PUDDLE PUZZLE

I recently test drove a vehicle and noticed when I came to a stop on the forecourt with the engine idling, as I spoke to the dealer a large puddle of water had formed beneath it. I know this usually happens because of the air conditioning running, but why exactly does it happen and why do some cars have this trait and some not? Silly as it sounds, although harmless, I felt less likely to purchase this car because of it.

DAMON SINCLAIR

Via email

You’re quite correct, it is harmless and is the result of the condensation forming on the air conditioner evaporator. The ventilation fan draws in outside air and directs it through the evaporator located in a housing within the ventilation system. Moisture in the incoming air condenses as the air passes through the evaporator and is cooled. The condensation collects at the bottom of the housing and is discharged underneath the vehicle via a rubber tube. The amount of condensate depends on the air conditioner cooling setting as well as the humidity. Selecting the recirculation setting recirculates the air within the vehicle – already dried – and blends in only a small portion of fresh air, thus also reducing the amount of condensation. If the engine is switched off immediately the vehicle is stopped, very little condensation will be discharged; however, when idling while stationary, condensation will continue to form and be discharged.