Why a Split System That Heated Fine All Winter Can Still Struggle to Cool

It is one of the more common calls we get in the first genuinely warm week of the year. The split system ran all winter without complaint, heated the room fine, and then on the first hot day it either does not cool properly, takes hours to make any difference, or starts smelling of something damp within a few minutes of being switched over.

Nothing has broken over the last fortnight. What has happened is that the machine has been asked to do the harder of its two jobs, and the things that were quietly wrong with it all winter have stopped being survivable.

Cooling and heating are not the same demand

A reverse-cycle unit moves heat rather than making it, and the direction it moves it changes which conditions help and which hurt.

In heating, the machine collects heat from cool outdoor air and delivers it into a cold room. The room is the easy end — a room at 14 degrees will take heat readily, and a unit running below its best still makes an obvious difference that you feel immediately.

In cooling, it has to reject heat outdoors into air that is already hot. That is working uphill, and it is at its hardest exactly when you need it most, on a 38-degree afternoon. On top of that, cooling produces condensation. Pulling a room’s temperature down means the indoor coil runs below the dew point, water forms on it continuously and has to be drained away. Heating mode does none of this. So cooling loads a whole set of components — the coil surface, the drain pan, the drain line — that spent the entire winter dry and untested.

The filter that coped in heating mode

Filters load up gradually and there is no moment where the machine tells you. Through winter, a partly blocked filter shows up only as a unit running a bit longer than it used to, which nobody notices.

In cooling, restricted airflow costs you twice. Less air across the coil means less heat carried out of the room, so output drops. And a coil that is not getting enough airflow over it runs colder than intended, which on a humid Melbourne day can put it close to icing, at which point performance falls away sharply rather than gradually. Pulling the filters out and washing them is the one part of this anyone can do, and it is worth doing before the first hot day rather than during it.

The indoor coil, and what has been growing on it

Behind the filter is the coil, and this is usually the real answer when a unit heats acceptably and cools poorly.

Fine dust gets past every filter over a season. In heating mode that coil is warm and dry, so the dust just sits there inertly and nothing much happens. Switch to cooling and the same surface becomes cold and permanently wet, and the dust layer becomes damp organic material sitting in the dark in a warm house. That is where the smell that appears on the first cooling run comes from — it is not the ductwork and it is not the outdoor unit, it is a coil that has just been wetted for the first time since autumn.

It also matters for output. The coil is the surface that actually does the heat exchange, and anything coating it insulates it from the air passing over it. A split system clean is largely about getting that coil and the blower barrel back to bare surface, because those two parts are what the machine’s capacity actually depends on.

The drain that has not carried water since March

The condensate drain is the component most likely to fail in spring for the simple reason that it has been sitting unused and dry for six months.

Over that time the drain pan collects dust that turns to sludge the moment water arrives, and the line itself can host a slow biological build-up that only becomes a blockage once flow starts again. Insects nest in dry outlets. The result is water that has nowhere to go, and it announces itself as drips from the bottom of the indoor unit, a stain appearing on the wall beneath it, or a unit that shuts itself down.

This is the specific reason it is worth running a split system in cooling for a decent stretch on a mild spring day: it puts water through the drain while it is still merely a service call, rather than discovering the blockage during the first heatwave.

The outdoor unit after a winter of leaf litter

The outdoor coil is where heat actually leaves the building in cooling mode, and it needs unobstructed air through the fins to do it.

Winter is when that gets compromised without anyone looking — wind-blown leaves packed against the fins, a plant that has grown across the airflow path since March, litter caught underneath. In heating mode a partly blocked outdoor coil is largely tolerable. In cooling on a hot day it is the difference between a unit that keeps up and one that does not, because rejecting heat into already-hot air has no margin in it. Clearing a good arm’s length of space around the unit and getting the leaf litter out from between the fins costs nothing and is worth doing now.

The order to check things before the first hot week

There is a sensible sequence to this, and it runs from what you can do yourself to what needs a technician. Working through it on a mild day tells you which category your unit is in while there is still time to book.

If it cools properly after the easy steps, you are done. If it still struggles, the answer is nearly always the indoor coil, the drain or a refrigerant issue — and the last of those is not a DIY item at any point. Our HVAC team handles the coil, blower and drain work across Melbourne, and spring is a considerably easier time to get a booking than the week after the first 38-degree day.

Frequently asked questions

Why would the same unit heat well but not cool well?

The two modes are not equally demanding. In heating, the machine is helped by the temperature difference it is working with; in cooling it is working against it, and it also has to deal with the moisture it pulls out of the air. Anything restricting airflow or fouling the coil costs you far more in cooling mode than it did in heating.

What is the smell when I first switch a split system to cooling?

It usually comes from the indoor coil. In cooling mode that coil runs cold and wet, and any dust and organic material sitting on it becomes damp and starts to smell within minutes of the first run. Heating never wets the coil, which is why the smell appears in spring and not in June.

Should I run the unit before summer even if I do not need it?

Yes, and spring is the time for it. Run it in cooling for a decent stretch on a mild day, listen to it, and check that water is leaving the drain outside and not appearing inside. A fault found on a mild day is a booking; the same fault found in a heatwave is a wait.

Does cleaning the filter fix a unit that is not cooling?

Sometimes, and it is always the right first step because you can do it yourself. But a filter only catches what reaches it. The coil behind it is the surface that actually transfers the heat, and once that is coated, cleaning the filter alone will not restore the unit’s output.