Do I need to top up the pressure in my boiler?
Updated: May 19
You may need to check and top up the pressure in your central heating system regularly. Find out why, and use this guide to learn how to top up the pressure in your boiler.
Older heating systems may have used a small storage cistern in the loft to supply water into the heating circuit. But most modern (and some older) heating systems are completely sealed and need to the pressure topped up manually.
What is boiler pressure and why does it matter?
The pressure shown on your boiler is determined by the volume of water in your heating system. It’s essential to get this right to ensure your heating system runs correctly and that components are not put under undue strain resulting in premature failure.
What pressure should my boiler/heating system be at?
System and combination boilers usually have a pressure gauge built into the appliance itself to indicate the pressure within the heating system. As a general rule, heating systems should run at a pressure of around 1 bar (when cold). It’s normal for this pressure to increase slightly when the system gets hot, but if the pressure in your heating system is over 2.5 bar, you may already have a fault, or a fault might be developing.
When should I top up the pressure in my boiler/heating system?
The pressure should be checked periodically, checking it a few times a year is usually sufficient but there is no harm in checking it more often. When the pressure in a heating system falls below 1 bar, it needs to be topped up. This is done using a filling loop. Filling loops come in various designs but the most common is called a conventional filling loop and looks like this:
Can’t see something like that on or around your boiler? Filling loops on modern combination boilers are often built into the appliance, in which case you’ll need to consult the manual, the appliance manufacturer or your service engineer to check how the system can be re-filled.
Step-by-step: topping up boiler pressure
When using a conventional filling loop to top up your system you must first ensure that the system is cold and the boiler is turned off. You can then add water (and therefore pressure) by turning the lever(s) or flat headed screws (this depends on the design of the loop itself) located at either end of the flexible braided hose. The levers or screws should be turned individually. They will likely only turn a quarter of a rotation from the off to on position, and you should observe the pressure gauge while you are turning each one.
TOP TIP: As with most valves used in plumbing, when the lever (or screw) is in line with the pipe onto which it is installed, this indicates it’s fully on. When the lever or screw is perpendicular to the pipe, it’s fully off.
Slowly allow water into the heating system until the pressure gauge on your boiler reaches 1 bar. The lever/s (or screws) should then both be turned fully off again to prevent the system pressure from getting too high.
Once the system pressure is at 1 bar, bleed the radiators throughout the system and top up again as necessary (you might need to do this process multiple times). Once air has been removed from all the radiators and the system pressure is at 1 bar, the boiler can be turned back on and normal operation resumed.
If you accidentally overfill your heating system, contact your service engineer to remedy the issue.
How often should I top up my boiler pressure?
It’s worth noting that although it is normal to top up a heating system a few times per year (four or five is reasonable), if you find that you’re having to do this on a regular basis, it may indicate a leak in the system or a fault with you boiler. If this is the case, contact your service engineer to investigate the cause of the issue.
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