Drivers are being urged to hold off on cranking up the air conditioning the moment they climb into their cars, with the UK bracing itself for another heatwave from Wednesday, July 7. The Met Office is forecasting temperatures as high as 35C once more, prompting searches for ways to stay cool.
What Is the '4-20 Rule'?
When it comes to keeping comfortable in your vehicle, experts suggest one simple technique could chill the cabin faster while easing the strain on the system. The method, dubbed the '4-20 rule', is straightforward and should work for any make or model. It involves winding down all four windows and driving for roughly 20 seconds before closing them again and activating the air conditioning.
The principle is to let the scorching air trapped inside the car escape naturally before firing up the system. By expelling the hottest air first, the system faces less of a challenge and can achieve a pleasant temperature more swiftly.
Fuel Economy Benefits
Experts say this approach can also cut down on needless fuel consumption since the air conditioning isn't labouring as hard to cool down extremely hot air. The US Department of Energy highlights that allowing hot air to escape before engaging the air conditioning lessens the burden on the system, while also recommending drivers avoid idling with the air conditioning running before pulling away.
Using air conditioning is among the most significant factors impacting fuel economy during warm weather. The Department of Energy states that running air conditioning in extremely hot conditions can slash a conventional petrol or diesel vehicle's fuel economy by over 25%, especially on shorter trips.
How to Use the Rule Effectively
After the hot air has been expelled, motorists should wind the windows back up and let the air conditioning cool the now less-heated interior. At higher speeds, this approach is also more fuel-efficient, as driving with windows down increases aerodynamic drag.
Specialists also suggest using the recirculation setting once the interior starts to cool. This enables the system to recycle cooler air already present in the vehicle instead of continuously drawing in warmer air from outside, assisting in maintaining a lower cabin temperature.
Additional Tips for Keeping Cars Cool
Where feasible, motorists should also park in shaded areas or use a reflective windscreen sunshade to minimise heat accumulation while the vehicle is parked. Even leaving the windows slightly open when it's safe to do so can help reduce cabin temperatures before returning to the car.
While the '4-20 rule' is frequently cited by motoring specialists, the precise claim that it cools a vehicle 'twice as fast' varies depending on the car, weather conditions and air conditioning system. The broader guidance to vent hot air before activating the air conditioning is backed by automotive specialists and official energy advice as an effective method to cool a hot car more efficiently.
