Boiler losing pressure: causes and what to do
Most boilers need a pressure of about 1 to 1.5 bar to work. If yours keeps dropping, water is getting out somewhere, or a part that controls the pressure has worn. You can top it up yourself, but if it keeps falling, the cause needs finding.
Updated 02/11/2026 · Express Contractors
Topping up the pressure gets the boiler going again. It doesn't fix the reason it dropped. If you top up more than once or twice a year, you'll need a boiler repair from a Gas Safe engineer to find the fault. This guide is part of our common boiler problems series.
What should my boiler pressure be?
On most combi and system boilers, the gauge should read about 1 to 1.5 bar with the heating off and cool. It rises a little when the heating is on. Your boiler manual gives the exact figure for your model.
Combi and system boilers run on a sealed system. That means the water in your pipes and radiators is kept under a little pressure. The boiler shows it on a gauge or on its screen, in "bar".
| Gauge reading | What it means | What to do |
|---|---|---|
| Below about 1 bar | Too low. Many boilers stop and show a fault code. | Top it up (steps below) |
| 1 to 1.5 bar, heating off | Normal for most homes | Nothing |
| A bit higher, heating on | Normal. Hot water takes up more room. | Nothing |
| Jumps a lot when the heating comes on | The expansion vessel may have lost its air | Call an engineer |
| Around 3 bar | Too high. A safety valve lets water out. | Check the filling loop is shut (see below) |
Some gauges have a green zone to make it easy. If you have an older boiler with a tank in the loft, it may not have a gauge at all.
Why does my boiler keep losing pressure?
Boiler pressure drops when water gets out of the system, or when a part that controls the pressure wears out.The usual causes are a leak, bleeding radiators, a worn relief valve or a tired expansion vessel.
- A leak. Even a slow drip lets pressure out. Look for damp under radiator valves, around pipe joints, under the boiler and on ceilings below pipes. Some leaks are hidden under floors. See our guide to a leaking boiler.
- Bleeding the radiators. Letting air out lowers the pressure. This is normal. Just check the gauge afterwards and top up.
- The pressure relief valve. This is a safety valve. It lets water out if the pressure gets too high. It drains through a small copper pipe, usually out through the wall. If that pipe drips, the valve may be worn. Or the pressure may be rising too high.
- The expansion vessel. This is a small tank with air on one side. It soaks up the extra pressure as the water heats. Over time it can lose its air. Then the pressure jumps when the heating comes on. The relief valve lets water out, and the pressure drops low again once it cools.
If you think a small leak is coming from a radiator valve, our plumber page covers radiator and valve repairs.
Safety first
Smell gas? Call the National Gas Emergency line on 0800 111 999 straight away. Open doors and windows. Don't use light switches or anything that could spark. Turn the gas off at the meter if you can do it safely, then go outside.
Headaches, dizziness, sickness or a lazy yellow flame? These can be signs of carbon monoxide (CO), a poison gas you can't see or smell. Turn the boiler off, open windows, get everyone outside and call 0800 111 999. See a doctor if anyone feels unwell. Our carbon monoxide advice has more.
Never open the boiler case. Only a Gas Safe registered engineer should take the cover off a gas boiler. The checks in our guides are all ones you can do from the outside.
How do I top up my boiler pressure?
To top up boiler pressure, turn the boiler off and slowly open the filling loop valves while you watch the gauge.Close both valves fully once it reaches about 1 to 1.5 bar. The filling loop is usually under the boiler.
A filling loop is a short silver hose with a valve at each end. Some boilers have a built-in tap or a lever instead. Check your manual first, as each make is a bit different.
- Turn the boiler off and let it cool down.
- Find the filling loop and make sure both ends are attached.
- Open the valves slowly. You'll hear water hissing into the system.
- Watch the gauge. Stop at the pressure your manual says, usually about 1 to 1.5 bar.
- Close both valves fully. This step matters. A valve left open can push the pressure too high.
- Take the hose off if your manual says to. Keep a towel handy for drips.
- Turn the boiler back on. You may need to press reset.
Keep an eye on the gauge for the next few days. If it stays put, you may have just lost a little water when you bled a radiator. If it drops again, the fault is still there.
Is it OK to keep topping up boiler pressure?
Topping up once or twice a year is fine. Regular top-ups hide a fault, and they can cause new ones. Each top-up adds fresh tap water, which thins out the liquid that protects your system.
That liquid is called an inhibitor. It slows down rust and sludge inside the pipes and radiators. As it gets weaker, sludge can build up. Over time, that can lead to cold radiators and worn parts.
When to call a Gas Safe engineer
Call a Gas Safe engineer if you top up more than once or twice a year, or the pressure drops again within days.These signs point to a leak or a worn part that needs finding.
- you need to top up more than once or twice a year
- you can see water leaking from the boiler or its pipes
- the small copper pipe outside keeps dripping
- the pressure jumps a lot when the heating comes on
- the pressure drops again within a few days of topping up
These faults often need a part inside the boiler checking or replacing. Only a Gas Safe engineer should do that. The engineer first runs a thorough check to find the fault and the parts needed. See our boiler repair page for the faults we fix.
A yearly boiler service can catch worn parts early, before they cause a breakdown.
What if my boiler pressure is too high?
If the gauge reads close to 3 bar with the heating off, the pressure is too high. A common cause is a filling loop valve that wasn't closed fully.
- Check both filling loop valves are shut.
- With the heating off and cool, bleed a radiator. This lets out a little water and brings the pressure down.
- Check the gauge again. If the pressure keeps rising, call an engineer.
Other boiler guides
- Heating works but no hot water: low pressure is one cause, but not the only one.
- Frozen condensate pipe: why boilers stop on freezing days.
- Cold radiators: what cold at the top or bottom means.
- Leaking boiler: what to do first, and where the water is coming from.
- All common boiler problems.
Still losing pressure? Book a boiler repair and we'll find the cause.
Common questions
What should my boiler pressure be?
In most homes with a combi or system boiler, the gauge should read about 1 to 1.5 bar when the heating is off and cool. It rises a little when the heating is on. Your boiler manual gives the exact figure for your model.
Why does my boiler keep losing pressure when there is no leak I can see?
The leak may be hidden, under a floor or inside the boiler. The pressure relief valve may be letting water out through a pipe outside. Or the expansion vessel, a small tank that soaks up pressure as water heats, may have lost its air charge. These need an engineer to find.
Is low boiler pressure dangerous?
Low pressure on its own is not dangerous. Most boilers simply stop working until the pressure is back up. A leak can damage your home, though, and water must be kept away from the electrics.
How often is it normal to top up boiler pressure?
A sealed heating system should hold its pressure. Topping up once or twice a year, or after bleeding radiators, is not unusual. If you need to top up every few weeks, something is wrong and it needs looking at.
Can bleeding radiators lower the pressure?
Yes. Bleeding a radiator lets trapped air out, and the pressure drops as it goes. Check the gauge after bleeding and top up if it is below 1 bar. How to bleed a radiator.
Pressure still dropping?
Call us to book a boiler repair. A Gas Safe registered engineer will find out where the pressure is going and fix it.