What happens when a heater is connected

Topping up the heating water - instructions & important information


If the water pressure in the heating system is too low, this leads to reduced heating output and rising energy costs. The living comfort decreases and in the long run the components can be damaged. If you want to top up the water in the heating system, you should rely on the expertise of a professional. Otherwise there is a risk of damage to the heating system or - even worse - health consequences. If you have been instructed by an expert and have refilled the heating water a few times under professional supervision, you can certainly do it yourself. Find out here step by step how to refill your oil or gas heating and get it going again.

If you are not 100 percent sure about topping up the heating water, let a professional do the work. The money that you can save by filling it yourself is out of proportion to the material and health damage that can result from incorrect work.

1st step: Checking the water pressure

There is a pressure gauge on the heating system in the boiler room. Before you top up the heating water, you must check the current pressure. Most pressure gauges have a marker that shows you the ideal range. Alternatively, the operating instructions for the heater serve as a source of information for the recommended water pressure. A rule of thumb is that the pressure in single-family homes should be between 1 and 2 bar. A drop below the 1 bar mark can already lead to damage and malfunctions.

The height distance between the heating system and the highest radiator in the house is a guide to the ideal pressure. There should be an increase in pressure of 0.1 bar per meter of altitude.

Many heating systems have self-protection and switch off when the pressure is too low. In any case, top up with water first before switching the heating on again.

Step 2: Vent the radiators

In order to achieve the optimal functionality of the heating system, you should vent the heating. Since heating water has to be refilled each time the air is vented, it is advisable to combine these two processes.

Venting takes place directly on the individual heating elements. In the hardware store you can buy the appropriate keys with which the radiator valve can be opened and closed. Turn the valve and let the air escape. As soon as water comes out (catch it in a mug or bowl) close the valve again.

Avoid contact with the escaping water, as it has previously flowed through the heating pipes and can contain metal parts and other particles.

3rd step: Switch off the circulation pump

Before you top up the water, you must turn off the circulation pump. Also set the thermostatic valves on the radiators to the highest level.

4th step: Connect the heating system to the tap

Case 1:
Some systems have a permanent connection between the heating system and the fresh water system. Since this interface must not be permanently open, the corresponding taps are closed. In this case, you can top up with water directly by opening the taps.

Make sure you only leave the connection open for a few seconds and never add too much heating water. Always keep an eye on the pressure gauge.

Danger: It is forbidden to keep the connection between the heating water circuit and the drinking water circuit permanently open.

Case 2:
If no hose is installed, you must select the hose that is suitable for the type of heating in question. The exact requirements are described in the operating instructions. Most of the time, a simple garden hose is sufficient. Rinse the hose to remove any debris. For the connection you need a system separator, which is a special hose connection. It is necessary to prevent contaminated heating water from entering the drinking water pipe.

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First connect the hose to the filler neck of the heating system, which can be attached to the flow or return. The filler neck remains closed for the time being. Then connect the other end to the tap. Turn the hose loosely on the tap so that any air in the hose can escape.

Step 5: Let the air escape

Air should not get into the circuit. Slowly turn on the tap, which will cause the water to flow into the hose. Since the hose is only loosely connected, the air is forced out through the water. Hold a bowl or rag under the tap to catch any water that may leak out. As soon as water comes out, the air has escaped and you can firmly connect the hose.

6th step: Refilling the heating water

Open the filler neck on the heating system. Now turn on the tap. As soon as enough water has been refilled, close the tap again. Then make sure that the previously opened valves and cocks are closed. If air accidentally got into the system during the procedure, you must vent the heater again. Remove the hose and test the system.

In order to guarantee the constant functionality of the heating, it makes sense to conclude a maintenance contract. Here, the heating installer of your choice subjects your heating system to an overall check at regular intervals (with a conventional gas heating system, maintenance should take place annually) and carries out routine measures such as topping up your heating system with water.

How high does the pressure have to be in the heater?

As a rule, the specialist company should determine the necessary water pressure for the heating system. But there are initial guidelines that you can use as a guide.
The minimum pressure of the heater should be at least 1.0 bar if the height difference between the heater and radiators is around 10 meters. If you then add 0.3 bar as a safety buffer for possible pressure losses, you come to a solid guide value of 1.3 bar. However, if you want to determine the precise water pressure for your heating system, it is best to hire a plumber.

Important questions and helpful answers

  • How often does water need to be topped up?
    How often water needs to be topped up depends on the circumstances. Regular checks are important. It should take place every 2 to 3 months.
  • What points do I have to pay attention to with the water?
    Softened water is best.
  • Should I call in a professional?
    The advice of a specialist is recommended for the first implementation. He is very familiar with all regulations and ensures smooth filling.