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Thermostatic valves (often abbreviated as TRV) are an indispensable part of heating or cooling systems. They serve to regulate thermal output – the thermostatic valve protects against scalding from hot water and also reduces the risk of boiler damage due to low-temperature corrosion. This fitting will therefore save you thermal energy and money.
The task of the thermostatic valve is to maintain a constant temperature. Specifically, it mixes hot and cold water so that the set output water temperature is always ensured at the outlet. It is a three-way valve that does not require electrical energy for operation. Inside the valve, a thermostatic cartridge acts as the control element. The thermostatic valve operates on the principle of thermal expansion of material. This is a physical phenomenon where a solid body changes its linear dimensions when heated/cooled by a certain temperature – thus the volume of the body changes, with most substances expanding when heated.
In the case of thermostatic valves, the thermal expansion of material is utilized in a bimetal element, where two metal strips with different thermal expansions are firmly bonded. When the temperature changes, a defined change in dimensions occurs.
When the temperature increases, pressure is exerted on the spindle (pressure needle) in the thermostatic valve, and the valve closes. Conversely, when the temperature decreases, the volume of the body reduces, the spindle of the valve is released, and it opens again.
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Thermostatic valves differ in size, type of thread, connection dimensions, flow rate, or cartridge replaceability. These wear out over time and need replacement, thermostatic cartridges for fixed-setting valves are offered by BOLA as a commonly available spare part.
In brass valves with adjustable temperature, the cartridge usually cannot be replaced. However, complete renovation kits are supplied. Using a renovation kit, the entire core of the valve can be refurbished.
Bola.cz offers a wide selection of thermostatic valves. When choosing them, we recommend focusing on the temperature range and opening temperature – the set temperature is the one at which the valve begins to open. However, it is fully open only at a temperature 10 °C higher.
Thermostatic valves are also distinguished by their use. Cast iron/brass with a fixed mixing temperature for heating, or for filling the boiler with hot water, help protect against low-temperature corrosion. There are also brass valves with adjustable temperature for domestic hot water, where the valves are used mainly as protection against scalding. Lastly, there are high-flow valves, which are used for underfloor heating.
Since every rule has an exception, at Bola.cz you will also find a thermostatic filling valve VTC422, which is used to protect solid fuel boilers, but has an adjustable temperature.
From the above, it follows that thermostatic valves are also divided into fixed and adjustable. Those that are fixed are referred to as filling – they are used to fill boilers or storage tanks with hot water. The thermostatic cartridge can be easily replaced with a cartridge with a different opening temperature.
Filling the boiler with hot water has two reasons here. The first is to maintain the efficiency of the boiler, which accounts for warm filling. The second reason is protection against low-temperature corrosion.
Low-temperature corrosion is caused by certain components of flue gases with water vapor present in the flue gases, forming acids at a certain temperature. The vapors of these acids then condense when the surfaces of the boiler exchanger cool to a temperature lower than their dew point. This creates an aggressive acidic condensate. It attacks the material of the boiler, which, once compromised, quickly succumbs to corrosion.
Another accompanying phenomenon of operating heat exchange surfaces of the boiler below the dew point temperature of the flue gases is the adhesion of ash and the formation of deposits – known as sooting. Such a situation can clog the flow cross-section for flue gases, significantly reducing the lifespan of the boiler.
The thermostatic valve allows by mixing cold water from the boiler inlet with hot water from the boiler outlet to maintain its walls at the desired higher temperature.
Low-temperature corrosion is a problem for the vast majority of boilers. Depending on several variables (mainly fuel and boiler design), boilers require a minimum return temperature value for nominal performance. In the case of sulfur-free fuels, the range is between 40 and 70 °C.
Filling units with pump are a system of a thermostatic valve, pump, and thermometers. They are used to ensure and maintain a high return water temperature to the boiler. This parameter contributes to protection against low-temperature corrosion.
The advantage of the filling unit over a standalone thermostatic valve is that you get an overview of the temperatures on individual branches. Since it includes a circulation pump, there is no need to purchase an additional pump separately. It is therefore a cost-effective package.
Thermostatic valves with adjustable temperature are used in domestic hot water systems. This type of valve allows the output temperature of the mixed water to be set within a given range. The main advantage is the ability to change the output temperature by turning the control knob. This principle can be used, for example, as protection against scalding – for example, for children in schools. In buildings with central and long hot water distribution systems, the temperature is maintained at a higher level due to heat losses. At individual draw-off points, the water needs to be cooled to a comfortable level, which can be achieved using thermostatic valves with adjustable temperature.
In the catalogue, you will also find thermostatic valves for circulation balancing, which are essential for even distribution of flow rates throughout the heating system. For the proper functioning of individual elements of the heating system, it is necessary to balance the flow distribution, which is what these valves are for.
Balancing heating systems is the focus of balancing fittings, which can be equipped with thermo-electric actuators. Learn more HERE.
Finally, we bring you some questions that may arise when purchasing or using thermostatic valves.
o Thermostatic valves of the VTA series have a wide range of temperature ranges, depending on the manufacturing material – in brass, it is 20 to 43 °C, 35 to 60 °C, 45 to 65 °C, 50 to 75 °C with an output temperature stability of +- 2 °C. If the valve is made of nickel, the temperature range is between 32 – 49 °C, 35 – 50 °C, 35 – 60 °C, and 45 – 65 °C. The stability of the output temperature is +- 1 °C (for the VTA300 series) and +- 2 °C (for the VTA500 series).
o The brass series is considered basic, where the supply of hot water is available in 7 to 8 seconds. In the nickel series, it is available in 2 – 3 seconds.
o During disinfection heating, the water in the heating system must be heated to 60 °C. This prevents the multiplication of Legionella Pneumophilla bacteria, which cause the so-called legionnaires' disease or (milder) Pontiac fever. However, such a high temperature for effective bacteria elimination could cause scalding. Therefore, a thermostatic mixing valve (VTA) needs to be installed after the hot water heater to limit the water temperature according to the set limit.
o The thermostatic valve VTA322 with a temperature range of 20 – 43 °C can cover 40 to 80 m2. The thermostatic valve VTA522 with the same temperature range can cover 85 – 160 m2. The valve VTA572 20 – 43 °C can regulate an underfloor heating area of 130 to 240 m2.
The manufacturer states that the given parameters are based on the assumption of a maximum temperature difference of 7 K and an underfloor heating output of 50 W/m2.
§ It is necessary to ensure that the minimum temperature difference between the hot water supply and the mixed water outlet is at least 10 °C. Also, check if one of the ports is under too much pressure, for example, from a circulation pump.
§ There are two possibilities – either this problem is caused by low pressure on the supply branch, or a non-functional thermostatic cartridge. If you remove the thermostatic cartridge from the valve and place it in water at least 10 ºC higher than its triggering temperature, the cartridge should open shortly. In that case, the problem lies in the pressure balance of the fitting.
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