How to drain and refill central heating system with a combi boiler?

What is a combi boiler?

Combination boiler or combi boilers are the most commonly type of boilers which are used in UK for a reason. Unlike the system and conventional boilers which requires a hot storage cylinder, the combination cylinder is an all in one unit which produces hot water for taps and central heating. Also it saves a great space in your homes and its installation is easy and faster comparative to any other boiler.

It usually takes the water from the main supply and results in stronger water pressure. It provides you plenty of hot water. Thus, no longer waits for the tank to refill again and again.

Modern high efficiency unit boilers are the stainless steel condensing boilers with its built-in heat exchanger to provide hot water to a home or a building. During the cold season, combi boiler runs when needed to heat the structure and provide hot water to various hot appliances. Whereas in hot summer days, combi boiler works whenever there is a need for domestic hot water.

There are two main reasons behind the combi boilers great and powerful efficiency i.e. these units have high operating efficiency and low internal water volume. This means when in summer days you don’t need space heating you are only paying to heat hot water which is domestically required.

Sometimes it happens that we need to drain and then refill the system. You might be thinking of how to drain and refill central heating system with a combi boiler?

  1. Before you start the process keep in mind to turn off the boiler and disconnect it from the power supply. So that cut off couldn’t be there.
  2. Now let the system water to cool down and reduce the overall pressure.
  3. During the draining system it I important to maintain the minimal temperature of the boiler

During the drainage of the boiler

  1. First locate the drainage point in the system, usually it is located at the lowest point of the lowest radiator in the property.
  2. Once found the drainage point, connect a hose securely using a jubilee clip.
  3. Tighten the clip over the hose to prevent the leakage
  4. Now move the other end to the hose of the drainage point.
  5. Ensure that every valve on each of the radiator is open to know that system is drained completely.
  6. Open the drainage head with the help of spanner or wrench.
  7. You should see water flowing out of the hose.

Note – you can speed up the replacement of water with air by opening the vents at the top of the radiators.

During refilling of the boiler

  1. Firstly close all the air bleed valves of your radiator and then close the drainage valve and remove the hose.
  2. Find the braided filling loop beneath your boiler. This connects the cold feed and the return feed.
  3. Now open the filling loop and allow the boiler to refill to the recommended level by the manufacturer.
  4. Open the bleed valve of each radiator and allow the air to escape.
  5. Once you have released the air from the radiator, return back to the filling lop and top up the system’s pressure.
  6. Also keep a check there should not be any leaks in your boiler systems.
  7. As soon as the boiler heats up, you need to bleed the air-bleed-valves. This process has to be repeated several times so as to ensure all the air is removed from the system.
  8. Now top up the pressure as required.
  9. The water temperature increases connections causing water to escape.


These steps may seem very easy to perform but in practice it can be difficult. So, begin with the process once you are sure about what you are doing else contact an engineer for the same.

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