During a serious heatwave — outdoor temperatures above 35°C for multiple consecutive days — even well-specified air conditioning systems may struggle to maintain cool temperatures without some help. Here is how to get the best performance from your system in extreme heat.
Realistic Expectations in Extreme Heat
An air conditioning system is sized to maintain a target temperature — typically 21-23°C — at a design outdoor temperature. For UK residential installations, this design temperature is typically 28-32°C. When outdoor temperature exceeds 35°C for extended periods, the system is working outside its design envelope and will take longer to reach target temperature or may run continuously without fully reaching it.
This is not a fault — it is physics. The system is removing heat as fast as it can, but the heat ingress from the extreme outdoor temperatures exceeds the design capacity. Some warmth is normal and acceptable in truly exceptional heat.
How to Help Your System in a Heatwave
Close curtains and blinds on sun-facing windows during daylight hours. Solar radiation through glass is a major heat source. Closing curtains or blinds significantly reduces the heat load the air conditioning must overcome. External blinds or shutters are even more effective — they prevent heat entering the glass in the first place.
Minimise heat sources inside. Cooking generates significant heat — use the microwave instead of the oven, cook in the evening rather than at midday. Turn off lights and equipment not in use. Every kW of heat you do not generate indoors is a kW less your air conditioning must remove.
Pre-cool before the hottest part of the day. Switch on air conditioning at 8-9am before the outdoor temperature peaks. Getting the room cold while it is still possible is much easier than trying to cool a very hot room at 2pm.
Keep doors closed in air-conditioned rooms. Each time you open the door, cooled air escapes and warm air enters. In an ongoing heatwave, keeping air-conditioned rooms sealed makes a meaningful difference.
Clean the filter before a heatwave period. A blocked filter reduces airflow and cooling capacity by 15-20%. A clean filter ensures the system operates at full capacity when you need it most.
Check the outdoor unit. Ensure nothing is blocking airflow around the outdoor unit — seed heads, leaves or garden furniture are common summer obstructions. The outdoor unit must be able to reject heat freely to operate efficiently.
When to Call an Engineer
If the system is running continuously but the room temperature is significantly higher than the target temperature — more than 4-5°C above the set point after several hours of operation — the system may have a fault rather than simply being overworked. Refrigerant low, dirty coils or a fan fault can all prevent proper operation in hot conditions. Call 07833 053749 for emergency diagnosis.