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Boiler Repair

How to repressurise your boiler

Lack of heating and hot water? Immediate problem or do you simply want to know what to look out for with your boiler pressure? Our simple and helpful guide explains what boiler pressure is, what causes the fluctuations, how to check the pressure and ultimately how to repressurise your boiler.


What is boiler pressure?

A boiler is an essential and core component of any home. It is responsible for heating cold water including that which flows around the pipes and radiators in your home.

To function correctly a boiler needs a balance of water and air, to ensure that the pressure in the system is adequate for when water, for heating or hot water, is demanded from the boiler. Too little pressure, caused by not enough water within the sealed system, will result in an inadequate flow of water to heat radiators or water. The water pressure needs to remain at a stable level. A drop or even a rise in pressure needs to be acted upon.

What causes a drop in boiler pressure?

Boilers ultimately lose pressure over time, which is nothing to be worried by. However, there are two main reasons why your pressure might otherwise fall. If you have recently bled your radiators, this may well have lowered the pressure when the air was released. This is quite normal and not something to worry about. You simply need to check your boiler pressure and repressurise accordingly (see the steps below).

The other reason for a drop in pressure is a little more concerning. A leak. Even a small leak if unnoticed could lead to a gradual drop in pressure over time. Have a good look around your home and look for signs of any dampness around radiators, pipes and of course the boiler itself (do not look inside the boiler though – only a Gas Safe Registered engineer such as Aren Heating & Plumbing should do this). If you do find a leak, we can come and fix this for you and get you flowing back to normal again.

How to check the boiler pressure?

You can normally check the pressure by simply looking at the water pressure gauge on the front of your boiler. Ideally the pressure wants to be in the green area, usually between 1 and 2 bar. If the pressure is below 1 bar, then you need to repressurise boiler by using the filling loop and adding water to the system.

It is important to also be aware when there is too much pressure. If the pressure shows above 2.75 bar, then this may mean you need to bleed a radiator to bring the pressure back down.

What to do if the pressure on the boiler is low?

Depending on your type of boiler, repressuring will either be via an external filling loop or a repressurise key (also known as an ‘internal filling loop’). We have provided the instructions for both types below.

External Filling Loop

Filling loops do vary in design, but here are the basic repressurising steps to follow:

  1. Identify the filling loop, usually a flexible pipe with a valve at either end.
  2. Open both valves, to allow cold mains water into the system (you should be able to hear it).
  3. Wait for the pressure gauge to read 1.5 bar, just inside the top of the green area of the gauge.
  4. Close both valves, one at a time.

Repressurise Key (Internal Filling Loop)

  1. Switch your boiler off and allow it to cool down. Ideally allowing it to rest for 4-6 hours.
  2. Find the concealed tray underneath the boiler and locate the key and slot.
  3. Then, insert the key into the slot, making sure it lines up with the ‘open padlock’ signal.
  4. Then firmly push the key in and turn to the ‘closed padlock’ sign.
  5. Turn the white nut counter-clockwise (lefty-loosey) so that water flows into the boiler, you should be able to hear this.
  6. Keep this going until the pressure rises to 1.5 bar.
  7. Turn the white nut clockwise (righty-tighty) until the water stops.
  8. Turn the key back to the ‘open padlock’ sign. Be aware that some water may come out at this point. This is quite normal and nothing to worry about. (However, if it continues to come out, check the white nut is tight)
  9. Turn the boiler back on.

There is a slight variation of the Internal Fitting Loop that does not require a key. Simply, pull down the blue lever until the pressure is back over 1 bar.

Need further help?

If you are in doubt or have tried the above to no avail, our experienced and knowledgeable Gas Safe registered engineers are well equipped to take on the pressure and get you back to normal operation.

Have any questions or would like to discuss something with our team? Please head to our Contact page and fill out the online form or alternatively call us directly on 020 8287 7194.

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