Yes it's a clock, sort of. The low model units have a mechanical timer that runs the defrost cycle. The timer can (should) be adjusted so that the defrost cycle happens in the middle of the night when you are less likely to be raiding the ice box. As you move up in models the defrost cycle gets more complicated with units that attempt to know when the best time to defrost is and units that guess when and how long a defrost cycle should take place based upon compressor cycling and coil temperatures.
Chipmonger - Maybe it's because I haven't had the need to buy a freezer in a while, but your response to tekochip through me off. A "simple" refrigerator now has a clock too? My "even simpler fridge has the feature you mentioned but without a clock set to it. Periodically you can hear the frost dripping off and being vaporized. At least that is what I've been told that I'm hearing.
@tekochip - yes you are correct the heater is for the "automatic defrost" feature. The simple fridges use a clock motor that triggers a switch that turns on the heater every 24 hours. We use to turn the motor's shaft so that defrosting would take place at night when the familys in bed.
@DW - The optional heater is to keep the refrigerant from turning into a liquid. This prevents damage to the compressor since it is designed to compress gas and not liquid refrigerant. I have split air conditioners and they have these heaters in the ouside compressors that draw many watts of power 24 x 7.
@Charles Murray - Leaving the fridge door partly ajar does not defrost the fridge but actually makes it worst. The fridge will continue to run and actually run longer since it senses a higher than normal temperature. With the door open, outside moisture is drawn in which then forms ice on the cooling coils cutting off circulation. In the end, the cooling coil becomes completely encased in ice ncessitating a manual defrost.
Freezers that are non-self-defrosting still exist, and I actually have one in my garage, Tekochip. You're right about the process of defrosting the fridge manually -- it indeed happens when the door doesn't close. And, yes, manual defrosting is a mess. To fully appreciate the value of self-defrosting fridges, you have to do it manually once.
Tekochip: that's pretty much the same thing my repairman told me. I get it, but it seems to be wildly inefficient.
DW: I also have an old unit in my unheated garage. I'm in NJ, so it gets pretty cold. I hope I'm not putting the whammy on myself, but I've never had an issue with that refrigerator and it has no extra heater.
Here's more than you wanted to know. When I bought my currewnt frig 12 years ago, I sold the old one to the next door neighbor for their garage for $100. They moved, and the new neighbors didn't want it, so we took it back. Please don't tell anybody about the 100 bucks.
I live in Central Minnesota and it does get a little colder up here, -40C/F is not unheard of. We have a fridge in our barn to store eggs from our hens until they are sold. Like an AC unit a fridge only moves heat from one area to another, when it is below the freezing point things will freeze in a fridge. So every November when the temps finally fall below 32F I simply slip a 9W CFL in the fridge, it generates enough heat in the fridge to keep everything from freezing. And if it starts to warm up beyond the setpoint of the fridge thermostat then the fridge does it's job and moves the heat inside to the outside. I even have to keep my liquid hand soap and other hand cleaners in the fridge during the winter, it's hard to wash your hands with a block of frozen soap! Come April I can finally put out the CFL. Just part of the fun living living in the great white north.
I purchased a backup/extra storage refrigerator to put in my garage last fall. The garage is unheated space. One day in winter, I noticed the garage refrigerator was not cooling - the compressor was not running. After looking at the owner's manual and talking to the appliance service department, I learned that the refrigerator requires an optional heater if it is installed in an unheated space where the temperature may fall below 40 deg. F. This is not the defroster heater which is required to prevent ice build-up regardless of where the refrigerator is installed. Perhaps it is a booster to the defrosting heater? I had never heard of this before last winter. Does anyone know why this optional heater is required for a refrigerator installed in an area where the temperature may fall below 40 deg F? Has this always been a requirement or is this a result of replacing freon with other coolants?
The purpose of the defrosting heater is to melt the ice off the coils that has accumulated due to moisture from opening the door and allowing the warm, moist air to enter. Any ice on the coils makes them less efficient, so you like to keep them clean. Back when I was a young lad (there was a time when I was) you'd have to defrost the freezer yourself by turning it off and maybe placing some hot pots inside. Water would run everywhere and you had a big mess. It was a bit of a pain and you typically wouldn't do it until enough ice built up that the door wouldn't close or you couldn't put anything in the freezer. Freezers were smaller back then, though, because frozen foods were not as popular. Anyway, now refrigerators have heaters that melt the ice off, and then there's a tube that allows the water to collect in a pan near the floor. Warm air from the compressor blows across the pan and allows the accumulated water to evaporate. Simple refrigerators have a mechanical timer to control the defrost cycle, but more expensive appliances have clever units that monitor when the door is open and try to defrost while Mr. Sandman is tugging at your eyelids. Even more sophisticated units try to guess how much ice is on the coils, since running the defroster is a huge waste of energy.
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