Among the numerous equipment that is involved in the operation of underfloor heating systems, you can find a small device that plays a crucial role in controlling and adjusting the heating system. This is a servo drive, an electromechanical device, without which automatic temperature control for a warm water floor is not possible.

The device is based on an electrothermal reaction to a change in the heating temperature of the coolant in the main supply pipe and the subsequent mechanical action, which, in combination, ensures the opening or closing of the flow of hot water into the heating circuits. Servo drives or servomotors, officially in the language of professionals, the device is called an electrothermal servo drive, today they are present in almost all autonomous heating systems. New suburban residential buildings, cottages and dachas equipped with underfloor heating are equipped with underfloor heating, which is controlled by servo drives. It is the servo drive that is installed on the collector for the underfloor heating that performs the task of adjusting the flow of the coolant in the water floor heating system.

Existing types of servo drives today

Among the currently existing regulators that have become widespread in everyday life, there are the following servo drives. All devices can be divided into several types. Each variety differs in the principle of operation and functionality. According to the type of device design, there are two types:

  • closed;
  • open.

By the names you can judge the principle of action. Enclosed servos are characterized by an open position in the absence of power. The incoming signal activates the mechanical part, blocking the access of water to the system. For open type devices, the principle of operation is reversed. In the normal state, the servomotor is closed, only when a signal is received, the mechanical part is activated, opening the flow of water into the pipeline. You can judge which type is best for domestic use by evaluating the capabilities of your own heating system and the climatic conditions outside the window. Normally open servo drives are most often used in our country.

On a note: if the device fails, the coolant in the pipeline continues to circulate, leaving the floor warm for a certain time. This feature is especially relevant for country houses located in a cold climate zone.


According to the method of power supply, servomotors are divided into devices powered by a constant current of 24V and devices connected to a conventional 220V AC power supply. Servo drives powered by 24V are equipped with inverters.

Often, consumers use another, rather rare type of device. We are talking about devices that are set to the normal position depending on the technological requirements of the heating system. Such servos are called universal and can change functionality from a normally open state to a normally closed state, and vice versa.

All three types of servomotors can be connected to the collector. The only condition is the correct setting, balancing and operating conditions of the heating system.

Criteria for choosing the type of servo drive

In this section, we will try to answer the question. What is the basis for the choice of devices of one kind or another.

If you decide to equip your «warm water floor» heating system with servo drives, consider the operating parameters of your heating. In what position should the valve be most of the time. In a situation where underfloor heating is the main option for heating residential premises for you, when hot coolant constantly enters the pipeline, rely on a normally open servomotor. This view is ideal for a long heating season.

On a note: in case of interruptions in the electrical supply, the failure of the device will not stop the circulation of warm water in the heating water circuits. The warm floor will continue to be supplied with prepared water as a coolant.


For regions with a warm climate, a normal closed servomotor is suitable. If you are not afraid of defrosting the heating circuit, and you periodically turn on floor heating, this device will cope with its functions quite well.

Important! The servo drive for underfloor heating with smooth adjustment has an electronic type regulator. Such devices more accurately respond to changes in the temperature of the coolant flow, smoothly moving the stem to the desired position. Continuously adjustable servomotors are designed for underfloor heating, in which it is often necessary to dose the volume of the incoming flow.

In most cases, such devices are not used in home heating systems with underfloor heating. Therefore, when buying, pay attention to whether or not the installation of an electronic regulator is required for the device. If the instructions say that such equipment is necessary, then you are dealing with an electronic servo. Let’s say right away that it is inexpedient and unprofitable to use such a device at home.

Be sure to read: how to make a water floor from a gas boiler?

The device and principle of operation of servomotors

The main working element of the servo drive is the bellows. Those. the same part as in the three-way valve. Small in size, hermetic cylinder with an elastic body is filled with a substance that is sensitive to temperature. Depending on whether there is an increase or decrease in temperature, there is a corresponding change in the volume of the substance. Figure — the diagram clearly demonstrates the device of the servomotor, where the main place is occupied by the bellows.

The bellows is in close contact with the electrical heating element. Receiving a signal from the thermostat, the heating element is switched on from the network and put into operation. Inside the bellows, the substance is heated and increases in volume. Thus, the enlarged cylinder begins to put pressure on the rod, changing its position and blocking the path to the coolant flow. Evaluating the operation of the servo drive, we can conclude that the device is not equipped with any motors, it does not have any gears and transmission links. The usual working relationship is «thermal energy and electricity». Hence the common name of the devices, thermoelectric controllers.

In order for the valve to become open again, the whole process is repeated only in the opposite direction. Lack of power causes the heating element to stop working. Consequently, the substance inside the cylinder cools down, decreasing in volume. The pressure on the stem decreases, it rises, acting on the valve, and, consequently, access of hot water to the system opens.

On a note: the substance placed inside the cylinder is toluene, which has high thermodynamic characteristics. Nichrome filament acts as an electric heating element.

Having become acquainted with the principle of operation of the device, it is important to remember that a certain time is required for the mechanical action of the valve. Despite the fact that when a signal is received from the thermostat, the heating element begins to heat the substance inside the cylinder. The time required for changes in the physical state of the liquid is 2-3 minutes, so the valve is not actuated immediately.

For reference: when choosing a servo drive model, pay attention to the parameters of the heating element and the liquid heating time indicated in the device passport.


Unlike heating, the cooling of the liquid is slower. For the reverse process, i.e. it will take not 2-3 minutes to close the valve, but 10-15 minutes. When overheated, each servomotor should automatically shut down. To do this, the design provides an emergency shutdown mechanism.

For example: not all servo drives used in the work of the collector group are equipped with cylinders and cylinders with a substance. There are models in which thermoelements play this role, resembling a spring or a plate, which are heated under the action of the same heating element. Expanding, these parts again act on the stem, ultimately bringing the valve into working condition. You can determine in what position the valve is located by changing the appearance of the servo. The retractable element indicates the operation of the device. If this does not happen, then your appliance is not connected correctly or the heating system is intermittent.

For reference: a servo motor that is hot to the touch means that in this case the device is closed and switched off. If the device is cool to the touch, therefore, the valve is open, the coolant circulates normally through the water circuits of the warm floor.

Servo drive installation. Features and nuances

Before installing the servo drive, decide what type of thermostat the device will have to interact with. In cases where the thermostat controls the operation of one water circuit, both devices are directly connected by wires. When it comes to using a multi-zone thermostat, a device that serves several pipelines at once, the connection of servomotors is carried out as follows.


In order to properly connect all the wires and terminals, a floor heating switch is used. The function of this device is to connect and connect devices for various purposes into a single circuit. In addition to the distribution and linking function, the switch also plays the role of a fuse. In situations where all shut-off valves of the water circuits are closed, the switch turns off the power to the circulation pump.

The switch is very convenient in cases where underfloor heating is powered by an automated autonomous gas boiler. The figure shows how thermostats and servomotors are connected to a single control system.

Actuator installation location, manifold mounted thermostatic valve.

Important! When the heating system is operating, underfloor heating from a solid fuel boiler, such a switch function as turning off the pump is fraught with stopping the heating device itself. Installing a bypass and bypass valve will allow you to avoid stopping the pump and running the heater at idle.


It should be noted that thanks to the emergence of modern devices and devices, the management and adjustment of underfloor heating has become an ordinary and simple process. The design of many devices used for the operation of heating circuits is not particularly difficult. The principle of operation of many components and assemblies is also clear. This can be said with certainty about servos. The devices are mostly reliable, practical and easy to use. Thanks to servomotors, it became possible to fully automate the control system for underfloor heating, to make the conditions for using heating equipment simple and understandable.

Choosing a simpler option, you can get by with the installation of conventional control valves. Automatic regulators, thermal sensors and servo drives, a category of devices that work for your comfort and safety. Installing additional devices, such as a switch and a bypass valve, will make your heating system as efficient and safe as possible.


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