The capillary tube in the refrigerator thermoregulation system is the thinnest element on which the stability of maintaining the temperature in the chambers directly depends. It is through this channel that the refrigerant or inert gas transfers the thermal effect to the sensitive element of the thermostat, causing it to operate at the right moment. Understanding how it works thermostat capillaryis necessary for every technician who repairs household appliances, since damage to this tube is one of the most common causes of failure of the entire climate control system.
Visually, the capillary is a long copper tube twisted into a spiral small diameter, enclosed in a protective shell or hanging freely inside the refrigeration chamber. Its task is to hermetically transfer the change in the volume of the working fluid (gas or freon) to the membrane of the thermostat bellows. Capillary break or a violation of its tightness leads to an instant loss of functionality of the sensor, which requires immediate intervention. In this article, we will analyze in detail the design, principle of operation and diagnostic methods of this critical component.
The principle of operation of the capillary thermoregulation system
The basis of the functioning of the thermostat lies in the physical property of substances to change their volume under the influence of temperature. The capillary tube here plays the role of a conductor connecting the sensitive flask located in the cooled volume with the actuator - a bellows located in the thermostat housing. Inside the system there is a working substance, which can be freon (depending on the brand of refrigerant, for example, R134a) or an inert gas under a certain pressure.
When the temperature in the chamber rises, the working fluid in the flask expands, and this expansion is transmitted through capillary tube to the bellows. The bellows, in turn, extends and mechanically opens the electrical contacts, starting the compressor. This process is cyclical and ensures automatic maintenance of the set temperature. It is important to understand that the length of the capillary is strictly regulated by the manufacturer, since the accuracy of pressure transmission depends on it.
There are different types of thermostats, where different filling principles are used. In some cases, the capillary is filled with liquid refrigerant, in others - only steam. Steam-filled systems are less sensitive to changes in ambient temperatures outside the refrigerator, but have their own characteristics in the settings. The mechanical integrity of the entire “bulb-capillary-bellows” chain is a critical factor in safety and operational efficiency.
⚠️ Attention! Violation of the tightness of the capillary system leads to leakage of the working substance. Some older models of thermostats may contain freon inside, so if the tube is damaged, good ventilation of the room and compliance with safety precautions when working with refrigerants is necessary.
Design features and types of capillaries
The capillary tube is made of copper or copper alloys, which provides the necessary flexibility and thermal conductivity. The diameter of the tube is usually a fraction of a millimeter, and the length can vary from 0.6 to 2.5 meters depending on the model of the refrigerator and the type of thermostat. The surface of the tube is often protected by plastic braiding or metal armor, especially in areas where contact with sharp edges or moving parts is likely.
One of the key characteristics is capillary length. It is selected in such a way as to ensure the correct pressure in the bellows at given temperature thresholds. Shortening the tube even by a few centimeters can lead to a change in the calibration of the device, which is why the refrigerator will start to work incorrectly - either turning on too often or not turning off at all. Manufacturers strictly control this parameter during assembly.
At the end of the capillary there is a sensitive flask, which is attached to the evaporator or suspended in the air flow of the chamber. The location where the flask is attached is also important: it should provide maximum thermal contact with the object being cooled. Modern models often use thermostats with short capillaries, as electronics can compensate for some physical limitations, but mechanical relays with long tubes are still widespread in the budget and mid-range segment.
- 🔹 Material: Oxygen-free copper for high ductility and corrosion resistance.
- 🔹 Protection: Polymer insulation or steel braiding to prevent mechanical damage.
- 🔹 Filling: Gas, liquid or adsorption, affecting response inertia.
When replacing thermostat, it is important to pay attention not only to the electrical parameters of the contacts, but also to the length of the capillary tube. Installing an analogue with a shorter capillary will require careful installation so as not to damage it, and a longer one will have to be rolled into a coil, which must also be done carefully, without bending the tube at an acute angle.
Typical malfunctions: breakage and deformation
The most common problem faced by refrigerator owners and craftsmen is capillary break. This can occur due to mechanical impact during inaccurate defrosting, when the user tries to break off the ice with a knife or screwdriver. The thin copper wall cannot withstand the impact, and the tightness of the system is broken instantly. As a result, the working substance evaporates, and the thermostat stops responding to temperature changes.
Another common cause of failure is tube deformation. If the capillary is laid at too sharp an angle or is twisted, the lumen of the tube may decrease or close completely. This disrupts the transfer of pressure from the bulb to the bellows. In this case, the thermostat “sticks” in one position: either the compressor works continuously, freezing the food, or does not turn on at all, leading to the defrosting of the contents of the chambers.
Corrosion can also become the enemy of the capillary system, especially in conditions of high humidity or in the presence of aggressive chemicals in the air (for example, in a restaurant kitchen). Copper becomes thinner over time, and a fistula forms at the point of corrosion. Micro cracks may not be visible to the naked eye, but lead to a gradual loss of working gas and a drift in temperature settings.
⚠️ Attention! If you find that the capillary has rubbed against the sharp edge of the housing or other element, do not ignore it under any circumstances. Vibration of the compressor will sooner or later lead to a complete rupture of the tube. Insulate the contact point with a rubber gasket or heat shrink.
Diagnostics of the condition of the capillary system
You can determine the malfunction of the capillary by indirect signs of the operation of the refrigerator. If the unit hums continuously and does not turn off, or, conversely, remains silent for days while the food spoils, first of all you should check the thermostat. However, for an accurate diagnosis it is necessary to ring the contacts and inspect the handset itself. A visual inspection often reveals obvious creases, traces of impacts or corrosion.
For a deeper inspection, craftsmen use the heating method. The thermostat flask (end of the capillary) is clamped in your hand or lowered into warm water, listening to the clicks of the contacts. If there is no click when heating, and there is no click when cooling (putting in the freezer), most likely the system is depressurized. Checking the tightness capillary is a key step before installing a new unit.
It is also worth paying attention to the electrical resistance. Although the capillary itself is a dielectric (if it is not damaged and does not touch live parts), failure of the insulation can lead to breakdowns. The table below shows the main symptoms and their probable causes associated with the capillary system.
| Symptom | Probable cause (Capillary) | Action |
|---|---|---|
| The refrigerator does not turn off | Capillary break, gas leak | Replacing the thermostat |
| Refrigerator does not turn on | Clot of tube, blocking pressure transmission | Checking integrity, replacement |
| Temperature jumps | Partial deformation, corrosion | Diagnostics, replacement |
| Sparking inside the chamber | Damage to capillary insulation | Urgent replacement (dangerous!) |
| The compressor operates jerkily | Impaired circulation of the working fluid | Replacing the thermostat |
Using a multimeter in the continuity mode allows you to verify the integrity of the electrical circuit of the thermostat contacts, but will not directly indicate the condition of the capillary itself. Therefore, visual and thermal tests remain the main verification methods.
Replacing a thermostat with a capillary with your own hands
The process of replacing a thermostat requires accuracy and a minimum set of tools. Before starting work, be sure to disconnect the refrigerator from the power supply. Remove the protective panel behind which the thermostat is hidden and disconnect the wires, having first memorized or photographed the connection diagram. The old thermostat is removed along with the capillary tube, which must be carefully pulled out of its seat in the chamber.
Installation of the new element begins with the placement of the sensitive flask. It must be pressed tightly against the evaporator or installed in a standard hole, if the design of the refrigerator requires this. Attaching the flask often done using a special clamp or clip. It is unacceptable to leave the flask hanging in the air if the manufacturer provides contact with the heat exchanger, as this will lead to incorrect reading of the temperature.
☑️ Algorithm for replacing the thermostat
The most crucial moment is laying the capillary tube. It should not be pulled, twisted into knots or bent at an angle of less than 45 degrees (bending radius of at least 5 tube diameters). The excess length is carefully rolled into a ring and placed in the free space behind the control panel or in a specially designated channel, making sure that the tube does not touch moving parts and hot elements (for example, a condenser, if it is located at the back).
⚠️ Attention! It is strictly forbidden to shorten the capillary tube of a new thermostat. This will change the system volume and pressure, causing the device to not work correctly. If the tube is too long, it can be carefully rolled into a coil with a diameter of at least 3-4 cm.
After mechanical installation, the electrical contacts are connected. Make sure the wires are snug and not loose. Then reassemble the body in reverse order. Before fully assembling the control panel, it is recommended to conduct a test run to make sure that the compressor starts and stops at the command of the thermostat.
Frequent errors during installation and operation
One of the biggest mistakes when replacing is ignoring the type of thermostat. There are models for single-chamber and double-chamber refrigerators, with and without a defrost signal. Installing the wrong one analog may result in the refrigerator simply not starting or operating in emergency mode. Always check the markings of the old and new devices.
Often, when installing a new capillary, craftsmen or owners forget to check the condition of the seat in the chamber. If there are sharp burrs from the old fastener or pieces of insulation, the new capillary may be damaged immediately after the compressor starts vibrating. It is necessary to clean the installation site and, if necessary, lay an additional layer of insulation.
Another mistake is incorrect positioning of the bulb. In some models of refrigerators (for example Indesit or Atlant), the flask must be immersed in a special sleeve on the evaporator. If you simply tie it to the tube with a clamp without heat-conducting paste or tight contact, the thermostat will “lie”, reacting to the temperature of the air, not the evaporator, which will cause a large temperature delta and excessive energy consumption.
Is it possible to restore the tightness of the capillary?
Theoretically, it is possible to solder the copper tube. However, it is almost impossible to restore the working pressure and amount of gas inside the system in domestic conditions without special equipment and a vacuum station. Therefore, repairing the capillary is impractical - a complete replacement of the thermostat is required.
FAQ: Frequently asked questions
Is it possible to use a thermostat with a longer capillary if you carefully roll up the excess?
Yes, this is acceptable. The main condition is not to damage the tube when twisting. The bending radius should be smooth (at least 30-40 mm) so as not to squeeze the lumen. The excess length is carefully placed in the free space behind the refrigerator body, avoiding contact with hot parts.
Why did the refrigerator start working longer after replacing the thermostat?
This may be due to several factors: insufficient contact of the flask with the evaporator, the use of an analogue with other temperature characteristics, or the presence of an air lock in the system (if a replacement was made unprofessional). It is also worth checking the tightness of the circuit.
What is the difference between the thermostat capillary and the capillary tube of the refrigeration circuit?
These are completely different elements. The thermostat capillary is the part of the control system that transmits gas/liquid pressure to open the contacts. The refrigeration circuit capillary (throttle) is a part of the cooling system that creates a pressure difference for freon circulation. They are not interchangeable and perform different functions.
How to determine that the problem is in the capillary, and not in the relay itself?
If, when heating the flask in your hands (or with warm water), the contacts do not click, and the continuity shows a break or, conversely, a constant short circuit, regardless of the temperature, the tightness of the capillary system is most likely broken or the bellows is jammed. The visual integrity of the tube also plays a role.