A properly constructed heating system is safe to use and does not pose a threat to household members or the durability of the residential building structure. The condition for the proper operation of the system is the proper installation and efficient operation of safety valves. What is the function of such valves? How to recognize a safety valve failure and repair the system? How to check the correct operation of the safety valve? You will find answers to these questions in the article below.
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How does a safety valve work?
Safety valves protect heating systems against dangerous, uncontrolled pressure increases. This is due to excessive temperature increases of the heating medium in the circuit. The safety valve prevents excessive pressure increases in the central heating system. The valves are installed in places where the pressure in the system can increase above a safe level, in practice usually before the boiler or the DHW system circuit. When the pressure exceeds the set value, the valve automatically opens and releases excess liquid, which causes a decrease in pressure in the system. The lack of a safety valve or its damage may lead to a sudden increase in pressure, even to the explosion of the boiler, damage to the system and scalding of household members. Safety valves therefore play a key role in ensuring the stability and long life of the system.
Central heating safety valves
The central heating safety valves are constructed 
from a body with a spring, a spindle and a plug. The internal chamber of the body is protected by a membrane, protecting the body from the flowing medium. The valves fulfill their function until the heating medium reaches a temperature of 140 degrees Celsius. The initial tension of the spring (factory setting) corresponds to an opening pressure of 1.5 bar to 6 bar. On what principle does the safety valve work? The valve is closed when de-energized. The medium in the system flows under the plug. An increase in pressure above the set point causes the valve to open and the medium to flow out, until the pressure in the system drops. When the pressure stabilizes below the set point, the valve plug closes and the medium flows out.
The detailed operation and type of safety valve depends on the type of central heating installation. In open installations, a safety valve and an open expansion tank are used, through which excess water flows out. They are installed high, above the remaining elements of the installation, often in the attic or loft. This method is used in the case of installations cooperating with solid fuel boilers.
In the case of closed installations cooperating with condensing furnaces, heat pumps and gas boilers, the safety valve is connected to a pressure vessel and excess water is drained to the sewage system.
DHW safety valves
DHW safety valves are installed at domestic hot water tanks and are very important from the point of view of building safety. Their task is to prevent a sudden increase in pressure inside the tank, which may lead to its damage or explosion. A safety valve for domestic hot water tanks should be installed at the water inlet to the tank or at the hot water outlet. Such a valve is designed to open automatically in the event of a water pressure increase above a certain value, usually 6 bar.
Why is the safety valve leaking?
Sometimes, water drips from the safety valve. In the case of the valve in the DHW circuit, this does not necessarily mean a failure. Dripping may occur during use of the heater and is related to the volumetric expansion of water as a result of heating - it indicates that the valve is working properly. In such a case, it is usually enough to close the cold water supply, turn on the tap and drain the excess hot water from the installation, and then turn on the cold water supply.
Leaking of the central heating valve usually means a lack of an expansion vessel, its installation in the wrong place or failure to adjust its parameters of pressure and water temperature after installation in the system. Sometimes the cause is too high pressure in the water supply network.
How to check the safety valve?
We should periodically 
check the correct operation and tightness of the safety valves. This is done by lifting or turning the valve knob. Dripping water indicates its damage. In this case, you should call a plumber and order an inspection of the installation. The specialist will check whether the valve should be replaced, or whether it will be enough to clean it of sediments or regenerate it.
The installation is inspected and any faults are repaired before the heating season begins, to protect against heating failure in the middle of winter.