How Frost Free Fridges Work

Seems like a great idea, freezers that never need defrosting due to the fact that the frost doesn’t build up in the first place and it is a great idea too but, there is no doubt about it, frost free causes a lot of problems that are not easily sorted.

  Basic Principle

In a normal freezer, the evaporator i.e. the parts on which the frost forms is exposed, these are the pipes you can see that often actually form the shelves. The frost builds up on these and, if left, will completely take over the space where you should be able to place your food. When defrosted, usually just by switching the appliance off, the ice melts and drips all over the base of the freezer hence the need for towels etc. In a frost free appliance, the cooling evaporator is concealed, often behind the cover at the back of the inside of the freezer or in a compartment at the top. The evaporator is formed of pipes, similar to a normal freezer, but with fins attached. An electric fan draws air from the cavity (i.e. the food stored inside), through the finned evaporator and then back into the cavity again for the process to continue. Frost will therefore still build up on the evaporator but, due to its compact nature, it can be carefully monitored and defrosted by a small heater with the water running neatly down a drainage hole onto a tray mounted above the compressor from where it will evaporate. If the appliance is a fridge freezer then the fridge compartment may have no working parts in it at all, refrigeration will take place by simply opening a mechanical flap from the freezer which will close once the temperature is correct.

Do not confuse frost free with auto defrost fridges, they are completely different.

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Although the video above is a promotional item for Fisher & Paykel’s “Active Smart” refrigeration range it does offer a lot of insight into the general workings of a frost free fridge freezer.

  Parts Of A Frost Free Fridge Freezer

As well as the normal refrigeration system, i.e. a compressor, evaporator and condenser, frost free appliances will have a number of special features with which to control the defrosting.

As mentioned above, most will have a fan which may be visible from inside the freezer and will be heard running. If your fan is not running, don’t automatically assume it is faulty as they may switch off when the door is opened and when the correct temperature is reached. In general, evaporator fans don’t cause a lot of problems and rarely fail although often they become noisier than they should be and are changed for that reason.

  Freezer Defrost Heaters

The defrost heater will be wrapped around the evaporator and melts the ice build up when energised. Another heater will be placed around the drain hole where the defrosted ice water will run through, without this heater the drain will freeze over and become blocked. These may be protected by a thermal fuse which cuts the circuit should the heater become to hot.

  Fridge Freezer Temperature Sensors

These usually are thermistors, resistors that change their resistance value with temperature changes. These are usually located on the evaporator and another one somewhere inside the freezer cavity. In the case of fridge freezers, there will also be another one within the fridge. These all feed information back to the controller which will react accordingly. I.e., switching on or off the compressor and fan as necessary to maintain the correct temperature.

  Fridge Freezer Control Module

This is the main controller, nowadays an electronic PCB, often with a user display showing actual and set temperature. This controller will collect and compare information from the thermistors and activate the necessary action. It will also control the defrost programme. Some more basic, earlier frost free appliances use a mechanical thermostat and timer rather than an electronic version.

  Automatic Fridge And Freezer Defrosting: How It Works

Most frost free freezers are usually defrosted after a set time period. In the case of Hotpoint Mistral types, this is every 10 hours. The defrost heaters are turned on and will remain on until the thermistors record a temperature of, in Hotpoint’s case 20°C at which point the evaporator heater is turned off while the drain heater remains on for a further 5 minutes to allow for all the water to drain off. The compressor will then start and once the evaporator thermistor is reading -5°C the fan will re start.

  What Goes Wrong?

With all these electronic devices there is a lot that can fail. Diagnosing is not easy as any engineer will testify.

The most common problem is insufficient cooling, often in the fridge first before the freezer itself warming up. This is usually due to the build up of ice on the evaporator not being defrosted and building up to such an extent that the air passage through the fins, so vital to the fan action, is completely blocked. Manually defrosting will restore normal use but, unless the cause is rectified, the fault will return, in time. In general, the biggest problem we find is with the thermistors themselves giving false or no information to the controller. Not possible to test but usually relatively cheap to replace as a first try, although some may be foamed in to the actual insulation of the appliance and, in some cases, are non replaceable. Defrost heaters can also fail, but not so common these days and can be checked for continuity and earth faults with a test meter. As stated above, these are often protected by thermal fuses which are more likely to fail and again can be easily tested. If failed though, they may indicate another fault that may have caused overheating.

The control module itself is often the cause of problems, either missing defrosts or not re starting after defrosts are common faults. Not possible to test without replacement which may be costly.

  Fridges And Freezers Need Adequate Ventilation

Nearly all frost free appliances require a healthy air flow to operate correctly. Ensure that the appliance is not installed in a sealed unit unless it is designed to be and make sure any vents are not blocked.

To sum up, frost free is not simple to either work on or diagnose faults and once defrosted, the fault may not show up again for a long period. We would advise expert attention where possible. If you do look yourself, make safety your number one concern. All circuits are 230v mains and, coupled to the likelihood of water being around can and will kill you! So make sure its all unplugged before removing any panel. Also watch out for the fins on the evaporators as they are RAZOR sharp. Do not attempt to defrost with any tool as puncturing the system will wreck your freezer. Do not be alarmed if your appliance does not start up immediately when turned on. This may be timed to protect the compressor and may take up to 30 minutes before starting.