Is it more efficient to leave the fridge briefly open when I am pouring milk for my tea or to close and reopen it, given warm air is introduced whenever the fridge door is opened?
Nick Baker
Colchester, Essex, UK
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As the door opens, the cold, heavy air will flow out onto the floor, drawing in the room鈥檚 warm air from above. Within a few seconds, there will be a steady down-flowing convection current. Even quicker than that, the warm surfaces of the room will start radiating energy (in the form of infrared radiation) to the surfaces of the fridge鈥檚 contents. The rate of heat gain by these two mechanisms is of the same order of magnitude, around 100 watts. Closing the door will stop these instantly.
Air has a very low volumetric heat capacity, so the amount of heat in the trapped warm air will be tiny compared with 鈥渃oolth鈥� (i.e. the opposite of warmth) stored in the cold contents. But even if it were significant, leaving the door open would just lead to several fridgefuls of warm air rather than one or fewer, plus more radiant gain. Doing this would be like drilling a hole in the hull of your leaky boat to let the water out!
Howard Bobry
Port Townsend, Washington, US
It makes very little, if any, difference because any exchange of cold fridge air for warm room air is insignificant. The interior of the fridge includes solid metal, glass and plastic components and containers, as well as various foods and liquids. These all have a far higher heat capacity than air, so even a complete exchange of all the chilled fridge air for warmer room air would result in negligible change in food and drink temperatures.
Maurice Gallagher
London, UK
It is more efficient, better for your health and more environmentally friendly to give up on milk and drink black coffee.
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