Contact us

Why do I have to top up the heating system with water?

Picture: © Zvone /

In order for the heat to reach all rooms from the boiler room or utility room, the heating system needs a certain system pressure (also known colloquially as heating pressure). If this is too low, it can have a negative effect on the heating output. Possible consequences include cold heating surfaces and annoying noises from the heating system. Lastly, if the system pressure is too low, the heat generator will no longer work optimally, resulting in unnecessary heating costs. For this reason, it is important to top up the heating system with water when the system pressure drops too low.


What role does water pressure play in heating?

A heat generator is needed to heat the rooms to comfortable temperatures even in the depths of winter. This warms the heating water to the necessary flow temperature before it is transported to the rooms. The heating pump is responsible for transporting the water from the heat generator (usually on the lower floor) to the radiators (usually on the upper floors). It must build up a certain pressure so that the heating water can overcome this height difference. The unit used for this is called bar. One bar equates approximately to the air pressure on the Earth's surface. In addition to the height difference, there are pressure losses in the pipes, fittings and heating surfaces themselves, which also have to be compensated for.


Why does the heating system lose pressure in the first place?

A heating circuit is theoretically a closed system. In practice, however, air enters the heating pipes via various routes and bubbles form. The air bubbles reduce the heating output and also cause annoying noises. To eliminate this, the radiators need to be bled every now and then. However, an amount of heating water always escapes in the process. This means that the system loses water every time it is bled, causing the system pressure to drop. Therefore, the heating system has to be topped up with water every now and then. Other possible reasons for pressure loss in heating systems include an undersized safety valve (rather rare), an incorrect or faulty diaphragm expansion vessel (DEV) and other leaks.  

Experience has shown that the most frequent cause, apart from a leak, is that the pre-charge pressure of the expansion vessel is no longer correct (this is checked and corrected during maintenance) or that the diaphragm is faulty and the DEV is full of water.

Picture: © Dmitry Kalinovsky /

How do I calculate the optimum system pressure?

How high does the system pressure need to be, or how many bars must the heating system be filled to, is different in each individual case. However, the formula required for this is relatively simple: to determine the optimum system pressure, the difference in height between the heat generator and the highest radiator must be multiplied by 0.1. Add 0.5 bar to the result to compensate for any pressure losses.


System pressure = height difference (metres) x 0.1 (no unit) + 0.5 (bar)

Here's an example

The height difference between the heat generator and the highest radiator is 10 metres. Based on this therefore, the minimum system pressure should be 1.0 bar (10 x 0.1). When the tolerance of 0.5 bar is added, the result is a system pressure of 1.5 bar.

Topping up the heating water in 5 steps

If you wish to top up the heating water and thus increase the system pressure, it is best to call in a contractor. This is because if it is not done properly, the heating system could be damaged by contaminated water. A Viessmann trade partner, on the other hand, knows exactly what to look out for when topping up the heating water. It is essential to observe the following points:

  • The water must be clear and free of deposits and must meet the specifications of VDI Guideline 2035
  • The requirements of the Drinking Water Ordinance [Germany] must also be met
  • Under no circumstances must the heating water be allowed to enter the DHW circuit during filling

The following instructions for topping up the heating water are intended for system owners who have carried out this work several times before.

  1. Switch off the circulation pump and fully open all thermostatic valves. You can now bleed all the radiators. For properties spread over several floors, it is best to start from the bottom.
  2. Check the water pressure – either on the user interface of your heat generator or on the pressure gauge. The latter usually has markings that highlight the ideal system pressure in colour.
  3. Fill the corresponding water hose with water until it overflows. Now connect the heating system to the water connection. Use special fill valves from Viessmann for this to ensure that no heating water enters the DHW circuit.
  4. Now open both valves and top up with heating water until the ideal system pressure is reached. Use the above formula to calculate the system pressure.
  5. When the user interface or the pressure gauge shows the required value, you can close the valves and turn off the tap. To make sure that no air has been introduced to the system, you can bleed the radiators again.

Important: the system pressure should remain constant after filling and must not be below 1.3 bar. If this is the case, the job was successful. If the pressure keeps dropping, there may be a technical fault. In this case, contact your heating contractor.

How often should I top up the heating water?

How often the heating system needs to be topped up with water must be decided for each individual system. In addition to the condition of the heating system itself, other factors such as frequent bleeding or technical components such as the expansion vessel also play a role. During the heating season, it is advisable to check the heating system pressure regularly.

Take out a maintenance contract and enjoy peace of mind

Whether it's topping up the heating water or checking safety equipment – in many cases it's worth signing up to a maintenance contract with your local contractor. A heating specialist will then call by at regular intervals to make sure your system continues to work properly. When necessary, the engineer will adjust the system pressure to the ideal value.

How can we help?

Tap on the button below to WhatsApp us at +65 8028 0468 during office hours (Mondays – Fridays, 9am – 6pm), or select Contact Form below to provide us with details of your enquiry. We will get back to you as soon as possible.

Contact us