Rated for 35 degrees, working at 45
Our air conditioning coped fine in spring. Why does it give up every February once it gets past 40?
Published 02/10/2026. Written by the Pristine Cooling team, Kalgoorlie.
Almost every piece of air conditioning and refrigeration plant is sized against an assumed outdoor temperature of about 35 degrees. Kalgoorlie spends a fair part of each summer above that, so from about December the gear on your roof is working outside the conditions it was chosen for. Nothing breaks at 45. The plant just gets slower at the job it was bought to do. You pay for that in run hours and repairs rather than all at once, which is why a system that was fine in spring gives up in February.
What the rating on the side of the unit means
When a manufacturer publishes a capacity for a unit, so many kilowatts of cooling, that number was measured under a set of fixed conditions. One of those conditions is the temperature of the air going over the outdoor coil. Change the condition and you change the number.
So a unit sold as ten kilowatts is ten kilowatts at about 35 degrees outside. At 45 it will do less. The second number does not get printed on the box, because there is one for every temperature.
None of that is a marketing problem. The 35 degree figure is a genuine measurement taken under conditions somebody had to pick. It just sits below what we get here for a good part of the year.
Why hot air makes a condenser struggle
A refrigeration system does one job. It picks heat up in one place and drops it somewhere else. The dropping off happens at the condenser, which is the coil outside with the fan on it.
Heat only ever moves from hot to cold. So for the condenser to shed heat into the outside air, the refrigerant inside the coil has to be hotter than that air. At 30 degrees outside there is a wide gap between the two, so heat pours out of the coil without much effort.
At 45 that gap has closed up. The coil is the same size and the fan moves the same volume of air, but each cubic metre of that air arrives hot, so it carries less heat away than it did in spring.
The system's answer is to raise its own pressure. Push the refrigerant to a higher temperature and the gap opens back up, so the heat can still get out. That is what a technician means by high head pressure. Most of what goes wrong on a hot day starts there.
What high head pressure costs you
The compressor works harder for the same result. It is pushing against a bigger pressure difference, so it draws more current and turns more of your power bill into heat instead of cooling.
The unit delivers less. Capacity falls away as the outside temperature climbs, so a room that just held temperature in October cannot hold it in February. Nothing has broken. The machine has less to give.
It runs for longer. Less capacity, plus a bigger heat load on the building outside, means the compressor is on most of the day rather than half of it. Run hours are what wears a compressor out.
It runs hotter inside. High pressure means a high discharge temperature where the refrigerant leaves the compressor. The oil in there is what keeps the compressor alive. Heat degrades it. By the time you can hear any difference it has usually been going on a while.
Then it protects itself. Every system has a high pressure cut out. When it trips the unit stops, sits there until it has cooled off, then starts again. From the floor that looks like a fault that comes and goes for no reason.
The heat usually is not the fault
When something fails on the hottest afternoon of the year, everyone blames the heat. Usually the heat was not the fault. It just took away the last of the margin the system had.
A condenser with a light coating of dust on it loses a small amount of capacity. At 28 degrees you would never know. The unit is only working at part of what it can do, so a small loss vanishes into the slack. At 45 there is no slack left. That same light coating is now the difference between holding temperature and tripping out on high pressure.
So the fault was already there in August. February only made it visible. A lot of what we attend in a heatwave is like that: something that had been sitting there for months until the ambient got high enough to show it up.
Why the same unit works harder here
Put the same unit on two buildings, one in Perth and one here, installed the same way. The one here does more work for the same result. Peak temperature is not the main reason, because Perth gets hot days too.
It is about hours. The Goldfields stays hot, which the coast does not. A long run of days over 40, with nights that do not drop far enough to give the plant much of a rest. Every one of those hours is a compressor running at high pressure with a high discharge temperature.
How much of that a given unit will put up with comes down to the brand and the model and how well it has been looked after. The workload is the part that does not vary. The same machine is being asked for more here than on the coast.
Dust does the same thing by a different route. A coated condenser behaves like a smaller one. A smaller condenser raises head pressure the same way hot air does. Out here you get the heat and the dust together.
What we do about it
We select for the ambient we actually get here rather than the one in the catalogue. In practice that often means more capacity than a straight kilowatt calculation suggests. Sometimes it means a larger condenser on the same compressor.
Where the condenser goes gets a good deal of our attention, because it costs nothing to get right at design stage and a lot to fix later. A unit tucked into a corner is breathing air well above the shade temperature. So is one pressed up against a wall or sitting in another unit's discharge. The real ambient at the coil is 45 plus whatever the position adds.
We leave room to get at it. A condenser nobody can get a brush and a hose to is a condenser that will not get cleaned, whatever the maintenance schedule says.
On the maintenance side we set the service interval to this climate rather than to the manual, which was written for average conditions somewhere else.
What you can do before summer
Get the condensers cleaned before it gets hot rather than during it. October is a better time for it than January.
Walk around your outdoor units and look at what has changed since the install. Pallets stacked against a unit, a shed extension, a new fence, a shrub that has done well. Any of those will raise the temperature of the air going into the unit.
Take intermittent faults seriously in December. A unit that keeps stopping and restarting on its own has run out of margin. That is worth looking at before February.
If a room creeps up in February but behaves the rest of the year, do not assume something is broken. That is a capacity conversation rather than a repair.
None of this means the gear is faulty. It means it was chosen against a temperature we go past most summers. Worth knowing before the next one, whether you are buying new plant or working out why the old lot struggles every February.
Ring us and we'll tell you when we can be there
Same number either way. Ring it if something is down right now or if you want a quote, a service or a maintenance contract set up.