The arrow on a split filter shows airflow, and it has to point the way the manufacturer designed, which is with the air moving into the coil. At handover, watch the filter come out and go back. After you wash it, the same way is the only way. A filter in backwards, or a panel that does not click home, starves the coil, drops the cooling, and can ice the unit even at a sensible setpoint. That is not fixed by setting 16C. It is fixed by turning the filter around.
The arrow points with the air
Room air is drawn through the filter before it hits the cold coil. The mesh or the frame is shaped for that direction. An arrow moulded on the frame points with the flow, toward the coil, not at random. Some filters also have a coloured face that must sit outward. The manual for your model wins if it disagrees with a guess. Photograph the filter in place on handover day, before anyone cleans it. The first week is too soon to experiment. If the installer skips the demo, ask them to pull one filter before they leave. If someone else will clean the filters, they are the person who needs to see the arrow.
Washing, seating and a filter turned round
User filters are usually rinsed, dried fully, and refitted. A damp filter back in the unit grows a smell and restricts air. Let it dry. Do not use a knife, a brush that sheds, or a vacuum set so hard it tears the mesh. If there are two filters, do not swap a left for a right if the shapes differ. Arrows still have to agree with airflow. A filter that will not seat is the wrong way around or the wrong model. Forcing the panel bends the clip and lets air bypass the mesh, which is how coils silt up before the first season's service.
Weak throw, a light icing, a unit that never reaches a normal setpoint, and a whoosh that sounds starved are the clues. Check the arrow before you assume the refrigerant is low. Do not ask for a regas as the first reply to a backwards mesh. If the pipes do need opening, that is work for an F-gas certified company.
Leaves on the outdoor coil are a different job
Outdoor coils have fins too, and they are not this filter. Leaves on the outdoor unit are a separate job: clear them without knifing the fins. The indoor arrow does not tell you how to scrape the condenser.
Frequently asked questions
Which way does the arrow point?
With the airflow, toward the coil, unless your manual shows otherwise. Photograph the installer's fit and copy that. Do not guess from a different brand.
Can I run it while the filter dries?
No. Running with the filter out dumps dust on the coil. Fit a dry filter the right way, or leave the unit off until you can.
The arrow has worn off. What now?
Use the manual for that model, or the photo from handover. If you have neither, send the model number and a picture of the frame to the installer before you button it up.
Cooling collapsed after a clean. Is the gas low?
Check the arrow, and that the panel clicked home, before you assume the refrigerant is low. A new install that was demonstrated correctly and then cleaned wrongly is a household fault. A new install that was never shown, and arrives weak, belongs with the installer.
Core buying guides
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Related guides
Sources and further reading
How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.