The thermostatic radiator valve (TRV) is also called thermostat or thermostatic valve. In this article I introduce you to the function of a thermostatic radiator valve. First of all, however, a differentiation must be made, because a thermostatic radiator valve consists of two components: the thermostatic head and the radiator valve.
Table of Contents
Figure 1 shows the two individual parts of a thermostatic radiator valve: on the left a thermostatic head (Oventop Uni LH*) and on the right a radiator valve (Oventrop AV6*) with six-stage presetting. In the following I will explain the function of these two components.
The thermostatic head is the visible part of the thermostatic radiator valve and has, among other things, the function of controlling the room temperature. In recent years, the classic thermostat has developed rapidly, so that a modern thermostat has many more functions than the classic control of the room temperature.
In addition to the ability to control the room temperature, smart thermostats can reduce and raise the room temperature at a set time or be remotely controlled by app, tablet and personal computer. A smart thermostat thus represents an important pillar in modern energy management.
An improvement to the smart thermostats is the smart heating control, which makes it possible to control several radiators and rooms centrally with an app or via a control unit. This means that you no longer have to program each smart thermostat individually, instead you can enter all data centrally for specific rooms and zones. Furthermore, most smart heating controls are already compatible with voice controls such as Amazon Alexa* and Google Assistent[/atkp_product] as well the software framework Apple Homekit.
In the following I will introduce you to both forms, i.e. the typical conventional thermostats as well as smart thermostats.
Conventional radiator thermostats
With the help of a conventional thermostatic head (see Figure 2) it is possible to set a desired room temperature manually – level 5 can correspond up to 28 °C, level 3 corresponds to approx. 20 °C and level * corresponds to approx. 7 °C – frost protection. A thermostatic head is thus a sensing element.
The thermostatic head is then able to control the temperature in the selected range and react to deviations of +/- 2 Kelvin. Inside the thermostatic head there is a sensing element with an expansion mass or a liquid which expands under heat and contracts under cold. The expansion of the mass inside the sensing element moves a transmission pin which opens or closes the valve (see Figure 3)
To understand the function of a thermostatic head a little better, the control process of a conventional thermostatic head can be described as follows (see Figure 4):
- First, we assume that the thermostat has been set to level 3. According to the instructions, we have learned that level 3 corresponds approximately to a room temperature of 20°C.
- When the desired room temperature of 20 °C is exceeded, the expansion mass in the thermostatic head expands. As a result, the transmission pin of the sensor spindle  in the thermostatic head presses on the stuffing box pin  in the radiator valve, which closes the radiator valve .
- When the desired room temperature falls below 20 °C, the expansion mass in the thermostatic head contracts again. The transmission pin of the sensor spindle in the thermostatic head  therefore moves back to its original position and takes the pressure from the stuffing box pin in the radiator valve , causing the radiator valve to open again .
In order to be able to better allocate the components of a thermostatic valve, the cross-section of a thermostatic valve is shown in Figure 5. The red border shows the thermostatic head and the blue border the valve body. In this example the cross-section of a Danfoss RA 2000* thermostat and a Danfoss RA-N* valve.
In the example, there is a steam-filled corrugated tube  around the sensor spindle , which reacts to the temperature differences in the room. The sensor spindle transmits the kinetic energy to the stuffing box pin , causing the valve plug  to move and open or close the valve.
Smart Radiator thermostats
An improvement on conventional thermostats are smart radiator thermostats or completely retrofittable smart heating controls. Compared to conventional thermostatic heads, these can additionally save up to 30 % of thermal energy through time and temperature control.
The 30% heating cost savings are based on a worst case scenario. This scenario can look as follows:
- a building has no night reduction for heating
- the room temperature is above average (approx. 24 °C)
- During the day the conventional thermostats are not turned down manually.
If the room temperature is lowered by one Kelvin, approx. 6 % of thermal energy is saved. If the room temperature is now reduced by 3 Kelvin from 24 to 21 °C, approx. 18 % of thermal energy is saved through the temperature reduction alone. If now an additional night-time reduction and a reduction of the room temperatures to 17 °C during the day when no one is present (see Figure 7), we come very close to 30%.
If this is still not enough, the use of window contacts or the constantly improving “window open” function can be used as arguments, which have a significant impact on the savings potential.
The principle of electronically controlled radiator thermostats is the same as for conventional thermostats. However, electronic thermostats do not have an expansion mass or liquid as sensing element, but an electrical sensor which reacts to temperature fluctuations and then, as required, moves a motor to close or open the valve (see Figure 8).
The model shown here is one of the first electronic radiator thermostats from the company Thermy. The central unit of the thermostat is an ATmega169 controller from Atmel. In this case the controller is used to measure the temperature and time and to control the motor to move the radiator valve.
The LC display shows all necessary information such as setpoint temperature, switching times or current operating mode. Furthermore, there are three buttons AUTO/MANU, PROG and temperature day/night, which can be used to carry out all programming steps for storing a heating profile. On the front of the thermostat is the setting wheel for adjusting the target temperature.
In order for electronic thermostats to work, they are usually equipped with batteries. These normally last for about 2 heating periods. I only know of one manufacturer who offers electronic thermostats without batteries. This is the EnKey system from Kieback&Peter, which uses the so-called Energy Harvesting, in which thermal energy is converted into electrical energy.
As with thermostatic heads, a lot has happened in the development of radiator valves. These differ in non-setting valves, presetting valves and presetting valves with differential pressure control. Globe valves and angle valves are commonly used as construction forms.
Non-settable and presettable radiator valves
Non-setting radiator valves only have the function of opening and closing the valve. As it has become more and more important in the last decades to save heating energy, good hydraulics, meaning an optimized flow behavior of heating water in a heating system, plays an increasingly important role. This is where presettable radiator valves come into play, which have a further function besides opening and closing. With the help of presettable radiator valves it is possible to do hydraulic balancing.
By presetting the radiator valves with an Presetting key* (see Figure 10) the exact amount of water for each radiator can be determined. This is done by reducing the cross-sectional flow area at the valve (see figure 11).
This prevents radiators from being oversupplied or undersupplied with hot water. Figure 5 shows the reduction of the flow cross-section in 6 steps. Shown is a pre-set radiator valve from Oventrop Durchgangsventil, Baureihe ."AV 6" DN10-3/8"* with 6 pre-setting stages. Of course, there are many other manufacturers for presettable radiator valves. These include Danfoss, Heimeier or Honeywell.
Presettable radiator valves with differential pressure control
An extension of pre-settable radiator valves are pre-settable radiator valves with internal differential pressure control. The internal differential pressure control further refines the hydraulic balancing in the partial load range of the heating system.
The calculation of the volumetric flow rates for a hydraulic balancing is always carried out for the full system load case, so that the hydraulics do not always function optimally in the partial system load case. This has to do with different volumetric flow rates in part-load and full-load operation, which results in different pressure conditions in the heating system.
Using a presettable valve with internal differential pressure control, the differential pressure across the valve is kept constant and a possible oversupply of individual radiators is prevented. In the following video from Danfoss the function of the Dynamic Valve is explained very clearly.
Radiator valves with internal differential pressure control are of course not only offered by Danfoss, but also by IMI Heimeier (Eclipse valves) and Oventrop (AQ series) and in my opinion they represent an important development of pre-settable radiator valves. This makes it possible to further refine hydraulic balancing in order to release further energy saving potential.
The “Valve Jungle”
Over the past decades, many valve manufacturers have brought many valves to the market in order to establish the valves of their own brand. The result is that when radiator thermostats from other manufacturers are retrofitted, they do not always fit the existing radiator valve.
The solution is either to replace the valve as well or to use valve adapters. In the meantime, the standard thread size M30 x 1.5 according to the factory standard for radiator thermostats has become established, so almost all new radiator thermostats have this threaded connection. Only the Danfoss company continues to work with its own thread dimensions. In order not to lose the overview, you will find all important information about valve adapters in my article “The big overview: Adapters for thermostatic valves“.
For me personally, it is very exciting to see how radiator thermostats and radiator valves have developed and continue to develop. It will be interesting to see how things continue and whether alternative systems, such as the Geniax System, will become established in the future.
I hope this article has helped you to understand a thermostatic radiator valve and its components a little better. If you have questions, suggestions or criticism about this article, please use the comment function.
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