Why is the pressure dropping in the central heating system and how to fix it
Falling pressure in central heating is one of the most common signals that customers address, especially during cold months, and often leads to the need for immediate intervention. If you notice that the pressure gauge on the boiler drops below the critical range of 1.0 to 1.2 bar, it means that water is leaking from the system or other physical changes are occurring that require professional intervention.
Main causes of pressure drop and their identification
The most common cause of pressure loss is mechanical water leakage, which can occur at visible locations such as radiator valves or at hidden joints in walls and floors. In older systems, it often happens that corrosion gradually damages welded joints or valve seals, leading to a slow but constant leak that the homeowner does not notice until the pressure drops to zero.
- Water leakage at thermostatic heads or radiator valves.
- Damaged seals on the expansion tank or safety valves.
- Corrosive damage to pipe networks or radiators due to poor water cleaning.
- Draining of water during repair or replacement of individual system components.
The role of the expansion tank and air in the system
The expansion tank functions as a buffer for pressure fluctuations during heating and cooling of water, with its internal air chamber needing to be properly set. If air stops flowing into the membrane of the expansion tank or if the membrane ruptures and fills with water, the tank will lose its function and the pressure in the system will fluctuate rapidly with temperature, causing regular pressure drops when heating starts.
An important part of maintenance is regular air bleeding from the system, as accumulated air in the upper part of radiators creates air locks that hinder circulation and can cause localized overheating or errors in the pressure sensor. Regular air bleeding through radiator valves and automatic air vents is essential for maintaining stable operating pressure.
The influence of temperature and thermal expansion of water
Theoretically, pressure in a closed system should only drop when cooling, as the volume of water decreases, but a real problem arises if the system is not properly filled or if the expansion tank does not have sufficient capacity. In typical households, it is recommended that the cold pressure be set to a value of 1.2 to 1.5 bar, which ensures that even at full heating, the pressure does not exceed the safety limit of 3.0 bar.
Experience shows that in new buildings with insulation and low-temperature systems (floor heating), pressure behaves differently than in older systems with high-temperature radiators. If you have floor heating that operates at lower temperatures, pressure may drop less noticeably, but if the expansion tank is improperly sized, a rapid pressure drop can occur during the first winter start.
Repair procedure and water refilling into the system
Repairs always begin with a check of the entire system, including a visual inspection of radiators, pipes, and the boiler to rule out visible water leaks. If no leaks are found, you can safely proceed to refill the water using a filling valve, which is part of most modern boilers or separate filling kits.
When refilling the system, it is crucial to follow an exact procedure: open the filling valve slowly, monitor the pressure gauge, and stop the water flow immediately after reaching the desired pressure of 1.2 to 1.5 bar. It is important not to overdo the pressure, as excessive pressure can trigger the safety valve and result in water leaking out of the boiler.
- Check that the filling valve is completely closed after the process is completed.
- After refilling the water, start the boiler and monitor the pressure for several hours.
- Ensure that all air vent valves are properly closed.
Why pressure drops again after refilling
If you have just refilled the water and the pressure drops again after a few days, it is a clear sign that there is still a leak in the system that you were unable to locate, or that the expansion tank is not functioning properly. A common phenomenon is also the so-called "breathing" of the system, where air in the water dissolves during heating and is released during cooling, which can cause a slight pressure drop, but not so drastic that it requires constant refilling.
If the pressure drops regularly every few days, it is necessary to call a specialist for diagnosis, who will use special leak detectors or measuring instruments to check the functionality of the expansion tank and safety valves. Ignoring this problem can lead to boiler damage due to lack of water and subsequent disruption of thermal insulation.
Prevention and long-term system maintenance
Long-term prevention involves regular inspection of water quality in the system, which includes chemical treatment against corrosion and sedimentation. Using high-quality corrosion inhibitors and anti-fouling additives extends the life of the entire system and minimizes the risk of microscopic leaks at joints and in radiators.
We also recommend annual inspection of the expansion tank, especially before the start of the heating season, where the air pressure in the tank and the condition of the membrane are checked. Proper maintenance not only keeps the pressure stable but also increases boiler efficiency and reduces energy consumption, which ultimately results in a lower heating bill.
Effect of air on pressure stability in the system
Air accumulation in the system creates local "air pockets" that hinder proper circulation and may cause false pressure drops on the pressure gauge, as the sensor only registers the water phase.
Frequently asked questions
Can I add water to the system myself?
Yes, if you have access to the filling valve and know where the pressure gauge is located, you can add water yourself according to the instructions in the sections above.
Is it normal for the pressure to drop in summer?
No, the pressure should remain stable even in summer, unless water was drained during maintenance or there is a leak that only becomes apparent when the system cools down.
What to do if the pressure drops below 0.5 bar?
Immediately turn off the boiler to prevent damage, and check the system for water leaks or contact a service technician.
A reasonable approach to maintenance and timely response to signals such as pressure drop will allow you to use your heating system trouble-free for a long time, without the need for expensive repairs or component replacement.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us - we are happy to help.