed at the ceiling, can feel weak even when the unit is working well.
- The remote. Try new batteries. Most units beep when they receive a command.
- Multi-splits. On a standard multi-split, the indoor units running at the same time have to be all heating or all cooling. If one room is set to heat and another to cool, one of them usually stops and waits.
Pauses that are normal
- Warming up. In heating mode the indoor fan may stay off or run slowly for a few minutes until the coil is warm, so it doesn't blow cold air.
- Defrosting. On cold, damp days the indoor fan stops for a few minutes while the outdoor unit clears frost from its coil, as explained in why the outdoor unit defrosts in cold weather.
- Restarting. After the system is switched off, the compressor usually waits a few minutes before it will start again, to protect itself.
- Nearing the setting. As the room gets close to the temperature you set, an inverter system slows down, and the air from it feels less cold or less warm.
- A heat-soaked room. On a very hot day, walls and furniture hold a lot of heat, and it takes time to bring the room down.
The room and the weather
Open doors and windows, sun through unshaded glass, and heat from people, computers and cooking all add to what the unit has to deal with. A unit sized for one room will struggle if it's also cooling a hallway or stairwell through an open door.
Most wall units measure the temperature at the indoor unit, high on the wall where the air is warmest. In heating mode, that means the room can feel cooler where you sit than the setting suggests. Some remotes and apps can measure the temperature where you are instead.
In very cold weather a heat pump's heating output and efficiency fall, and in extreme heat its cooling output falls. Every model has an outdoor temperature range it's designed for, listed in the manual. A system that has never kept up in ordinary weather may be too small for the room. What size air conditioner do you need explains how size is judged.
Checks you can safely make
- Filters. Clogged filters cut the airflow. How to clean the filters takes a few minutes.
- The outdoor unit. Clear leaves, snow and anything stored against it.
- Power. Check the isolator switch is on and the circuit breaker hasn't tripped. If a breaker trips again after you reset it, leave it off and call an engineer.
- Error codes and flashing lights. Note the code and look it up using the model number.
- Ice. Ice on the indoor unit or the pipes, or thick ice that stays on the outdoor unit, isn't normal. It can come from blocked filters or a fault, and if it comes back once the filters are clean, call an engineer.
Symptoms and likely causes
| Symptom | Possible causes | Who can fix it |
|---|---|---|
| Air moving but not cold or warm | Wrong mode, setting reached, low refrigerant | Check settings, then an engineer |
| Weak airflow | Dirty filters, low fan speed, dirty fan or coil | Clean filters, then a service |
| Starts well, then fades | Setting reached, sleep or eco mode | Check settings |
| Stops blowing for minutes while heating | Warm-up or defrost | Normal |
| One room stops on a multi-split | Rooms set to different modes | Use the same mode |
| Never copes on hot or cold days | Unit too small, heat gains, weather | Talk to the installer |
| Runs constantly with ice or a hiss | Refrigerant leak or other fault | Engineer |
| Nothing happens at all | Remote, isolator or tripped breaker | Check power, then an engineer |
Refrigerant isn't used up, so a system that's short of it usually has a leak, or occasionally was given the wrong amount at installation. Finding and repairing a leak, or correcting the amount, needs an engineer with the right F-gas qualification. Faults in the compressor, fans, sensors or circuit boards are work for an engineer too.
Related guides
- The modes explained: cool, heat, dry, fan and autoCool lowers the temperature and takes out some moisture, heat warms the room, dry mainly removes moisture with gentle cooling, fan only moves air, and auto chooses between cooling and heating for you.
- What size air conditioner do you need?Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.
- Error codes and flashing lightsError codes and flashing lights usually show that the system has detected a problem, but they mean different things on different makes and models. Look the code up using your model number, and give it to the engineer if you need one.
- When to call an engineerCall an engineer when simple checks don't fix a problem, and switch off straight away for burning smells, smoke, a circuit breaker that keeps tripping or signs of a refrigerant leak. Refrigerant, electrics and anything inside the casing are work for a qualified engineer.