Climbing into a car that has sat in the August sun is an ordeal: the cabin is at 50C, the steering wheel is too hot to hold and the air is barely breathable. You switch on the air con and hope it gets to work quickly. Yet almost every dashboard has a button that few people ever bother with: recirculation mode cools the cabin far faster and takes the strain off the engine or the battery.
Why recirculation cools faster
With recirculation off, the air conditioning draws air from outside. At 35C, that means trying to chill a constant stream of scorching air. With recirculation on, the system reuses the air already inside the cabin, which gets a little cooler every time it passes over the evaporator. Chilling air that is already down to 25C takes far less energy than endlessly chilling air at 35C, so the cabin hits your target temperature in a few minutes rather than a quarter of an hour. The compressor runs less hard, and for less time, which means less fuel burned in a petrol or diesel car and less range lost in an electric one.
In electric cars the difference is even more striking, because the climate control runs off the same battery that drives the car: how you use the air con is one of the biggest factors in summer range. Drivers who charge at home can also pre-cool the cabin while the car is still plugged in, so they set off in a cool car without giving up any of the battery's driving range.
When to switch it back off
Recirculation should not stay on indefinitely. Because the system keeps recycling the same air, humidity and carbon dioxide gradually build up in the cabin: the windows start to mist up and the air turns stale, which can dull your concentration at the wheel. The best approach is to run recirculation for the first few minutes to knock the temperature down fast, then switch back to the fresh-air intake to keep the cabin cool and properly ventilated. Recirculation also earns its keep in underground car parks, or when you are stuck behind a vehicle belching exhaust, by keeping fumes and bad smells out.
The trick before you set off
One last habit makes a real difference. Before turning on the air conditioning, open the windows on both sides for a few seconds to let the trapped hot air escape, then close them and switch on recirculation. A sunshade across the windscreen when you park will also knock several degrees off the starting temperature. Getting the 50C air out before you start cooling is the simplest way to avoid wasting energy chasing a temperature the system cannot reach. The same small habits pay off at home too: for more, our guide on how to save electricity is full of useful pointers.