Seasonal HVAC Maintenance Checklist: What to Do Before Summer & Winter

Almost every emergency callout we attend in a heatwave was preventable in April. Systems rarely fail without warning — they fail because small, cheap problems were left to become big, expensive ones at exactly the moment the system works hardest. Two short maintenance windows a year prevent nearly all of it.

Pre-Summer Checklist (April–May)

Do this before the first hot week, not during it — engineer availability in a heatwave is the worst of the year.

• Filter inspection & replacement — the single highest-value job on this list. Clogged filters choke airflow and trigger most of the failures below.
• Refrigerant pressure check — verifies there's no slow leak quietly killing efficiency (and, under F-Gas rules, leaks must be found and fixed, not topped up).
• Electrical connections cleaned & tightened — loose terminals and tired capacitors are a leading cause of dead-on-arrival systems on the first hot day.
• Condenser coil cleaning — a winter's worth of grime on the outdoor coil can cost you 10–25% efficiency.
• Thermostat calibration — a sensor reading 2°C off means the system chases a temperature it never reaches.
• Test run in cooling mode — a full cycle under observation catches problems while they're still bookable repairs.
• Drain line flushed — blocked condensate drains are the number-one cause of "my AC is leaking water down the wall".

Pre-Winter Checklist (September–October)

Essential if you have a heat pump — this is its main season.

• Heat pump defrost cycle test — confirms the unit can clear frost from its outdoor coil; a failed defrost in January means no heating when you most need it.
• Filter replacement — heating airflow matters as much as cooling airflow.
• Outdoor unit debris removal — leaves and debris around the unit restrict airflow and trap moisture against the casing.
• Electrical safety check — heaters, crankcase elements and contactors get their first workout in months.
• System runtime monitoring — a system that's short-cycling or running long is flagging a problem before it becomes a breakdown.

What You Can Do Yourself — and What You Shouldn't

Safe DIY: cleaning or replacing accessible filters, keeping the outdoor unit clear of leaves and vegetation, wiping indoor unit grilles, and paying attention to new noises or smells. These cost nothing and genuinely help.

Leave to us: anything involving refrigerant (legally requires F-Gas certification), electrical work, coil chemical cleaning, and pressure diagnostics. This isn't gatekeeping — refrigerant work without certification is unlawful, voids warranties, and occasionally hospitalises the optimistic.

The Faults Behind Most Breakdowns

1. Dirty filters — the most common trigger by far, cascading into frozen coils and overworked compressors.
2. Low refrigerant from gradual leaks — efficiency bleeds away for months before cooling visibly fails.
3. Electrical failures — capacitors and contactors wear out quietly, then fail on the hottest day when demand peaks.
4. Frozen evaporator coils — nearly always a symptom of the first two problems, not a fault of its own.

The pattern is obvious: routine servicing catches every one of these early. In our experience, an annual service prevents the overwhelming majority of emergency breakdowns — the industry rule of thumb says as many as 95% of them.

One Service or Two?

Cooling-only domestic systems: one annual service (spring) is usually enough. Heat pumps and commercial systems working year-round: spring and autumn visits, matching the two checklists above. Commercial premises with F-Gas leak-check obligations should build servicing into their compliance calendar anyway — see our F-Gas guide.