Why boiler pressure drops
If your boiler pressure keeps falling, water is leaving the heating system somewhere, or a part that controls pressure has worn out. Most modern combi and system boilers run on a sealed system that needs about 1 to 1.5 bar on the gauge when cold. Below about 0.5 to 1 bar, depending on the model, the boiler locks out and shows a fault code. The good news is that the first two checks are safe to do yourself.
Low pressure is a water problem, not a gas problem. If you can smell gas at any point, stop, leave the boiler alone and call the National Gas Emergency Service on 0800 111 999.
Read the gauge properly
Find the pressure gauge on the front or underside of the boiler. It is either a dial with a needle or a number on the display. Read it with the heating off and the system cold, because pressure rises a little as the water heats up.
| Reading when cold | What it means |
|---|---|
| Below about 0.5 to 1 bar | Too low. The boiler may lock out. |
| About 1 to 1.5 bar | Normal for most homes. |
| Above about 2.5 bar | Too high. Do not top up. Call an engineer. |
Your manual is the final word. Some models run at slightly different figures.
The common causes
- Bleeding radiators. Letting air out also lets out a little pressure. This is normal, and a top-up fixes it.
- A small leak. A weeping radiator valve, a joint under the floor, or a drip from the boiler itself.
- The pressure relief valve. This safety valve lets water out through a small pipe, usually running outside, if pressure gets too high. Once it has opened, it can wear and keep dripping.
- A flat expansion vessel. This is a tank inside or next to the boiler with an air cushion that soaks up the water’s expansion as it heats. When the air charge goes, pressure shoots up when hot and drops when cold, and the relief valve keeps opening.
- An internal leak in the boiler. Less common, and always a job for an engineer.
Safe check one: top up using the filling loop
If the pressure is low and nothing is obviously leaking, you can usually top it up. Follow the instructions in your boiler manual, because filling loops vary between makes. In general:
- Turn the heating off and let the system cool.
- Find the filling loop. On many combis it is a silver braided hose with a valve at each end under the boiler. Some boilers have a built-in key or lever instead.
- Open the valves slowly, one then the other, and watch the gauge rise.
- Close both valves when the needle reaches the level in your manual, usually about 1 to 1.5 bar.
- If your loop is a removable hose, disconnect it as the manual describes, keeping a cloth handy for drips.
- Reset the boiler if the manual tells you to, then turn the heating back on.
If you overfill it, do not try to let water out from the boiler. Bleeding a radiator brings it back down, a little at a time.
Safe check two: look for leaks
With the pressure topped up, give it a day or two and watch the gauge. While you wait:
- Check under every radiator and around its valves for drips, damp patches or rust marks.
- Look at exposed pipework, especially joints and anywhere it runs into the floor.
- Look underneath the boiler for drips or water staining, without opening the casing.
- Find the small copper pipe that runs from the boiler to the outside wall. If it is dripping or there is a damp patch below it, the pressure relief valve is letting water out.
- Check ceilings below upstairs radiators for new stains.
Note where you find any drip, and take a photo. It saves the engineer time, and time is what you pay for.
When to stop and call an engineer
Top up once or twice. If you are topping up every week, something is wrong, and repeatedly refilling a leaking system pulls in fresh oxygen and water that speed up corrosion and sludge. Book a Gas Safe registered engineer if:
- The pressure drops again within a few days of a top-up.
- The relief valve pipe outside is dripping.
- The pressure climbs above about 2.5 bar when the heating is on.
- You can see water coming from inside the boiler.
- You cannot find the filling loop or the manual does not cover it.
Anything inside the boiler casing, including the expansion vessel and the relief valve, is a job for an engineer. The boiler is a gas appliance, and opening it up is not a DIY task.
What the fix usually costs
Most pressure repairs start with a diagnostic visit at a typical £80 to £120. Replacing a valve is usually in the £150 to £350 range, checked October 2026. A leak under a floor depends on how hard it is to reach. See boiler repair cost for more typical prices.
If your boiler is old and the expansion vessel, valves and pump are all starting to go, read repair or replace your boiler before you pay for parts one at a time. And if the pressure is fine but the boiler still will not start, try the checks in boiler not firing up.
City Boiler connects you with independent local engineers who are Gas Safe registered. See the boiler repair page for what a repair visit involves.
Common questions
What should my boiler pressure be?
On most sealed systems the gauge should read about 1 to 1.5 bar when the heating is cold, rising a little when it is hot. Your boiler manual gives the exact figure for your model, so check that first.
Is it safe to top up boiler pressure myself?
Yes, if your manual says it is a user task and you follow its instructions for the filling loop. Topping up only involves water, so it does not touch the gas side. If the pressure keeps dropping after you top it up, stop and book an engineer.
Why does my boiler lose pressure but there is no visible leak?
The water may be escaping through the pressure relief valve to a pipe outside, or leaking slowly under floors or inside the boiler. A flat expansion vessel can also cause the pressure to swing. These need an engineer to diagnose.
Does bleeding radiators lower boiler pressure?
Yes. Letting air out of radiators also lets a little pressure out of the system. Check the gauge after bleeding and top up if it has dropped below the level in your manual.