Low boiler pressure is one of the most common reasons heating stops working. Here's how to read the gauge, top it up safely and work out why it keeps dropping.
Gas Safe registered 25 years in the trade 12 years running the business
Written by Chris Gill, C Gill & Son Plumbing & Heating ·
What boiler pressure actually is
Most homes now have a sealed heating system, where the water in the radiators and pipes is held under a little pressure. That pressure helps the boiler circulate water properly, and the boiler has a sensor that stops it running if pressure falls too low, to protect itself.
This only applies to sealed systems — combis and system boilers. If you have a regular boiler with a tank in the loft feeding the heating, pressure works differently and the advice below may not apply. Your manual will tell you which you have.
What should the gauge read?
Always go by your boiler's manual, because manufacturers differ. As a general guide, many boilers like to sit around 1 to 1.5 bar when the heating is cold. It's normal for the reading to rise a little when the heating is on and the water warms and expands.
If the needle is well below the level in your manual, or close to zero, the boiler may lock out and display a fault code. If it's very high — into the red zone — don't keep topping up; that points to a different problem that needs an engineer.
How to repressurise, following your manual
Most sealed systems have a filling loop: either a flexible silver hose with two small valves under the boiler, or a built-in key or lever on newer models. The steps generally go like this, but follow your own boiler's instructions first:
Turn the heating off and let the system cool down.
Check both ends of the filling loop are firmly connected.
Slowly open the valves (or turn the key) and watch the gauge rise.
Close the valves once it reaches the level in your manual.
Reset the boiler if it has locked out, then turn the heating back on.
Common causes of pressure loss
Topping up once after bleeding radiators is normal. If it keeps dropping, water is leaving the system somewhere. These are the usual culprits:
Recently bled radiators — letting air out also lets pressure out, so a top-up afterwards is expected. See our cold radiator guide.
A small leak on pipework, a radiator or a radiator valve. Look for damp patches, stains, green or white crusting on joints, or drips under the boiler.
The pressure relief valve letting by. It's designed to release water if pressure goes too high; if it's faulty, it can drip outside through a small copper pipe.
A failed expansion vessel. This tank inside or near the boiler absorbs the water's expansion as it heats. If it loses its charge, pressure swings high when hot then drops when the relief valve releases water.
Quick checks you can make
Walk round the house and look at every radiator valve and visible pipe joint for signs of water. Look outside for the small copper pipe that comes from the boiler through the wall — if it's dripping or there's a damp mark below it, mention that when you call. Make a note of the pressure when cold and hot; a big rise when the heating is on can point to the expansion vessel.
When it needs an engineer
If you need to top up more than every few weeks, or the pressure drops right after you repressurise, it's time for a proper look. A relief valve or expansion vessel needs a Gas Safe registered engineer, and a hidden leak under floors can cause damage if it's left.
Book a visit under our boiler repairs service for this kind of slow fault. If the boiler has stopped altogether and you can't get it going again, see emergency boiler repair for safe checks and how to reach us.
Safety notes
Never remove the boiler's casing to look for a leak — only a Gas Safe registered engineer should work inside a gas appliance. Don't leave the filling loop valves open, and don't overfill the system. If water is leaking near electrics, switch off the power at the consumer unit if you can do so safely, and if you ever smell gas, call the National Gas Emergency Service on 0800 111 999. For anything you're not sure about, contact us.
Common questions
Boiler pressure FAQs
Clear answers before you arrange a visit.
How often is it normal to top up boiler pressure?+
Occasionally, such as after bleeding radiators. Needing to do it every few weeks suggests a leak or a faulty component.
Why does my pressure go up when the heating is on?+
Water expands when heated, so a small rise is normal. A large rise into the red can point to a failed expansion vessel.
Where is the filling loop?+
Usually under the boiler — a flexible hose with valves, or a built-in key or lever. Your manual shows where it is on your model.
Water is dripping from a small pipe outside. Is that bad?+
It may be the pressure relief valve discharging. Mention it when you call, as it often points to a valve or expansion vessel fault.
Can low pressure damage my boiler?+
The boiler normally locks out to protect itself, but running for long periods with problems can lead to faults. Get repeated pressure loss looked at.
