Bleeding the heating system – we explain just how simple it is

When it gets colder outside, it is also time to start putting on the radiators. It is not uncommon for annoying, and sometimes loud, flow noises or gurgling to occur. This is caused by air in the radiators. Apart from the noises, another clear indication of this is when your heating system does not fully heat up, i.e. radiators stay completely or partially cold. The correct action in this case is to bleed the heating system. If you do not do this, some rooms will remain cold and the thermal energy generated goes to waste because the air bubbles in the heating system are preventing the heat from being distributed properly. We'll explain with a few clear instructions how you can easily bleed the heating system yourself.  

Why should you bleed the heating system at all?

A heating system is, in principle, a closed heating circuit carrying hot water. The starting point is the boiler or heat generator. There, the water is heated before being fed via a system of pipes and lines to the appropriate rooms where it then flows through the radiators. These in turn emit the heat to the surrounding air. 

For trouble-free operation, the heating water must be able to be optimally distributed. But this is not possible if there is air in the system. This is because air conducts heat much less effectively than water. This in turn means that some radiators heat up more than others. An annoying side effect is the gurgling noise that deprives many homeowners of a good night's sleep. In addition, the small air bubbles impair the efficiency of the system as a whole. 

If you bleed the heating system, you will ultimately save money. Because a radiator that is not sufficiently supplied with heating water consumes more energy. To reach the required temperature, the thermostatic valve is usually turned up further incurring higher costs.

What do you need for bleeding the heating system?

Before you set to work bleeding your heating system yourself, you should have a few things ready. These can usually be found in every household.

You will need the following tools for the bleeding the heating system:

  • a glass, a cup or a mug
  • a rag or a cloth
  • a radiator bleed key

You can buy the bleed key at a reasonable price from your heating contractor or from a DIY or plumbing retailer. This is usually a standard quadrant key. It may also be possible to open some drain valves using a standard slotted screwdriver. You will need the rag and the container to catch any heating water that escapes.

Step-by-step guide

To bleed the heating system properly, all that's required are the aforementioned tools and the following 7 short steps. Once you have all the tools ready, you can start: 

  1. If you are the homeowner, you should turn the circulation pump off if possible. 
  2. Now wait approx. 30 to 60 minutes so that all the air bubbles can collect in the radiators.
  3. Before starting the actual bleeding process, first turn the radiators on to their highest level. 
  4. Place the rag under the radiator so that it will catch any water that leaks from the air vent valve. Alternatively, you can wrap the rag directly around the valve.
  5. Now insert the key and hold the container under the air vent valve. Slowly open the radiator valve with the key, but do not open it fully. It usually takes half a turn or less before you hear it hissing. Caution: the escaping air may be hot so keep a safe distance.
  6. When the hissing becomes quieter and finally stops, water will start leaking out. The radiator is now bled and you can close the air vent valve. This should be done quickly, otherwise too much water will leak out.
  7. If you shut off the circulation pump, don’t forget to turn it back on. Also check whether the water pressure in the heating circuit is still adequate.

Please note: you may need to carry out the bleed process more than once.


Bleeding the heating system in a rented apartment

As a tenant, you usually have the problem of no access to the boiler room. However, you do not have to turn off the circulation pump to successfully bleed the heating system. Simply proceed to bleed the radiators as described above, finishing with the radiator that is furthest away from the heat generator. 

Important: as a tenant, you are entitled to bleed the heating system. However, any further heating system maintenance measures are the responsibility of the landlord or the property management company. If the heat is not distributed properly even after bleeding, first contact the landlord who will then enlist the services of a heating engineer. 

Follow-up work after bleeding the system

Even if you have been able to bleed the heating system successfully, there are still a few things you need to consider afterwards. One particularly important task is to check the water or system pressure. When you bleed the heating system, some water is always lost. If it is a substantial amount, the water pressure in the heating system may drop. This can be seen on the pressure gauge. You may then need to top up the heating water. It's also worth doing a follow-up check. The heating system should be run for at least one hour after bleeding. The radiator furthest away from the boiler must then be checked again. If no more air escapes here, you have successfully bled your heating. 

Important: if problems continue to occur even though bleeding was successful, please contact a Viessmann partner who can inspect the system more closely on site. 


Does the heating system remain cold even after bleeding?

Sometimes the heating system remains cold or only gets half warm, even though it has been bled successfully. This may happen due to a variety of causes and is best determined for each individual case by an expert on site. For example, hydronic balancing may be necessary or the thermostatic valve may be stuck. It's also possible that other settings may need to be performed on the heating control unit. 

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