When the compressor in the refrigerator stops turning on or, conversely, it works without interruption, turning the chamber into a freezer, the owner’s first thought is often that the thermostat is broken. This small but critical unit is responsible for maintaining a given temperature by opening and closing the electrical circuit of the engine. Understanding what is in the thermostat tube and how exactly the temperature signal is transmitted helps not only in diagnosis, but also in the correct selection of replacement parts.
The internal structure of the temperature controller is built on the physical laws of expansion and compression of substances. Capillary tubewhich often It fails if handled carelessly and is a sensitive element of the system. Inside it is not air or water, but a special heat-sensitive substance that reacts to the slightest changes in heat in the chamber. It is the pressure of this substance that causes the mechanical contacts to operate at the right moment.
In this article we will analyze in detail the design of the unit, the types of refrigerants used and the features of fault diagnosis. You will learn why you cannot simply cut off the excess length of the tube and how to correctly install a new element. Tightness of the system This is the main condition for the performance of the entire refrigeration unit, and violation of the integrity of the capillary instantly disables the device.
The principle of operation of the thermostat in the refrigerator
The basis of the operation of any mechanical thermostat is a closed system consisting of a bellows (corrugated cartridge), a contact group and a long thin tube. Inside the tube and The bellows contains an inert gas or liquid with a high expansion coefficient, often freon or chloromethyl. When the temperature in the chamber rises, the substance in the tube heats up, expands and creates excess pressure in the bellows.
This pressure is transmitted to a mechanism that physically opens the contacts, starting the compressor. As soon as the temperature drops to a predetermined limit, the pressure drops and the spring returns the contacts to their original position. It is important to understand that mechanical forcecreated by gas expansion is quite large, which allows you to control powerful electrical circuits without the use of electronics.
⚠️ Attention: The capillary tube of the thermostat is made of copper or cupronickel and has very thin walls. Any microcrack or puncture will lead to leakage of the working fluid and complete failure of the sensor.
The accuracy of operation depends on the quality of filling the system and the absence of impurities. If a “plug” has formed in the system or the tightness has been broken, pressure ceases to be transmitted correctly. In such cases, the refrigerator either does not turn on at all or operates in emergency mode. Temperature range The contact actuation is set by the manufacturer and is regulated by a calibration screw, access to which is often closed with a seal.
Composition and properties of the substance in the capillary tube
Many people mistakenly believe that there is simply air inside the tube, but this is incorrect. Effective operation requires a substance that changes its state of aggregation or volume in a narrow temperature range characteristic of a refrigeration chamber. Most often used freon (various brands, for example, R12 or R134a) or chloromethyl. These substances are highly sensitive to heat.
The choice of a specific filler depends on the model of the refrigerator and the required temperature range. For example, freezer applications require mixtures that remain flexible and expandable at very low temperatures, down to -60°C. In household single-chamber models, less extreme compositions can be used. Gas inertness Also important: it should not react with the metal of the tube or bellows for years.
The volume of the substance is strictly dosed. Even a small change in the amount of working fluid (for example, when trying to lengthen the tube) will upset the pressure balance. The system is designed so that at normal evaporator temperature, the gas pressure is balanced by the spring force. Violation of this balance leads to “sticking” of the contacts.
There is a division of thermostats into types according to the filler:
- 🧊 Gas-filled - they work by changing the gas pressure when the temperature changes, they are less sensitive to changes in atmospheric pressure.
- 💧 Liquid - use the expansion of the liquid, provide more accurate adjustment, but are afraid of overheating.
- ⚡ Adsorption - they use vapors adsorbed in a solid filler, are less common and are specific to older models.
What happens if you release gas from the tube?
If the integrity of the tube is broken and the gas has evaporated, the bellows will stop responding to temperature. The contacts will either remain open forever (the refrigerator will not turn on) or closed (the refrigerator will freeze constantly). It is impossible to restore such a thermostat in domestic conditions.
Design features and types of thermostats
Externally, the thermostat is a plastic or metal case with a protruding rod for the regulator handle and a long “tail” - a capillary tube. Inside the case there is a contact group that is opened by a cam mechanism. Contact made of silver or special alloys that are resistant to sparking, since opening the circuit under load causes a spark.
The tube can be of different lengths, usually from 0.5 to 2.5 meters, depending on the model of the refrigerator. It is wound in a spiral for compactness. At the end of the tube there is often a thickening or “cone”, which is the main heat sink. It is this part that must be in close contact with the evaporator or placed in a special sleeve inside the chamber.
There are also electronic thermostats, where the role of the sensitive element is played by thermistor. They do not contain gas tubes, and the sensor resistance changes depending on the temperature. However, classic mechanical models with a tube remain the most common due to their reliability and low cost.
Comparison of the main characteristics of mechanical thermostats:
| Parameter | Value / Description | Impact on operation |
|---|---|---|
| Tube length | 0.5 - 3.0 meters | Determines the possibility of installation in a specific model |
| Working pressure | Depends on gas | Influences the contact opening force |
| Temperature range | -35°C ... +10°C | Determines which chamber it is intended for (freezer or main) |
| Contact type | Normal closed | When heated, they open, starting the compressor |
Diagnostics of thermostat malfunctions
You can determine that the problem lies in the thermostat by characteristic signs. The most obvious symptom is that the compressor does not start, although the light inside the chamber is on and there is voltage in the socket. If, when you turn the control knob to the “maximum” or “minimum” position, a characteristic click does not occur, most likely contact group it does not work due to loss of pressure in the tube.
The opposite situation: the refrigerator hums continuously, the food in the main chamber freezes, and the compressor does not turn off. This indicates that the contacts are “stuck” in a closed state or the sensitive element has ceased to perceive a decrease in temperature. It is also worth checking the condition of the tube itself: if it has kinks, creases or traces of corrosion, the tightness of the system is broken.
For an accurate check, you can use a multimeter in continuity mode. At room temperature the contacts should ring (if the thermostat is normally closed). If you place the sensitive bulb of the tube in a container with ice or, conversely, heat it with a hairdryer, you should hear a click of the circuit opening/closing. Lack of response confirms the malfunction.
⚠️ Attention: Before any diagnostics, be sure to disconnect the refrigerator from the power supply! Working with live electrical components is life-threatening.
Instructions for replacing a thermostat with your own hands
Replacing a thermostat is a procedure accessible to a home handyman with basic skills. The first step is always to find the exact model of the part. The number is usually stamped on the body or indicated in the passport of the refrigerator. There are few universal solutions, since the length of the capillary tube and the seat may differ.
The replacement process requires care. The new thermostat tube is often longer than the old one. Under no circumstances should you cut it! The excess part must be carefully laid in a zigzag in the free space of the case, avoiding sharp metal edges that can damage the thin walls.
Attaching the sensitive cone is a critical moment. In modern models, it is inserted into a special plastic sleeve on the back wall of the camera. In older models (“Atlant”, “Biryusa”) the tube is wound with a clamp directly to the evaporator. The contact should be tight, sometimes thermal paste is used for better heat transfer.
☑️ Checklist before replacing the thermostat
After installing the tube and connecting the wires (it is important to remember or photograph the connection diagram), you can carry out a test run. If the compressor started and turned off after a while, the replacement was successful.
Frequent errors during installation and operation
One of the most common mistakes is an attempt to lengthen the tube by welding or soldering. Tightness it is impossible to ensure such a system at home, and solder getting inside the channel is guaranteed will damage the part. If the tube is short, look for another thermostat model.
Another mistake is incorrect positioning of the tube. If the bulb does not fit tightly to the evaporator or hangs in the air, the thermostat will “lie”, reading the temperature of the air in the chamber, and not the temperature of the cooling element. This will lead to incorrect compressor operating cycles.
Also, users often ignore the condition of the door seal. If warm air enters the chamber, the thermostat will constantly try to cool the volume, causing the compressor to work hard. In this case, replacing the sensor will not solve the problem.
FAQ: Frequently asked questions
Is it possible to replace a mechanical thermostat with an electronic one?
Theoretically it is possible, but this requires a deep rework of the refrigerator control system. Electronics require different voltages, operating algorithms and sensors. It’s easier and more reliable to find an original mechanical analogue or a high-quality universal replacement.
Why doesn’t the new thermostat click when you turn the knob?
This may mean that the temperature range of the new sensor does not match the current temperature in the chamber. For example, if the room is +25°C, and the thermostat response range starts at -5°C, it will not click until you cool its sensitive element.
How long does a thermostat in a refrigerator last?
The average service life of a mechanical thermostat is 5-10 years. However, frequent voltage surges, compressor overloads and mechanical damage to the tube can shorten this period. When used correctly, they last a very long time.
Is the substance inside the tube dangerous?
Most household thermostats use inert gases or freons in microscopic quantities. If one tube is depressurized, the concentration of the substance in the room will be negligible and not dangerous to human health, but inhaling the vapors directly is not recommended.