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How to top up your boiler pressure

Updated: Feb 12

gas central heating filling loop
External filling loop

One of the most common boiler faults is low boiler pressure. It is normal for the central heating system to lose pressure over a period of time. If you have bled your radiators recently, pressure could be lost due to that. It may be that there is a leak somewhere in your heating system or your expansion vessel might need recharging or replaced.

Sometimes leaks can be so small that they’re unnoticeable. It is recommended that you check your boiler pressure at least once a month to help reduce the chances of unexpected boiler breakdowns.

Your central heating boiler must have an operating pressure between 1 bar and 2 bar. Being at this pressure keeps the central heating boiler and central heating system free from air. If the pressure drops below the normal boiler pressure level, a safety switch is activated that cuts the boiler off - this is done to prevent damage to the central heating system and boiler. It also prevents both the central heating system and boiler from running dry.

If your boiler has low pressure, you can top up your system and have your boiler working within a few minutes by following the steps below:

  1. Look for the filling loop under the boiler. It will connect between 2 pipes. There are a few variations of filling loops, and for your reference we’ve included an image of a common filling loop below

  2. On each end of the filling loop there will be a valve which should be closed. This can be checked as the valve handle has to be positioned horizontally across the Valve .

  3. Are the valves  in the same position? If they are not, operate them so they are both horizontal.

  4. Open one valve fully and then open the second valve slowly. Keeping your hand on the valve, you will hear the water entering the boiler. Watching the pressure gauge, you will see the black needle rising. Ideally the pressure should be between 1-1.5 bar depending on the manufacturers instructions,

  5. The boiler pressure gauge will normally be located on the front panel or underneath the boiler. The pressure gauge may also be digital on certain boilers, this can be found by pressing the information button continually until the screen displays a bar reading. 

We have included some helpful links on topping up the pressure for some of the most common boiler brands. If your boiler is not listed, enter your boiler make with the phrase  “how to top up boiler pressure.” For example: “ATAG boiler how to top up boiler pressure.”

Topping up pressure on a:

Worcester Bosch filling loop
Worcester internal Filling loop

Please note that if you use the following links from a third party, Surrey & Sussex Heating Ltd will not accept responsibility for the information given. If you are unsure or are not confident on how to top up the boiler pressure and have a service plan with us call Surrey & Sussex Heating Ltd on 0800 772 0032 or email at and a member of the team will be happy to help.

For information about our service plans and annual servicing, click here.

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