Letter Re: Observations on Chest Freezer Efficiency

Hi Jim,
I was reading Monday’s letter regarding “Sizing a Retreat AC Power Generator”, and a thought came to mind when the author mentioned super-insulating a freezer for extended cooling durations. There are basically 2 types of freezer; the upright and the box, (what we call around here, the “coffin” freezer). Given the same basic amount of insulation included with each type, to the point where both manage the loss of cooling at the same rate, the “coffin” appears to be more efficient during access.
Cold air sinks. When the door of an upright freezer is opened, the cold air inside will pour out, much like you would expect water would pour out of it in the same circumstances. The cold sinks and falls out the front, and is replaced by warmer air from above. While the contents of the freezer chill the incoming air immediately, and give the impression that things are staying cold due to that same recently-chilled air passing over your face, in reality, heat is being absorbed by everything inside the freezer.
When you open the door of a box freezer, the cold has nowhere to go. There is disturbance of the upper layer of air as the door opens, and there is also a heat exchange effect at the boundary of the two layers, the vast majority of cold air remains in the box. A box freezer thus saves on the energy needed to take the temp down to its set level after opening the door.
Here’s a tip for preserving low temps for those with upright freezers. Keep as much food as possible inside the freezer. The more frozen stuffs you have, the less space warm air has to occupy. Cold food loses temp much much slower than displaced air does, and with this practice in place, the door may remain open for longer periods as junior tries to decide on rocky road or vanilla (the only real flavor on earth…) ice-cream. The remaining low volume of air will chill much faster after the door has been closed, and the energy required to do this will be less as well. This is good for post-TEOTWAWKI as well as everyday living.
We prefer our “coffin” for bulk storage. It’s easier to keep our prey “on ice”. – Randy in Central California

JWR Replies: I agree wholeheartedly that it is important to keep a chest freezer full. Not only will it mean less cold air spilling out, but their thermal mass will also provide more of a time lag before defrosting, in the event of a power failure. Here at the ranch, we fill up any extra chest freezer space with used one-gallon plastic milk jugs that have been 3/4ths-filled with water.