The most common reason for condensing boiler breakdowns – loss of pressure

Probably the most common reason for a condensing boiler to fail to provide heat is due to loss of pressure. The heating circuit on most condensing boilers is a sealed circuit. It is usually pressurised to one bar. Most condensing boilers are designed to stop providing heat if the pressure in the sealed heating circuit drops below half a bar, so it is important to maintain the system pressure if you want to have heat.

Loss of pressure may have been gradual and you may have heard water sloshing around the radiators for a week or two earlier, or found some radiators just will not stay bled, after bleeding them.

The first check you should do is to look at the pressure gauge. If pressure is down to zero, or close to it then loss of pressure is the reason that you have no heat. If the pressure is around one bar then there will usually be some other cause for the failure. You might have to tap the gauge gently to check that the needle has not got stuck.

Once you have established the pressure is too low or non existent then you have two options. You can call a qualified plumber, who will re-pressurise the system for you or you can attempt to solve the problem yourself.

Re-pressurising the system is a job that needs care and attention so if you have any doubts at all, you should call in a professional. If you are determined to have a go at doing it yourself then the following is a guide only, covering most common pressurised gas heating boilers.

  1. Turn off the boiler.
  2. Locate the filling loop which is the device for re-pressurising the system. It will be very near the boiler. The filling loop is usually made of a stainless steel hose, and is joined to copper pipe or may have one end or even both ends detached from a pipe. Connect any disconnected end – the place to connect it will be obvious. Do not over tighten the connection.
  3. Then open the valves at both ends of the filling loop, and open them slowly. You are now connecting the system to the mains water and mains water pressure will re pressurising the system. Watch the pressure gauge (usually located on the boiler) and when the pressure gauge and when the gauge shows just over one but less than one and a half turn off both valves and disconnect the hose.
  4. Go around the house checking each radiator and bleeding any air in any radiator that you might find.
  5. Go to the boiler and re check the pressure. If it has dropped then repeat from stage 2.
  6. Once you are satisfied that the pressure is set at just above 1 bar and the radiators are bled turn the heating back on, and you should now have a warm house.

If your system keeps losing pressure you should call in a heating engineer to investigate. There will be a small leak or two that the engineer will have to locate and fix. Provided this is done you should now have a warm and trouble free system.

If you still have no heating then there are two possible other reasons for this. First, the condensing boiler will have a pressure vessel, either contained within it or separately attached to the heating system. The vessel may be faulty and may need replacement or re-pressurising. Secondly the system will have a pressure reducing valve, which may be faulty. These faults should be rectified only by qualified people.

25 Responses

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  3. When/if my plumber arrives to fix loss of pressure to zero I shall show him your instructions. Although he has all the qualifications he’s never heard of a pressure vessel. Olga

  4. Correct me if am wrong but a damage eat exchanger or leaking one can also result in the lost of pressure which can only be seen once removing the condensing pipe ?

  5. thank you for this info. British gas here yesterday service done to their service agreement. this am no hot water or heat B.G REFUSED to come out. IM Bald now. But maybe your instructions may save the day.

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  8. Hey There. I found your blog using msn. This is a very well written article.

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  9. pressure gauge above 3.5? bolier off less than zero… bleed radiators with boiler off, this correct?

    • Yes. 3.5 is too high, so you need to reduce pressure to whatever level your boiler manufacturer recommends.

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