How to Prepare Your Air Conditioning for Summer: Pre-Season Checklist

Every year around late May, our phones start ringing with the same call. "The air conditioning won't turn on." Or worse, "It turned on and now there's water dripping down the wall." Almost every one of those callouts could have been avoided with 30 minutes of checking in March or April.

If your air conditioning has been sitting idle all winter, it needs a bit of attention before you rely on it again. Systems that sit dormant for months develop problems. Dust builds up, condensate drains block, mould grows inside the unit, and components seize from lack of use. Catching these issues early is cheaper and less stressful than dealing with a breakdown on the hottest day of the year.

Here's the pre-season checklist we recommend to every customer. You can do most of it yourself in under half an hour.

The 10-Step DIY Pre-Season Checklist

1. Clean or Replace Your Filters

Dirty filters restrict airflow, force the system to work harder, and circulate dust and mould spores around your home. Open the front panel and slide the filters out. If they're reusable (most split systems have washable mesh filters), rinse them under warm water with mild detergent and let them dry completely before refitting. Never run the unit without filters.

If your filters are disposable or visibly damaged, replace them. As a rule of thumb, clean your filters every four to six weeks during heavy use and at least once before summer starts.

2. Check the Outdoor Unit

Head outside and take a look at the condenser unit. After a winter of wind and rain, it's common to find debris piled up around it. Clear away any leaves, cobwebs, or vegetation within about 60cm of the unit on all sides. The unit needs clear airflow, and anything blocking it forces the compressor to work harder.

While you're there, check for visible damage or corrosion on the casing. Have a look at the pipe insulation running between the indoor and outdoor units. If it's cracked, split, or missing, book a professional. Damaged insulation reduces efficiency and causes condensation to drip onto walls.

3. Test Both Cooling and Heating Modes

Switch to cooling mode and set the temperature three or four degrees below room temperature. Give the compressor about five minutes to kick in, as many units have a built-in startup delay.

After 10 to 15 minutes, the air should feel noticeably cold. Quick test: put a thermometer in front of the supply vent. The air should be 8 to 12 degrees cooler than the room. If the gap is less than 8 degrees, something isn't right.

Then switch to heating mode and confirm it works too. The changeover valve can stick after months of inactivity, so check both modes while you're at it.

4. Check for Unusual Noises or Smells

While the system is running, listen carefully. A healthy air conditioning unit hums quietly. Anything beyond that is worth investigating.

• Rattling or vibrating: Usually loose panels, covers, or debris caught in the outdoor unit fan. Check for anything obvious and tighten any loose screws.
• Grinding: This points to a bearing failure in the fan motor. Switch off and call an engineer.
• Hissing: Could indicate a refrigerant leak. Don't ignore this one.
• Clicking without starting: Often a failed capacitor. Needs a professional.

Now sniff the air coming from the indoor unit. A musty or damp smell means mould has grown inside the unit during winter. This is extremely common in the UK climate. A burning smell means an electrical issue, and you should switch off immediately and call for a repair.

5. Inspect the Condensate Drain

Blocked condensate drains are the number one callout we get after winter. Your air conditioning produces water as it cools, and that water needs somewhere to go. If the drain line is blocked with dust, algae, or debris, the water backs up and leaks from the indoor unit onto your wall or ceiling.

You can test this yourself. Pour a small cup of water into the drain tray inside the indoor unit and check that it flows freely out of the drain pipe outside. If it doesn't, try flushing the line with warm water. Persistent blockages need a professional drain flush.

6. Check the Remote Control Batteries

You'd be surprised how many "my AC won't turn on" calls turn out to be dead batteries in the remote. Swap them out for fresh ones. While you're at it, check that the clock and date are correct, especially after the clocks change.

7. Clean the Indoor Unit Vents and Louvres

Dust accumulates on the vents and louvres over winter, and it gets blown straight into the room the moment you switch on. Wipe down the louvres with a damp cloth and vacuum the grilles with a brush attachment. If you can see mould on the outside, there's almost certainly more growing on the fan barrel and evaporator coil inside, and that needs a professional deep clean.

8. Check for Water Stains or Dampness Around the Unit

Look at the wall around and below your indoor unit. Water stains, damp patches, or discolouration are signs of leaking, possibly from a blocked drain, cracked condensate tray, or poor pipe insulation. In older UK properties, condensation around the mounting plate is surprisingly common. If you spot dampness, get it sorted before summer. Running the system with an existing leak only makes the damage worse.

9. Make Sure Nothing Is Blocking Airflow

Over winter, furniture gets rearranged and curtains get changed. Check that nothing is sitting within about 15cm of the indoor unit's intake or outlet. Restricted airflow makes the system work harder and can cause the evaporator coil to ice up. Same goes for the outdoor unit. If you've stacked garden furniture or bins against it, move them clear.

10. Set Your Timer and Schedule

Don't cool an empty house. Set the AC to start 30 minutes before you need the room cool, and stop 15 minutes before you leave. UK buildings hold temperature well enough for this approach.

And resist the temptation to crank it down to 16 degrees. That doesn't cool the room faster. It just makes the compressor run longer. Aim for 22 to 24 degrees. Each degree lower costs roughly 8 to 10 percent more in energy.

When to Call a Professional

The checklist above covers everything you can safely do yourself. But some things need a qualified engineer. Call a professional if you notice any of these:

• The system runs but doesn't cool properly after you've cleaned the filters and checked airflow. Low refrigerant, dirty coils, or a failed reversing valve all need expert diagnosis.
• Refrigerant issues. Topping up or testing refrigerant legally requires an F-Gas certified engineer in the UK. This isn't a DIY job.
• Electrical faults. Tripping breakers, burning smells, or persistent unusual noises all need professional attention.
• A musty smell you can't fix. If cleaning the filters and wiping down the louvres doesn't shift it, mould has got into the internal components. A proper anti-bacterial deep clean is the only fix.
• Water leaking despite a clear drain line. This could mean a cracked condensate tray or a failed condensate pump.
• The system is over 10 years old and hasn't been serviced in the last 12 months. At that age, annual servicing isn't optional. It's what keeps the system running and your warranty valid.

Most manufacturers, including Daikin, Mitsubishi, Fujitsu, and Panasonic, require annual professional servicing to maintain warranty cover. A skipped service can void a five-year warranty, which is an expensive oversight.

UK-Specific Tips: Damp, Mould, and Condensation

The UK climate creates specific challenges for air conditioning that you won't find in guides written for drier climates. Our high ambient humidity means condensation risk on AC pipework and around indoor units is higher than average. This is why intact pipe insulation matters so much here.

Mould Is the Big One

Mould thrives in the UK climate. After months of winter dormancy, the inside of an AC unit that wasn't dried out properly is a prime breeding ground. The evaporator coil stays damp, the fan barrel collects moisture, and by spring you've got a fully established mould colony ready to blow spores around your home the moment you hit the power button.

If you can see mould on the louvres or filters, it's almost certainly worse inside. That needs a professional anti-bacterial deep clean, not just a wipe-down.

Here's a tip most people don't know: at the end of summer, run your AC in fan-only mode for an hour or two before shutting it down for winter. This dries out the evaporator coil and prevents mould from taking hold during the off-season. It's the single best preventive measure, and almost nobody does it.

Condensation in Older Properties

If your property suffers from condensation or damp generally, your air conditioning can actually help. Running the system in "dry" or dehumidification mode reduces indoor humidity. Aim for 40 to 60 percent relative humidity. But if the unit itself is causing condensation drips because of damaged pipe insulation, that needs fixing first.

When to Book Your Service

Book your annual service in March or April. By June, every HVAC engineer in the country is booked solid, and if your system breaks down mid-heatwave, you're looking at emergency callout rates. A spring service for a single split system typically costs between £60 and £120. An emergency callout in July can easily be £200 or more.

There's also a practical benefit to booking early. If the engineer finds something that needs a part, there's time to order it and fit it before you actually need the cooling. Discovering you need a new capacitor in August means days or weeks without air conditioning while the part ships.

Another good habit: run your air conditioning for 15 minutes once a month during winter, even if you don't need it. Use heating mode or fan-only mode. This keeps the compressor oil circulating, prevents seals from drying out, and stops mould from establishing. It prevents most of those "won't start after winter" problems we get called out to every spring.

What a Professional Pre-Season Service Includes

If you're wondering what you're actually paying for when you book a service, here's what a thorough pre-season check covers:

• Evaporator and condenser coil cleaning. These coils collect dirt over time, which insulates them and reduces heat transfer. A proper clean restores efficiency.
• Refrigerant pressure and level check. Low refrigerant means the system can't cool properly and the compressor is under strain. The engineer checks pressures and tops up if needed using compliant refrigerant.
• Electrical connection test. All terminals are checked for tightness and safety. Loose connections cause arcing, which is a fire risk.
• Capacitor and contactor inspection. These components wear out over time and are the most common cause of "won't start" faults.
• Condensate drain flush. The full drain line gets flushed to prevent blockages and leaks during summer.
• Airflow temperature check. The engineer measures the temperature difference between supply and return air. It should be 8 to 12 degrees. Anything less indicates a problem.
• F-Gas leak check. Legally required for systems containing 5 tonnes CO2 equivalent or more.
• System performance report. You get a record of the system's condition, which is useful for warranty purposes and for tracking any decline over time.

Think of it like an MOT for your air conditioning. You wouldn't drive your car all summer without checking the oil and tyres. Your AC deserves the same attention.

Ready to Book Your Pre-Season Service?

If you've run through this checklist and everything looks fine, brilliant. Your system is probably good to go. If you've spotted anything that needs attention, or if your system hasn't been professionally serviced in the last 12 months, get it booked in before the summer rush.

We offer full pre-season service packages across London and the surrounding areas. Get in touch to book yours in, and we'll make sure your air conditioning is ready for whatever summer throws at it.