a. The vast majority of people accept the ruling by most rabbis that a refrigerator can be used without paying attention to the thermostat and without fear that opening and closing the door will cause the compressor to be turned on or to stay on for a longer time. Note that this approach has a firm halachic basis.
b. Based on this approach, no-frost refrigerators, both new and old models, do not present any problem. These refrigerators have a tiny heating element for thawing ice which is controlled by the thermostat (similar to the defrost wires in a car), but this element does not glow and is therefore not bound by the halachic restriction of "burning" (except for some very specific models).
c. On the other hand, most modern refrigerators have fans in the freezer and in the main refrigerator compartment to guarantee an even distribution of cold air. When the door is opened the fan goes off (giving a marginal energy saving by decreasing the loss of cold air to the outside). This is typically controlled by a mechanical switch linked to the opening of the door, and this switch is usually quite easy to find.
d. Technicians can determine if a specific model has a fan or not (as noted, most modern refrigerators do). If so, it can be jammed open with a small object that applies constant pressure to it, or it can be covered with masking tape before Shabbat. Many manufacturers supply a standard cover for this switch.
e. "Digital" refrigerators are not recommended for use! They have a minicomputer. Opening the door starts a timer that sounds a buzzer if the door is not closed within two minutes. They also often have an electronic display which is directly influenced by the opening of the door. Disconnecting the display or any other signal light or buzzer does not solve the problem of the electronic mechanism, which continues to send the information to the computer whether the buzzer sounds or not.
In most cases, the element that senses the door position is the same switch which controls the fans and also turns the light on or off. If this is true, it is possible to jam the switch during Shabbat (with a small object or with masking tape) and thus to "fool" the refrigerator into ignoring whether the door is actually open or not. This also solves the problem of digital control. However, in some models the computer receives its own independent information about the status of the door from another sensor, often a magnetic detector (which closes a switch when the door is opened) that is not visible from the outside (it is usually on the inside of the doorpost). In this case, jamming the switch on the door will not solve the problem!
Some companies provide an external magnet that can be placed on the outside next to the doorpost, opposite the internal magnetic sensor. This "fools" the sensor into acting as if the door is still closed. But not all companies provide this service, and technicians often refuse to disclose full information about this subject. We therefore recommend not buying a digital refrigerator unless it can be determined in advance that it is suitable for use on Shabbat.
f. "Mehadrin" requirements – Some people are very strict and will not open the door of a refrigerator while it is not working, fearing that to do so will cause the thermostat to turn the compressor on immediately. In order to provide a solution for those who want to take this into account, companies have developed a "mehadrin Shabbat mechanism." In some cases, this is a setting that disconnects the thermostat, so that the compressor remains on all the time (except during defrosting). Such a mechanism is liable to cause icing in the freezer and interfere with the proper operation of the refrigerator. Other companies have developed a special Shabbat control that replaces the thermostat with a fixed on-off cycle. As noted above, The Zomet Institute feels that it is not necessary to purchase such a special mechanism (which will typically have a relatively high maintenance cost).
(1) When buying a refrigerator, it is important to make sure that the service company is willing to provide special mechanisms for the switches on the doors (both the refrigerator and the freezer).
(2) Digital refrigerators should not be bought unless one can be sure that all of the door-related sensors can be disconnected on Shabbat.
Note: The Zomet Institute does not maintain data about specific refrigerators, since new models are continuously coming on the market. We feel that it is sufficient to provide information about the general principles involved, as in this article.