How To Repressurise A Boiler: Step by Step

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Last updated: October 31, 2023

If your central heating system is not working as efficiently as it should, it could be due to low water pressure in your boiler. Insufficient heat in your radiators or lack of hot water are both signs of a problem with your boiler pressure. It may sound serious but re-pressurising your boiler and bringing your heating back on track is not as difficult as it seems. In fact, repressurising a boiler every so often is part of good maintenance to keep your boiler and central heating system in tip-top working order. And you don’t necessarily need a gas engineer.

We take a step-by-step look at what you need to do, when it comes at repressurising a boiler.

Why do I Need to Repressurise my Boiler?

Your combi boiler should be running at an operating pressure of between 1 and 2 bar, with 1.5 the ideal. A correctly pressurised boiler ensures your central heating and hot water are working at an efficient level and indicates there are no problems or faults with the system. A pressure gauge reading of below 1 bar indicates low boiler pressure due to lost water in the system or a leak somewhere and will be felt in lack of heat in the radiators and your water not quite running hot.

What is a Boiler Filling Loop?

To repressurise your boiler you need a filling loop, which temporarily connects your boiler to the mains cold water pipe so you can top up the heating with the necessary amount of water needed to pressurise the boiler system. Boilers have either an external or internal filling loop, depending on the model, with the internal filling loop coming as a keyed or keyless design.

How to Repressurise a Boiler with an External Filling Loop

An external filling loop is a hose on the outside of your boiler that is braided with compression fittings on both ends. One end attaches to the cold water system valve and the other to the boiler’s pressure relief valve. Here’s how to repressurise a boiler using an external filling loop:

  • Switch off your boiler and allow it to cool
  • Attach the hose securely to the cold water valve and the boiler valve
  • Once the filling loop is secured, open up the two valves, starting with the cold water and then the boiler
  • You should hear the cold water entering the heating system, which will be needed to increase the pressure
  • With the valves open, watch the indicator needle on your boiler’s water pressure gauge
  • Once the needle has reached 1-1.5 bar (or the recommended pressure as indicated in the boiler’s manual), make sure you securely close both valves, one at a time. The valves need to be tightly closed, otherwise they can leak over time
  • Switch your boiler back on and, depending on the model, press the reset button
  • Now you can remove the filling loop and your boiler is repressured
  • Clean the ends of the filling loop and store it safely and close to your boiler

Repressurise a Boiler with a Keyed Internal Filling Loop

Most combi boilers typically have an internal filling loop, as it is integrated inside as part of the combi system. If you have this type of system, you should also have a filling key, which may be located underneath the boiler. To repressurise your boiler with a keyed internal filling loop, you need to:

  • Switch your boiler off and allow it to cool
  • Locate where the key needs to fit, which does depend on the make and model, so always refer to your manufacturer’s instructions
  • Insert the key into the key slot, ensuring it lines up with the arrows or the ‘open padlock’ symbol
  • Push the filling key firmly and turn to the closed padlock sign – make sure the key is pushed fully down to do this
  • Now turn the white tap next to the key slot to let water to start flowing into the system
  • Watch the boiler’s pressure gauge until it reaches the required bar level for your heating
  • Once the boiler is repressured, turn the tap back until the water stops
  • You now need to open the loop, so turn the key back until it lines up with the open padlock or open arrows.
  • You can now turn your boiler back on

Repressurise a Boiler with a Keyless Internal Filling Loop

A keyless internal filling loop is one of the easiest ways to repressurise your heating system. If your appliance has this type of system and you want to rectify low pressure in your boiler, you need to:

  • Locate the keyless filling link – typically a blue lever at the bottom of your boiler
  • Now simply pull down the lever and watch your boiler’s pressure gauge
  • The indicator needle should slowly rise as more water comes back into the system
  • Keep the lever open until the gauge reads the correct pressure in bars for your boiler
  • Now release the lever to stop the repressurising
  • Your boiler should be back on track!

What To Do If You’ve Repressurised Your Boiler Too Much

It is possible to put too much water into your system, and so repressurise your boiler too much. High pressure in a boiler will also cause issues with your heating system so if you had over-pressured and the boiler pressure is too high, you may need to bring it back to the normal zone. And the easiest way to do this is to bleed the excess water out of your radiators. For this, you need your radiator bleed key, as well as some towels or bowls to catch the bled water. Now open the bleed valve slowly to let the water drip out and release the pressure. Then check your boiler gauge to watch the pressure rise back to the desired 1-2 bar level.

However, if you are in any doubt, or bleeding your radiators does not rectify the issue, always call out a registered engineer who will be able to assess the situation and do any necessary repairs.

How Often Should You Repressurise a Boiler?

How often you need to repressurise a boiler is not set in stone, and it should depend on your own combi boiler and central heating system. However, any boiler should be repressurised if its pressure drops below 1 bar. A pressure of less than 1 bar indicates water has been lost from the system and so it needs repressurising to ensure that water has been replaced.

And while occasional repressuring of a boiler should be part of its ongoing care and maintenance, if you find you are having to do it more often than you think you should, then this could be a sign that your boiler is actually leaking.  And this is where you need to get advice from a gas safe heating engineer to ensure you get your boiler professionally repaired and the pressure problems safely resolved.


  1. How to repressurise a combi boiler – Viessmann
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