Thermostats or thermal heads for heating radiators

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In today’s realities, the very fact of heating is not enough; heating should be comfortable and be able to be individually adjusted. In the context of hydraulic heating systems, the best way to improve the ergonomics of control is to use thermostatic valves, to which our review is devoted..

Thermostats or thermal heads for heating radiators

Operating principles of hydraulic heating systems

Any heating source needs regulation devices. Air conditioners, underfloor heating elements and convectors have a built-in mechanical thermostat that turns off the power to the device when the required temperature mark is reached. And what technical means are used in the radiator networks of hydraulic heating systems?

On the one hand, almost any heating boiler has a built-in sensor that monitors the temperature of the coolant. However, it cannot be considered the main means of regulating the air temperature, since the rooms heated by liquid systems differ in volume and amount of heat loss. Thus, the main function of the boiler thermoregulation system is to prevent the heat carrier from overheating. In addition, we must not forget about solid fuel boilers, most of which are simply not capable of changing combustion modes depending on the temperature of the working fluid..

Gas boiler heating temperature control

To ensure a comfortable air temperature in inhabited premises, it is required to control the intensity of heat release on the regulators themselves. For this purpose, there is a wide range of valves and fittings classified as thermal heads for hydraulic heating systems. They differ in the method of control and internal structure, while the basic principle of operation is quite simple to understand.

The essence of the work of shut-off and control valves

To use temperature control devices correctly, you need to understand how a hydraulic radiator works. The heat source, which is ultimately transferred to the room atmosphere, is a heat carrier circulating in a closed loop and saturated with heat when passing through the generating part of the system. When it enters the radiator, the coolant gives off energy to the body, and it, in turn, emits it in the infrared spectrum and also transfers part of the heat to the air stream passing through the finning system.


Thus, it is possible to distinguish two ways by which the transfer of energy from the coolant to the air is limited. The first and most common is to reduce the flow of the coolant in the radiator channels. If a smaller volume of working fluid flows through the radiator, accordingly, the amount of heat energy supplied to the heating device will be less. In practice, this is realized by artificially lowering the nominal pipe diameter at the radiator connection..

The second method of regulation is to normalize the temperature of the incoming heat carrier, which seems more logical, but in practice it causes additional technical difficulties. The only way to reduce the supply temperature is to mix it with part of the return coolant. However, this cannot be done with the actual differential pressure of the standard hydraulic system. Therefore, this method of adjustment requires the installation of a unit with a flow-mixing valve and an additional circulation pump, which in reality is relevant not for a separate radiator, but for a whole group..

Balancing issues

If the radiator network is built on the principle of a two-pipe connection with a return flow of the coolant, it requires balancing. The essence of the latter is to limit the flow through the radiators located closest to the thermal unit, so that the heated working fluid flows to the most distant radiators without additional efforts.

For accurate balancing, it is required that the flow rate of the coolant in each radiator remains unchanged, which is impossible with the first of the described thermostating methods. If thermal heads that regulate the flow are used, then some errors in setting up the hydraulic system will simply have to be put up with. It should be noted that with a limited number of radiators – about 10-12 in one wing, the effect of changing the flow does not significantly affect the operation of the system as a whole.

House heating system

However, for long circuits with a significant number of radiators, this approach cannot be applied. Even the slightest increase in the flow rate in the heating devices of the nearest group causes serious failures, therefore in such systems there are two alternative ways out of the situation:

  1. Division of the radiator network into several wings with the installation of individual circulation pumps.
  2. Heat transfer rate regulation by temperature control using flow-mixing units.

It is impossible to say unequivocally which of the options is the best. For example, piping configuration and radiator locations may simply not allow the system to be split into multiple wings. At the same time, the installation of flow-mixing units is more costly, therefore the design of systems is always carried out on an individual basis, taking into account the above requirements.

Types of thermostatic heads and their principle of operation

Shut-off and control valves are presented on the plumbing market with an impressive assortment, while the fundamental differences are not always obvious to the buyer, because in general, the appearance and general description of the devices are not much different. Nevertheless, for such products, a very specific classification is applicable according to the mechanism of action and type of temperature control..

Mechanical thermostat in sectionMechanical thermostat in section

The assembly of the control valves is represented directly by the adjusting head and the valve on which it acts. The thermostatic head can use the thermal expansion of the working fluid, such devices are called semi-automatic. Liquid, gas or solid can be used as a working medium. Liquid and paraffin thermal heads have the fastest response, but gas ones are characterized by a longer service life at the expense of a high reaction rate.

Radiator gas condensate thermostatRadiator gas condensate thermostat: 1 – flow damper; 2 – detachable connection; 3 – valve stem; 4 – bellows; 5 – thermal head body; 6 – stuffing box; 7 – crane box; 8 – valve cone; 9 – valve body

The electronic unit can also control the degree of pressure on the valve, in which case we are talking about digital thermal heads. Direct pressure on the valve is provided by a servo drive, therefore, a power source is required for the device to operate. The main advantage of digital fittings is their high ergonomics: the temperature is adjusted almost on the fly, in addition, there is the possibility of programming daily modes to set individual temperature points during sleep and absence from home. At the same time, the cost of digital heads is 1.5-2 times higher than semi-automatic mechanical action.

Digital thermal headDigital thermal head

Depending on the type of valve on which the thermal head is installed, different types of room temperature correction are in effect. The method, which consists in limiting the flow, is implemented using a two-way valve, a three-way one is used when the circuit is executed on the flow-mixing unit. Almost all types of valves are designed to fit all types of thermal heads, at least full compatibility is guaranteed within the price list of one manufacturer.

Electronic thermal head servoElectronic thermal head servo

An additional difference is the placement of the temperature sensor. In some thermal heads, it is located in the device case, in others it can be located remotely: for digital thermostats, the distance of the offset is practically unlimited, while for mechanical devices, the removal contributes to a shorter response time and therefore the sensor is usually located from the thermal control device no further than 1 –1.5 m. Additionally, we note that the possibility of remote location of the temperature sensor exists for fittings that control heating of both air and heat carrier.

Features of installation and settings

In the simplest version, the thermostatic head is installed on the radiator supply pipe. It is important to ensure that the arrow on the valve body corresponds to the actual direction of movement of the coolant. Most of the valves have a convenient arrangement of connections: an external thread at the outlet for screwing into the sleeve and an internal thread at the inlet for easy installation of a union nut fitting. If necessary, the thermostatic assembly can be used to replace the upper shut-off valve of the radiator, but for this the valve itself must have an American-style outlet connection.

Installing the thermal head on the radiator

Three-way valves are used for installation in the flow-mixing unit. In this case, the taps of the main flow cut into the supply line in accordance with the direction of movement of the coolant, while the secondary outlet is connected to the bypass tube on which the circulation pump is installed. Here, all the same types of thermal heads can be used as for installation on a radiator: with air or coolant temperature control and with a different location of the sensor, depending on whether the installation is carried out openly or in a technological niche.

Pump and mixing unit for underfloor heating

There are a number of simple but obligatory rules for the installation of thermal heads. For the most part, they relate to ensuring the correct operation of the thermostat: the head should be freely blown by indirect convection, it should not be placed in dead-end areas, under curtains, as well as in places exposed to through air currents or third-party heating, for example, by open sunlight. Naturally, when it comes to heads with a remote sensor, all of the above applies directly to the temperature-sensitive element. The horizontal position of the regulator is considered optimal, thus, air flows freely through the protective grill and blows the working fluid, and the heating from the connecting pipes has a minimal effect.

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