Frequently asked questions about control and safety valves
Why to consider control and safety valves as a whole
In practice, I often encounter the situation where the customer focuses on the boiler, radiators, piping, and circulation pump - but only remembers the valves that protect and regulate the entire system when something goes wrong. A vent valve, a thermal control unit, a check valve, or a safety valve may seem like small components compared to a boiler costing thousands of euros, but their failure can cause system malfunctions, reduced efficiency, noise in the piping, or in the worst case, damage to the boiler due to low-temperature corrosion. This article is compiled as answers to the questions our customers ask most frequently during the sale and servicing of heating systems - from choosing a specific type of valve, through installation, to solving common problems.
We will proceed step by step, starting with system venting, through thermal control units for solid fuel boilers, up to general maintenance and diagnostics. The goal is for you to not only know what to buy after reading, but also to understand why and how to install it correctly.
Ventilation of the heating system - why it is fundamental
Air is the biggest enemy of any closed heating loop. It enters the system during filling, during water topping up, through chemical reactions between water and metal (corrosion produces hydrogen), or is released from the water itself when heated, as hot water has a lower solubility of gases than cold water. An air bubble in a radiator or at the highest point of a riser hinders water flow, causes gurgling, cold upper parts of the radiators, and in extreme cases, the pump can run "dry" in part of the circuit, which damages it.
The solution is two types of valves - automatic and manual. In larger and more complex systems, it is common to combine both types: an automatic valve at the highest points of the system (on manifolds, on the heating branch at the boiler, on risers) and a manual valve directly on the radiators, where the user controls the venting themselves.
Automatic air vent - principle of operation
The automatic valve contains a float that floats on the water level inside the valve body. When air accumulates at the top of the body, the water level drops, the float drops with it, and mechanically opens a needle valve through which the air escapes into the atmosphere. After the air escapes, the water level rises, the float rises, and the valve closes. The entire cycle is fully automatic and does not require user intervention - this is its main advantage over the manual type.
When choosing an automatic valve, I recommend our Automatic air vent - 1/2", which is suitable for standard domestic and smaller commercial systems. If you are dealing with visible installation, for example in a boiler room with stainless steel or chrome-plated piping, where aesthetics also matter, there is also a version Automatic air vent - 1/2"; nickel with a surface treatment that better matches design radiators or visible piping.
Manual air vent - when to use it
A manual valve (often also called Entlüfter or air bleed cock) is operated manually - using a screwdriver or key, the needle valve is slightly opened and air escapes until water starts to flow. It is typically mounted directly on the radiator, on the side or top outlet. It is inexpensive, reliable, has no mechanical parts prone to wear from the float, but requires the user to actively service it. Our Manual air vent - 1/2" is the standard you will find on almost every new radiator.
A detailed comparison of the advantages and disadvantages of both types, including recommendations for specific types of systems, can be found in a separate article "Automatic vs manual air vent - differences and usage." Here I just summarize the practical rule I use in design: in all places where it is possible to walk regularly without problems (radiators in living areas), a manual valve is sufficient. In all places where access is complicated, where air is a recurring problem, or where it is a critical point in the system (the highest point of the riser, manifold, boiler outlet), an automatic valve is required.
Thermal control units - protection of solid fuel boilers
The second major group of valves we encounter very often in practice are thermal control units (often also called thermostatic mixing units or in English terminology thermal control unit). Their task is to protect solid fuel boilers - wood, coal, pellets in simpler boilers without electronics - from low-temperature corrosion.
What is low-temperature corrosion and why does it occur
Boilers for solid fuels, especially cast iron or steel ones without special protection, are prone to corrosion if the return water (water returning from radiators to the boiler) returns too cold - typically below 55-60 °C. At low water temperatures on the inner walls of the boiler body, combustion gases (especially sulfuric acid and other aggressive compounds formed during the burning of wood or coal) condense, gradually corroding the boiler material from the inside. The result is a shortened boiler lifespan, leaks, and in the worst case, a boiler body rupture.
A thermal control unit solves this problem simply and without the need for electronics - it is a fully mechanical, thermostatic solution.
How a thermal control unit works
The unit contains a thermostatic valve with a temperature sensor (wax-filled), which mixes hot water from the boiler outlet with return water from the radiators so that the water entering back into the boiler never drops below a set minimum temperature (e.g., 55 °C or 61 °C). While the water returning from the circuit is colder than the set value, the valve does not allow it to pass directly into the boiler, but circulates it in a small loop through the boiler and outlet, gradually adding warmer water from the boiler until the desired temperature is reached. The unit usually also includes a built-in circulation pump (for circulation in the small loop while the system is heating up) and a check valve, which prevents unwanted reverse water flow, for example during gravitational circulation after the pump is turned off.
Choosing the correct temperature - 55 °C, 61 °C, 72 °C or 80 °C
The opening temperature of the thermostatic unit is selected according to the type and manufacturer's recommendation of the boiler. In general, most modern cast iron and steel solid fuel boilers require a minimum return temperature of 55-65 °C. We offer two common variants:
- Thermostatic unit - with pump and check valve - DN 25; 55 °C - suitable for boilers where the manufacturer specifies a minimum return temperature of around 50-55 °C
- Thermostatic unit - with pump and check valve - DN 25; 61 °C - for boilers with a higher required minimum temperature, usually around 60 °C
With some larger or more sensitive boilers, we also encounter requirements for 72 °C or even 80 °C - a detailed guide on how to precisely choose the correct temperature according to the type of boiler and power can be found in the article "What temperature of the thermostatic unit is suitable for a solid fuel boiler". As a basic rule, remember: always follow the technical specifications of your specific boiler, not just general recommendations - manufacturers specify the exact minimum return temperature and failure to comply with this value may result in the warranty being void.
DN 25 vs DN 32 - choice of nominal diameter
The second parameter, in addition to temperature, is the nominal diameter (DN) of the unit, which must correspond to the boiler's power and water flow in the system. For smaller and medium-sized solid fuel boilers (up to approx. 25-30 kW), DN 25 is commonly used. For more powerful boilers, it is necessary to choose DN 32 to ensure the unit is not hydraulically undersized and does not create unnecessary pressure loss, which could limit water circulation. An undersized unit will show itself by the system not achieving the required performance, the boiler overheating, or the unit pump having to operate at the edge of its capacity. The exact calculation procedure and tabular recommendations according to boiler power can be found in the article "DN 25 vs DN 32 - how to choose the correct nominal diameter of the thermostatic unit."
| Boiler power | Recommended nominal diameter | Typical note |
|---|---|---|
| up to 25 kW | DN 25 | typical for family homes |
| 25-45 kW | DN 25 to DN 32 (depending on flow) | hydraulic calculation must be verified |
| over 45 kW | DN 32 | larger buildings, boilers with higher power |
Installation - what to pay attention to
Positioning of the air vent
An automatic air vent is always installed at the highest point of the system or a specific branch - air tends to rise upwards, so if the vent is lower than the actual highest point of the pipe, part of the air will never reach it and will form an air pocket somewhere else. Typical installation locations are: at the outlet of the boiler before the manifold, at the top of vertical risers, at floor heating manifolds, near the expansion tank.
The vent is mounted in a vertical position (body pointing upwards), because the float mechanism is designed for gravitational action - in a tilted or horizontal installation, the float may not respond correctly to the water level. It is recommended to install a shut-off ball valve before the vent so that the vent can be replaced or cleaned in the future without draining the entire system.
A detailed installation procedure, including recommendations for tightening torque, thread sealing and testing after installation, can be found in the article "Installation of an automatic air vent - procedure and placement in the system."
Installation of the thermostatic unit
The thermostatic unit is installed in the return pipe between the boiler and the manifold/radiator circuit, and must be connected exactly according to the manufacturer's scheme - it has a defined input from the boiler (hot water), an input from the return (cold water from the circuit) and an output towards the system. Swapping the inputs is the most common installation error we encounter - the unit may still "work" somehow, but it does not fulfill its protective function, because the mixing takes place in the wrong direction.
It is also important to ensure the correct flow direction according to the arrow on the body of the unit and not to forget the electrical connection of the built-in pump - the pump requires a separate power supply (usually 230 V), ideally via a separate circuit breaker or via the boiler's control unit, if the boiler allows it. A complete step-by-step procedure, including a wiring diagram for connection to an existing boiler circuit, is described in the article "Installation of a thermostatic unit with a pump and check valve."
Why the air vent is leaking or not working
This is one of the most common service questions we receive from our customers. In practice, the causes are mostly the following:
- Clogging of the mechanism with dirt - especially in older systems with sludge or rust in the pipes, the needle valve or float can become clogged and not seat properly. The result is either constant dripping of water, or on the contrary, a valve that does not allow air to pass at all.
- High pressure in the system - at pressures above the value for which the valve is structurally designed (usually up to 6 bar working pressure), leakage of the sealing can occur.
- Worn sealing - after several years of operation, the rubber sealing of the needle valve loses elasticity and loses the ability to seal completely.
- Incorrect installation position - as we have mentioned, the valve must be in a vertical position. In a tilted installation, the float does not function properly.
- Missing or non-functional shut-off valve - when replacing the valve without prior closure, the technician may, in an attempt to quickly tighten the new fitting, not tighten the thread sufficiently and a micro leak may occur.
If the valve is constantly leaking, it is not advisable to "just tighten it" by force - this usually does not solve the problem and there is a risk of damaging the valve body. It is better to replace the valve with a new one, because the cost of a new unit is significantly lower than the risk of a larger leak or damage to surrounding components due to moisture. A detailed diagnosis and solution according to the specific symptom can be found in the article "Why the air vent is leaking or not working."
Practical scenarios from everyday service practice
Scenario 1: Radiators gurgling after winter
A typical situation - the customer turns on the heating after summer, the radiators gurgle, the upper parts of the bodies are cold. The solution is simple - gradual air venting of all radiators using manual valves, starting from the lowest floor upwards. If the problem recurs every season at the same location, it is a sign that there is a microscopic leak in the system that is constantly replenishing the air (for example, a leaking expansion tank seal or porosity at a welded joint), and in such cases, I recommend adding an automatic valve directly at the problematic location, so that the air venting is done automatically without the need for repeated manual intervention.
Scenario 2: A wood-burning boiler corrodes after three years
A very common case in older installations where a solid fuel boiler was connected directly to the system without any protection of the return. After two to three seasons, increased fuel consumption, smell of combustion gases, and later visible corrosion on the boiler inspection ports appear. In these cases, we clearly recommend the additional installation of a thermostatic unit - the investment pays off through the extended boiler lifespan, which can be several times longer.
Scenario 3: New solid fuel boiler without anti-corrosion protection
When designing a new boiler room with a solid fuel boiler, a temperature control unit is practically a necessary component, unless the boiler already has its own integrated electronic return temperature regulation (which is only the case for higher category boilers). Many manufacturers even condition the warranty of the boiler on the use of such protection - I always recommend checking the warranty conditions of the specific manufacturer.
Maintenance and inspection of valves - how often and what to check
Control and safety valves are not maintenance-free, even though they may look like it at first glance. Recommended inspection scope:
- Drain valves - once a year, preferably before the start of the heating season, check the tightness of the closure and the functionality of the float (for automatic types, gently lift the closure to check for air/water leakage and the return closure).
- Temperature control units - check the functionality of the pump (noise, vibrations), verify that the return temperature does not actually drop below the set value (measured on the pipe), and visually check the tightness of the connections.
- Check valves - check that there is no reverse flow when the pump is off (typically indicated by nighttime overheating of the boiler or unexpected circulation).
A comprehensive guide to regular inspection of the entire set of valves, including a printable inspection checklist for service technicians, can be found in the article "Maintenance and inspection of control and safety valves in heating systems."
Pressure relief valve - why you should never underestimate it
Besides drain valves and temperature control units, I also encounter questions about pressure relief valves in service practice - and this is where the biggest mistakes can happen, because it is a component that is directly related to the safety of the entire system, not just its convenient operation. A pressure relief valve is the last safety measure against excessive pressure in a closed heating circuit. If the expansion tank fails, if the circulation is blocked, or if the boiler regulation for some reason does not stop the heating, the pressure in the system increases - and without a functional pressure relief valve, there is a risk of the boiler body or pipe bursting in the worst case.
How a pressure relief valve works
The mechanism is mechanical and independent of electronics or regulation - this is a crucial feature that distinguishes it from other safety components in the system. Inside the body, there is a spring that pushes the closing cone against the seat. The spring force is precisely set to the opening pressure value (most commonly 2.5 or 3 bar in standard home systems). As long as the pressure in the system is lower than this value, the spring keeps the valve closed. At the moment when the water pressure exceeds the set limit, the pressure overcomes the spring force, the cone lifts from the seat, and the excess pressure water escapes through the outlet into the drain pipe. After the pressure drops below the rated value, the valve closes by itself.
When selecting the opening pressure of the pressure relief valve, a simple but crucial rule applies - the value must exactly match the maximum operating pressure for which the boiler and other system components (expansion tank, circulation pump, radiators) are dimensioned. A pressure relief valve with too high an opening pressure does not protect the system in time enough, while a pressure relief valve with too low a pressure may unnecessarily open even during normal operating pressure peaks, causing unnecessary water losses and the need to repeatedly refill the system. That is why the pressure of the pressure relief valve is never guessed "off the cuff" - it always follows the maximum allowable pressure of the most sensitive component in the system, most often the boiler.
It is also important to correctly install the discharge pipe from the pressure relief valve - it must direct in a safe direction (typically into a floor drain or container), must not be blinded or narrowed, and must not be longer or have more elbows than the manufacturer allows, as this would increase the pressure loss during discharge and the valve may not reduce the pressure in the system quickly enough. Checking the functionality of the pressure relief valve should be part of every annual service inspection - the valve is gently turned or lifted with a test lever to verify that the mechanism is not clogged with deposits or corrosion.
Common questions
Do I need both a manual and an automatic air vent on my system?
It is not mandatory, but it is a common and recommended practice. An automatic vent is used in critical and hard-to-reach places (divider, risers, boiler outlet), while a manual vent remains on individual radiators, where you can manually bleed air as needed without having to buy a more expensive automatic one for each radiator.
Can I install a temperature control unit on any solid fuel boiler?
Practically yes, as long as it hydraulically matches the boiler's performance (correct DN size) and the temperature setting corresponds to the manufacturer's recommendation. For boilers with their own electronic return temperature regulation, it is necessary to check whether the duplicate protection is unnecessary or counterproductive - in such cases, contact the boiler manufacturer's technical support.
What happens if I choose an incorrect temperature for the temperature control unit (e.g., 55 °C instead of 61 °C)?
If you choose a lower temperature than the manufacturer recommends, the protection against low-temperature corrosion will be insufficient and the risk of boiler corrosion remains practically unchanged. If you choose a higher temperature than necessary, the system will circulate in the small circuit for a longer time before releasing heat to the radiators, which may cause a slower heating response, but the boiler will be better protected. In case of doubts, it is always better to choose the higher value of the two available options.
How often should I replace the air vent?
With normal use and good quality water in the system, an automatic vent typically lasts 8-15 years, less in harder or polluted water. At the first signs of leakage or malfunction, I recommend replacement - due to the low cost of the component, repair is usually not worth it.
Do I need a separate circulation pump for the temperature control unit?
No, our units (for example, DN 25 at 55 °C and 61 °C) have an integrated pump directly in the body, including a check valve. A separate main circulation pump for the system (e.g., for the radiator circuit) is of course still needed, but the pump for the small protective circuit of the unit is already part of the package.
Can the temperature control unit be used with pellet or gas boilers as well?
It is primarily intended for solid fuel boilers (wood, coal, briquettes), where the risk of low-temperature corrosion is highest due to the composition of the flue gases. In modern condensing gas boilers, on the other hand, a low return temperature is intentionally used for higher efficiency, so this unit would be counterproductive there. In the case of pellet boilers, it depends on the specific type - most newer models have their own electronic protection, but older or simpler types can benefit from a mechanical temperature control unit just like classic solid fuel boilers.
Conclusion
Control and safety valves belong to components that may seem like minor details at first glance, but their correct selection and installation determine whether the heating system will operate quietly, efficiently and without faults for many years, or whether it will suffer from gurgling, reduced performance and premature wear of more expensive components such as the boiler itself. When choosing a specific type and size, I recommend always referring to the technical documentation of the boiler and the actual performance of the system. If in doubt, do not hesitate to contact our technical support – when choosing between DN 25 and DN 32, or between a temperature of 55 °C and 61 °C, we can recommend a specific solution from our range based on the type and performance of your boiler.
Do you have a question on this topic?
Having trouble making a decision or dealing with a specific situation in your home? Write to us – we will be happy to help.