Should I completely fill my freezer?

Physicist Dave Ansell answered this question...

Dave - Yeah, there's a couple of things which might affect the efficiency of your freezer, depending on how much it's got in there. I guess, first, if you put something warm in there, then you're going to take a load of energy to pump the heat out of it. But assuming you've already got the stuff cold, which is best, if you have an upright freezer, when you open the door, all the cold air is denser than the warm air, so all the cold air falls out. You're essentially putting in heat by all the warm air which is taking its place, and you shut the door and it's got to re-pump that heat out again.

Chris - So the more stuff that's in there, the less room there is for warm air to go in and push cold air out, so the better then?

Dave - That is correct, up to a point. If you jam every corner absolutely full of stuff, you can then get other problems because the way the freezer knows how warm it is is you've got a thermostat in there and if the cold air can't get from the cooling elements to the thermostat then it's going to keep making them colder and colder. The bigger the temperature difference, the more the freezer's got to work from between the room and the temperature inside the freezer, the harder work it is and so the more energy it uses. So, if the cooling elements are actually at -30°C rather than -20°C, that's going to be costing your electricity. So fairly full, but not so full that air can't circulate at all!

Diana - So I guess it is a good idea to defrost your freezer occasionally so you don't get that build up of huge chunks of ice if you haven't got a frost-free freezer?

Dave - Yeah, especially as that is even worse than filling your freezer right up because it insulates the cooling parts of your freezer, so the cooling parts could get even colder which then causes even more ice. It is a horrible vicious circle and you end up running your freezer continuously and not actually ending up with a very cold freezer.