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How to Drain a Combi Boiler?


Throughout your combi boiler’s lifecycle, it will need to be drained on several occasions. Draining a boiler may sound like a complex and lengthy process that’ll cost you a lot of money. However, this is not true as the process is relatively simple, and you can do it yourself if you have the relevant knowledge. Nevertheless, you should never attempt to drain your boiler if you don’t know how to do it safely.

In this guide, we will show you the procedure for safely and efficiently draining your combi boiler. While at it, we will explore the reasons why you may need to drain your combi boiler. But before then, let’s answer the question on your mind: can I drain my combi boiler myself?

Can I drain my combi boiler myself?

The answer is yes. By following the steps outline in this guide, you will be able to safely and efficiently drain your combi boiler. However, this job is best left to a professional heating engineer who will achieve perfect results.

Moreover, if you attempt to drain your boiler on your own without the necessary skills, you may end up damaging your boiler or even causing flooding in your home. And, of course, the manufacturer will most certainly void your boiler’s warranty, leaving you exposed should your boiler develop issues in the future.

Why should I drain my combi boiler?

Apart from the yearly routine maintenance, there are a number of other reasons which may require you to drain your combi boiler. Some of these reasons include:

  • The need to prevent your pipes from freezing, especially if you plan to be away from your home for an extended period of time during the cold months.
  • To clear sludge blockages in your boiler and heating system. You can also drain your combi boiler to minimise the chances of sludge forming in your heating system.
  • To minimise the risk of your home flooding if you plan to repair or alter your radiators or pipework.

How often should I drain my boiler?

Apart from the routine maintenance that requires you to drain your boiler once a year, you can slip in a one-off if you are faced with the above situations. However, note that draining and refilling your boiler frequently will cause your boiler to develop scaling and corrosion issues.

How to drain a combi boiler

Now the most exciting part you have been waiting for. The step-by-step guide to safely drain your boiler. But before we delve into that, it is advisable that you consult your owner’s manual for the procedure for draining your particular model. If you don’t have a physical copy of the same, try to access it online on your manufacturer’s website.

For this particular job, you will need the following items:

  • Hose
  • Bucket
  • Jubilee clip
  • Spanner
  • Radiator valve key
  • Towels or rags

That said, here is the general procedure for draining a combi boiler.

  1. Switch off your boiler and disconnect it from the power supply

First, switch off your boiler and unplug it from the mains. Once done, allow the boiler enough time to cool down. This safety measure will protect you from being scalded by the hot water during the draining process.

  1. Find the drainage valve.

Once you are sure your boiler has sufficiently cooled down, locate the drainage valve as it will be the centre of this operation. The drainage valve is usually found on the lowest point of the lowest radiator in your home.

If you can’t find this on one of the downstairs radiators, then it is most likely located outside the property. And if this is the case, there is cause for celebration as you won’t be running any risks of potential water spillage.

  1. Attach the hose to the valve

Attach one end of the hose pipe to the radiator valve and secure it in place using the jubilee clip. Lead the other end of the hose pipe to somewhere you don’t mind getting wet. The reason for securing the hose pipe to the drainage valve is to prevent the water coming out of your heating system from leaking and damaging your floor or carpet.

You can also place the bucket underneath the drainage valve to catch any little water that still manages to leak from the valve during the draining process.

  1. Open the drainage valve.

Using the spanner, turn the valve anticlockwise to open it. Once the drainage valve is open, water will start to flow out of your central heating system through the hose pipe. Note that it will take a couple of minutes for the water to drain out of the heating system.

  1. Open the bleed valves starting from upstairs radiators.

As the water continues to drain from the heating system, you should move your centre of operation and now focus on your radiators. Using the radiator valve key, open the bleed valves on the upstairs radiators of your home. Locating the bleed valves won’t be difficult as they are always found on the top left or right of every radiator.

To ascertain that a bleed valve is fully open, continue turning it until you hear a sucking noise coming out of the radiator. Bleeding the upstairs radiators will ensure that the water in these radiators make its way down to the lowest radiator and out through the drainage valve.

Once the upstairs bleed valves are all open, proceed to the downstairs ones and open them for a faster and more efficient system draining.

  1. Close all the open valves

Once the system has completely drained and no more water is coming out of the hose, it is time to tighten all the valves in your home, from the drainage valve to all the radiator bleed valves.

How do I refill my combi boiler?

After draining your boiler, the next logical step is to refill it and restore your home’s heating and hot water supply.  To refill your boiler, locate the filling loop underneath the boiler. The filling loop looks like a flexible silver hose and connects the mains water supply pipe to the central heating pipe.

On each end of the filling loop is a valve. Open both valves to allow water to flow into your heating system. As the water continues to flow in, keep an eye on your boiler’s pressure gauge. Once it reaches the recommended value of 1.5 bars, close the valves to prevent the system from over pressurising.

If the boiler pressure rises beyond 1.5 bars, there is no need to panic, provided it doesn’t shoot beyond 2 bars. This is because most boilers operate at pressures ranging from 1.5 to 2 bar.

What should I do if my boiler continues leaking after I drained it?

Despite following your owner’s manual draining procedure or the above steps to the latter, there is still a chance that your boiler or heating system will leak after a draining session. The leaking is often due to one or more radiators not being properly closed. And this is the reason why draining a combi boiler is a job best left to a certified heating engineer.

Anyway, once you notice the earliest signs of a leak after a draining session, however small it is, the first thing you should do is turn off your system’s mains water supply. Next, contact a professional heating engineer and be sure to give them as much information as possible. From the information, the engineer will narrow down the possible causes of the boiler leak and advise you on what to do.

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