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Why is my boiler losing pressure?

an installer walking through the door shaking hands with another man in the doorway

One of the most common reasons behind boiler problems is low boiler pressure. Over time a boiler can lose pressure due to leaks and boiler faults, which can lead to no hot water or central heating. It is therefore important for homeowners to diagnose when a boiler has lost pressure and how to resolve it.

What is boiler pressure?

Most of your central heating components carry a volume of water, but unlike older style systems your new boiler doesn’t use a water storage tank (header tank) in the loft. Instead of the header tank your system will use an expansion vessel.

Because water volume increases and decreases with temperature the expansion vessel absorbs the pressure changes (pressure rise) of water when your new boiler heats up the system water.

For boilers to operate efficiently and safely they need to operate within a set range (0.7 bar & 2.5 bar). If the pressure occasionally falls below the set range it can usually be topped up using a filling valve located on the system.

installer carrying out work on a boiler

How do I check if my boiler pressure is low?

If you suspect that your boiler has lost pressure, find your boiler manual and check the front panel on your boiler. Your boiler will have a pressure gauge that indicates whether the pressure is high, adequate or low. Modern boilers will often also have a screen which flashes an error code to indicate the cause of the problem. Check in your boiler manual to see whether the error code for your model corresponds with low boiler pressure.

The pressure gauge will show certain bars that indicate the levels of boiler pressure. If the boiler pressure needed or digital indicator is below one bar this means that your boiler has lost pressure. If the pressure bar is marked as high according to your user manual, then the pressure has increased above normal operating levels and can be lessened by bleeding the connected radiators. Your appliance or system has a built in safety device which automatically releases pressure preventing the pressure becoming dangerous. However, you should still bleed your radiators if the pressure is showing as too high.

What causes a boiler to lose pressure?

The most common reasons for a boiler to lose pressure are:

  • Releasing air or water from a radiator when bleeding it.
  • A leak from the appliance or on the system most commonly from radiator valves or system components.

The circuit of pipes and radiators connected to the boiler system can corrode or loosen over time, eventually allowing water to escape the system and gradually decrease the boiler pressure as water is replaced with air.

How do I find a leak in my heating system?

Although a leak can happen throughout the circuit of pipes and radiators inside the walls, it will most likely be found around the connections on radiators. Try to find signs of damp patches around the radiators. The pipework can become wet due to condensation rather than a leak, so make sure to carefully wipe the pipe clean and place a plate underneath it to check for any drips.

Also check for discoloured paint or flaking on both the pipework and skirting boards, as well as water stains on the floor or ceiling. These are signs of water damage caused by leaks. Do not attempt to repair loose or broken pipework joints unless you are trained to do so. Instead arrange for a specialist to make the necessary repairs and check to see if the boiler stops losing pressure as a result.

Can a boiler lose pressure without a leak?

If you have found no leaks and your boiler is still losing pressure, the cause may instead be a boiler leak, a hidden leak on the system.

  • Boiler leak

    Boiler leaks can occur when parts inside the boiler corrode or loosen over time. Unless a boiler is regularly serviced, boiler parts can loosen with age and cause the system to lose pressure, usually these leaks will be a visible drip from the underside of the boiler. If you suspect that a boiler leak is at fault, arrange for a Gas Safe Registered (GSR) boiler engineer to inspect your boiler. Do not remove the boiler casing yourself.

    They will then advise you on whether the part can be repaired or replaced, or if the entire system needs cleaning or replacing. Check to see whether your boiler is under warranty, as this could potentially save you hundreds of pounds in repair costs.

  • Faulty boiler part

    Boiler parts can also simply become faulty over time and cause boiler heating problems. As above, do not attempt to identify the faulty boiler part yourself. Instead, call a GSR engineer to remove the boiler casing and identify the part for you. They will then let you know what needs to be repaired or replaced.

installer demonstrating a boiler to a woman in her kitchen

How do I fix low boiler pressure?

While fixing the leak or faulty boiler part issue is the long-term solution, you can also increase the boiler pressure yourself using the filling loop attached to your boiler. This is typically silver with a flexible connection, with a handle valve and an isolation valve on each end of a flexible pipe connection.

Using a flat head screwdriver, turn the isolation valve anti-clockwise until the line on the valve is in line with the pipe. Then turn the handle valve anti-clockwise to allow water to enter the system. Monitor the pressure gauge to ensure that the pressure increases to the recommended levels, before closing both the handle valve and the isolation valve by turning them clockwise.

You will know that the boiler pressure is back at recommended levels because either:

  • The fault code has disappeared
  • Or the boiler is functioning and heating water

If you are uncertain about how to top up the system pressure of your boiler, do not hesitate to ask a family friend, neighbour for assistance as topping up the pressure is common knowledge among many homeowners. If no one is available to assist you consulting a competent person or gas safe engineer is another option but is likely to be chargeable.


  • You can check if your boiler pressure is low by checking the pressure gauge and consulting the boiler manual.
  • Check for leaks around radiator pipe connections. Common clues include water stains and paint discolouration on pipes and skirting boards.
  • Call a qualified engineer to inspect your boiler if you suspect an internal part is at fault.
  • Familiarising yourself with the topping up procedure could prevent your boiler from failing to operate buying you valuable time until a Gas Safe Engineer can inspect your appliance or system.

If your boiler is still leaking water, contact your local Vaillant installer