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How to drain a combi boiler

Throughout your boiler’s lifecycle, it will require regular drainage. But, don't fret about scary bills or lengthy processes; it is a relatively simple job. Plus, you shouldn’t drain your boiler without knowing exactly how to do so safely!

Here we present a run-through of how to drain a combi boiler. However, if you are unsure of anything do not take any risks, seek the help of a professional.

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Why do I need to drain my boiler?

There are a number of reasons why you may need to drain a combi boiler. 

These include:

  • To prevent pipes freezing. If you’re planning to be away from your property for an extended period of time, it’s worth draining your boiler to prevent pipes from freezing.
  • If you’re about to alter radiators or pipework and want to avoid flooding.
  • To reduce chances of sludge - in this case you can have a chemical flush carried out on your whole heating system. If you already have sludge, you will need to bleed your radiators.

Should you drain your boiler yourself?

While you may be able to drain and refill your boiler yourself by following the steps below, it may be better to call out a professional engineer if you’re a novice or still unsure.

If you try to drain a combi boiler without the relevant skills, you could damage your whole heating system and cause flooding in your home.

Boiler need some TLC?

Let our experts take care of it with an emergency repair

Our Boiler Plan Gas Safe engineers will handle the job and point out any other work that needs to be carried out to keep your heating running smoothly and efficiently.

With our 24-hour emergency repair and 95% first time fix rate, we’ll have your boiler drained and your heating up and running again in no time.

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How often should I drain my boiler?

On average, you should aim to drain and repressurise your combi boiler once a year. If you’re needing to drain and top-up the unit more than this, it’s likely that your central heating system has a leak. If so, have it looked at promptly by a gas safe engineer.

Draining your boiler more than just once a year is not a good idea. The over-drainage will likely cause your unit to suffer scaling and eventual corrosion.

How to drain a combi boiler safely

The method needed to carry out the drainage will depend largely upon the model of your unit. Therefore, your first port of call should be to consult the manual. If you can’t find the physical copy, it is likely available online.

When researching how to drain a boiler, you should consult your model’s instructions carefully.

However, the process for draining a boiler is usually as follows:

  1. Before you start any of the draining, SWITCH OFF THE BOILER AND DISCONNECT IT FROM THE POWER SUPPLY.
  2. Wait until your hot water has fully cooled after switching off the unit and disconnecting it from the mains,
  3. Connect a hose into the drainage valve (typically the lowest point of the lowest radiator in your home).
  4. You then need to open the vents (bleed valves) of all the radiators in your home to ensure that the system drains properly. 
  5. Now use a wrench or spanner to open the drainage valve. Water should now be flowing out from the hose. 
  6. Once all the water is gone, these valves must be closed/disconnected.

Draining your boiler could be necessary if you’re having other maintenance work carried out to the central heating system or pipe work.

How do you refill a combi boiler after draining it?

  1. Ensure that the drainage valve is closed and you have removed the hose.
  2. Open the filling loop to repressurise to its recommended level.
  3. Eliminate any air trapped in the central heating system by bleeding all your radiators moving from those downstairs upwards.
  4. Open the filling loop again to correct the pressure level. 
  5. Switch the boiler back on.

If you are struggling with any part of this process, do not attempt the job yourself as there is a risk of damaging your whole central heating system if things go wrong. Save yourself time and money by opting for an engineer to carry out the necessary work or repairs.

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Take out a boiler care plan

With a chemical flush costing anywhere between £300 and £700, it’s worth protecting your boiler against this and any other unforeseen and expensive maintenance and repair work. With our range of affordable care plans, you’ll never have to sit in the cold again.

Whatever boiler cover plan you choose, each comes with an annual service to ensure your system is fully maintained and ready to keep your home reliably warm.

Choose from:

  • Service PlanBoiler service every year to ensure it’s working correctly
  • Gold PackageIncludes: 3 call outs per year, annual service, carbon monoxide test.
  • Platinum PackageIncludes: Unlimited call outs, all parts and labour, repairs to the entire heating system, £500 towards a replacement boiler.
  • Boiler Plan Plus Package -For those with a Boiler still in warranty. Includes: Unlimited call outs, all parts and labour, repairs to the rest of your entire heating system.
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