Heatwaves are when air conditioning earns its keep, and when it works hardest. On the hottest days, a correctly sized system may run flat out and still only hold the room a few degrees below outside. That isn’t necessarily a fault: it’s working beyond the conditions it was designed for.
This guide explains what to expect, how to set the system and help it with shading and good habits, how to look after the outdoor unit, how to protect vulnerable people and which signs mean it’s time to call an engineer.
What to expect on the hottest days
Systems are sized for a design outdoor temperature and a target indoor temperature. When it’s hotter than the design condition, the heat coming in can exceed what the unit can remove. The room will still be much more comfortable than without cooling, but it may not reach the number on the remote. If the air from the indoor unit is clearly cold, the system is working. If it feels barely cooler than the room, see air conditioning not cooling.
Before the heat arrives
- Wash the filters: see how to clean the filters.
- Clear leaves, plants and clutter from around the outdoor unit.
- Run the system in cooling for a while to check it works, before you need it.
- If it hasn’t been serviced in the last year, book a service early; diaries fill up fast once a heatwave starts. Our guide to preparing for summer has a checklist.
Settings that help
- Start early. Switch on in the morning, before the house heats up. It’s far easier to keep a room cool than to cool a hot one.
- Pick a sensible setting. Setting the lowest number doesn’t cool the room faster; the unit works at full output either way. It just keeps going longer.
- Use auto fan, or a higher fan speed while the room cools down, then lower it.
- Leave it running rather than switching on and off through the day.
- Close doors to rooms you aren’t cooling, and keep windows shut while it runs.
Keep the heat out
The NHS heatwave advice is to keep windows, curtains and blinds closed during the day and open them at night when it’s cooler outside. Sun through glass is one of the biggest heat loads a system faces, and external shading such as awnings or shutters stops it before it reaches the glass. Inside, avoid the oven at midday, switch off equipment you aren’t using and use the coolest parts of the house for the hottest hours.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Look after the outdoor unit
The outdoor unit gets rid of the heat taken from your rooms. If it can’t breathe, the whole system suffers. Keep at least the clearances in the manual around it, and never cover it. Shade can help, as long as it doesn’t restrict airflow or trap hot air. Don’t spray it with a hose or pressure washer; if the coil is dirty, that’s a service job.
Protect people at risk
The NHS lists adults over 65, babies and young children, people with serious long-term conditions and people on certain medicines among those most at risk. Check on them, and check the temperature of the rooms where they sleep. The NHS suggests fans only below 35°C. UKHSA and the Met Office issue Heat-Health Alerts, which are worth following. See elderly relatives in a heatwave.
When to call an engineer
- ice on the pipework or indoor unit;
- water dripping from the indoor unit;
- the outdoor fan not turning while the unit runs;
- error codes or flashing lights that return after a restart;
- tripping electrics, burning smells or unusual noises.
Switch off and call. For businesses where heat puts people, stock or equipment at risk, our 24/7 emergency line is 07833 053749. If you need temporary cooling, ask about air conditioning hire.
After the heatwave
If a room never got comfortable, it may need more capacity, better shading or a second unit. We can survey and tell you which. Larger homes often add rooms later on a multi-split.
Quick answers
Why can’t my air conditioning reach the set temperature in a heatwave?
Systems are sized for a design condition. In extreme heat, more heat comes in than the unit can remove. If the air from the unit is clearly cold, it’s probably working.
Does setting a lower temperature cool the room faster?
No. The unit runs at full output either way; a lower setting just keeps it running longer.
Should I leave my air conditioning on all day in a heatwave?
Usually yes, at a sensible setting, starting in the morning. Keeping a room cool is easier than cooling a hot one.
Can I spray water on the outdoor unit to help it?
No. Keep it clear and shaded without restricting airflow, and leave coil cleaning to a service.
Related services
- Air conditioning servicing
- Air conditioning repair
- Emergency air conditioning repair
- Air conditioning hire
More advice
- How to Prepare Your Air Conditioning for Summer: 5 Essential Checks
- Air Conditioning Not Cooling: Causes and How to Fix It
- Keeping Elderly Relatives Cool in a Heatwave
- How Much Does It Cost to Run Air Conditioning in the UK? 2026 Guide
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.