Why is my boiler losing pressure?

There could be numerous reasons why your boiler keeps losing pressure. A drop in boiler pressure could be caused by a hidden leak, but there are also other variables which can affect the heating system and subsequently disrupt boiler water pressure.

The different causes of low boiler pressure

What causes a boiler to lose pressure? Let’s take a look:

Radiators with too much air in them

Air bubbles developing in your radiators can prevent heat building up in your home. The bubbles
stop hot water from circulating efficiently, which limits the level of warmth you would expect from
your central heating system. These bubbles can develop for a variety of reasons, from incorrectly
topping up the boiler to your system being full of sludge.

Leaking solder joints

A weeping solder joint indicates there is a weakness in the pipework. This is more common in older
heating systems, as the joint connection can become vulnerable and weak over time. Even the
smallest of leaks from solder joints can cause a change in boiler water pressure and can damage
your home.

Leaking radiators, towel rails and valves

Leaking radiators, towel rails and valves will be visible to the eye and are one of the most common
explanations for a drop in boiler pressure. If you spot moisture or water on or around the radiator
valves you have likely identified the problem.

Faulty pressure relief valve

If your boiler’s pressure relief valve is faulty it will not shut off correctly and instead release
pressure. This leads to inconsistent and uncontrolled water pressure, which prevents the boiler
operating sufficiently. You can sometimes see whether the pressure relief valve is failing if there is
water escaping from the pressure relief drain.

Leaking auto air vents

Leaking auto air vents can be recognised if they are weeping with moisture or water, or stained with
limescale or rust. The role of an auto air vent is to keep central heating systems from retaining air, so
a leak would contribute to lower boiler water pressure.

Faulty expansion vessel/tank

An expansion vessel or tank is designed to protect water heating systems from building too much
pressure. If it is faulty it will be counterproductive and cause excessive boiler pressure.

Faulty pressure gauge

The pressure gauge indicates the live boiler pressure, but if it is faulty the reading will be wrong. This
is something you would not be able to determine yourself. To verify the gauge is faulty, a
professional would likely have to rule out all other reasons for the boiler’s change in pressure.

A low boiler pressure is a hindrance to any home or business owner, but there are of course always
ways to resolve the problem.

What can you do if your boiler pressure is low?

If you do recognise any of the problems above, you likely need to seek expert assistance.
That said, there are some quick fix actions which could provide you with a solution without the need
for professional help.

Repressurise the system

First you need to ascertain if you have a problem. Boilers will naturally lose a small amount of
pressure over time. If you find yourself topping the boiler up once or twice a year this is normal.
Anything more frequent and you may have a problem.

Bleed your radiators

Bleeding radiators is a simple solution to releasing air in central heating systems. This works better
when the boiler is running as the action shifts the air round to the top of the system (generally). Yet,
if the system has been drained down, it is advisable to bleed radiators whilst the boiler is off as
otherwise the air in the system can damage the pump, or it can run dry.

Use a bleed valve key to loosen the radiator valves (turn anti-clockwise up to half way). Prepare for
some moisture to leave the valves by using cloths and dry towels to protect clothing, furniture and
flooring. A hissing sound will indicate air is escaping from the radiators. When the hissing stops and
there is no more water present, re-tighten the valve. Turn the heating back on to test whether this
exercise has been a success. If there is still not enough heat, there may be another underlying

Tighten up radiator valves

Opposite from bleeding your radiators, there may be some loose connections which are causing
leaks and a drop in boiler pressure. Inspect your radiator valves and tighten them carefully if they
are weeping and evidently loose.

When to get professional help

Detecting a leak may ultimately require expert help if your boiler keeps losing pressure or there is no
water pressure in the boiler at all (and you’re unable to top the system up). Polygon can help to
provide clarity, source the leak and fix the problem.

How Polygon detect and test low boiler pressure

Polygon uses a variety of non-destructive leak detection methods.

Thermography – the use of thermal imaging cameras. An in-depth thermographic survey of
the property will:

  • Highlight hot or cold spots from leaking water
  • Pinpoint areas for further investigation.

Tracer gas - to drain the heating system and re-pressurise your pipework (a mix of 95% nitrogen and 5% hydrogen). Gas sniffers are then utilised to detect where the gas is escaping from the pipework. Tracer gas can also be used as a simple pressure test of the heating pipework over a period of time (with the boiler isolated) – as this can help diagnose whether there is a boiler issue.

A moisture survey - to pick up signs of water damage and identify the escape of water.

Fixing your boiler pressure

Once the cause of the leak in the boiler has been recognised, the Polygon technician on site will
determine whether the repair is something that can be dealt with during the visit. In the unlikely
event we cannot repair the leak, Polygon will deliver a comprehensive report analysing the best
approach to resolve the problem. This can be passed directly on to your plumber or insurer for

Losing boiler pressure? Give us a call - we can fix it: 0330 332 2958
We always offer competitive prices, so get in touch now.