A glance at your boiler’s pressure gauge and the needle is sitting low — or you topped it up last week and it has dropped again. Boiler pressure loss is one of the most common complaints Gas Safe engineers are called out for, and the good news is that many cases have a simple, free fix you can do in minutes.
The key question is whether you are dealing with a one-off drop (normal, especially after bleeding radiators) or a recurring pressure loss (which signals a fault that needs professional attention). This guide walks through every likely cause, what you can safely do yourself, and the clear signs it is time to call an engineer.
Pressure dropped suddenly and you need help now? Find a local Gas Safe engineer near you.
At a Glance: Pressure Loss Causes and Next Steps
| Cause | DIY Fix? | Engineer Needed? |
|---|---|---|
| Bled radiators recently | ✅ Yes — repressurise via filling loop | Only if pressure keeps dropping |
| Small radiator valve or bleed screw leak | ✅ Tighten the bleed screw | Yes, if tightening does not stop it |
| Filling loop left open or dripping | ✅ Close the loop fully | Only if the loop valve is faulty |
| Faulty expansion vessel | ❌ No | Yes — Gas Safe required |
| Heat exchanger leak | ❌ No | Yes — urgent, Gas Safe required |
What Should Boiler Pressure Be?
Your boiler’s pressure gauge — usually a round dial or digital display on the front panel — shows the water pressure inside the sealed heating system. There is a safe operating window for all modern combi and system boilers:
- Cold (heating off): 1 to 1.5 bar
- Hot (heating running): 1.5 to 2 bar
These figures are typical, but always check the label on your boiler casing — manufacturers sometimes specify a slightly different range.
Too low: If the gauge drops below 0.5 bar, the boiler will usually lock out and display a fault code. It is protecting itself from running dry.
Too high: If pressure climbs above 3 bar, the pressure relief valve — a pipe that exits through an outside wall — will open to release the excess. If you notice that pipe dripping persistently, that is a symptom worth investigating rather than ignoring.
A reading of 1.2 to 1.3 bar when cold is the ideal target when repressurising manually.
The 5 Most Common Causes of Boiler Pressure Loss
1. You Recently Bled a Radiator
Bleeding a radiator releases trapped air from the heating system — but it also releases a small amount of water. After bleeding one or more radiators, a modest pressure drop (often 0.2–0.5 bar) is completely normal. Simply repressurise via the filling loop (see the step-by-step section below) and the boiler will function normally.
If pressure drops again within a few days without any further bleeding, something else is causing the loss.
2. A Slow Leak at a Radiator Valve or Pipe Joint
Even a pinhole drip from a radiator valve, bleed screw, or a pipe joint can drain pressure noticeably over days or weeks. Check around every radiator in the house — look for damp patches on the floor, rusty staining on the valve body, or water marks on the skirting board.
A loose bleed screw (the small brass nipple at the top of a radiator) is often the culprit after bleeding. Close it firmly with a radiator key. If the valve body itself is leaking, that needs a plumber.
3. The Filling Loop Has Been Left Open or Is Faulty
The filling loop is the valve — or braided hose — that connects the mains cold water supply to your heating system. When the loop is open, water from the mains enters the system to raise pressure. If it is not closed fully, water continues to trickle in, causing pressure to fluctuate or gradually climb rather than drop.
Conversely, a filling loop with a worn internal valve can drip backwards, slowly bleeding pressure back to the mains. Check that your loop valves are fully closed and dry. If the loop fitting is weeping, a plumber can replace it for a modest cost.
4. A Faulty Expansion Vessel
Every sealed heating system contains an expansion vessel — a small pressurised tank, usually mounted behind the boiler casing, that absorbs the expansion of water as it heats up. Over time, the rubber diaphragm inside the vessel can fail, meaning the vessel can no longer do its job.
The tell-tale sign: pressure is fine after repressurising, but drops back to near zero again after a heating cycle or two, even with no visible leaks anywhere. Diagnosing and replacing an expansion vessel is a job for a Gas Safe engineer — the system must be drained and re-pressurised correctly, and the new vessel needs to be set to the correct pre-charge pressure.
5. A Heat Exchanger Leak
The heat exchanger is the core component of a combi boiler — it is where the burner heats the water. Pinhole corrosion or hairline cracks in the heat exchanger allow water to escape internally, sometimes evaporating before it drips visibly.
Symptoms include: a persistent damp smell when the boiler is running, rusty-coloured staining inside or underneath the boiler casing, or pressure loss that cannot be traced to any external leak. A heat exchanger leak is the most serious cause on this list. It requires urgent inspection by a Gas Safe engineer, and in older boilers it can be the deciding factor between repair and replacement.
How to Repressurise Your Boiler (Step-by-Step)
Repressurising is the one task that every homeowner can safely do, regardless of their technical knowledge. It does not involve gas pipework and requires no tools.
What you will need: the filling loop or filling key (supplied with your boiler) and access to the pressure gauge.
- Turn the boiler off and allow it to cool for 10–15 minutes if it has been running.
- Locate the filling loop. On most modern boilers it is a braided silver flexible hose with a valve at each end, underneath the boiler. Some newer models use a small built-in keyless valve instead — consult your manual.
- Open the filling loop valve(s) slowly. You will hear water entering the system. Watch the pressure gauge rise.
- Stop at 1.2–1.3 bar. Do not overfill — going above 1.5 bar creates a separate problem. Close the valve(s) completely.
- Check the loop is fully closed — there should be no sound of running water and no dripping from the loop fittings.
- Restart the boiler and monitor the gauge as it heats up. It should rise to around 1.5–2 bar when hot, then return to your target cold pressure once cooled.
If pressure drops again within 24–48 hours of repressurising, you have a leak or a faulty expansion vessel. At that point, stop repressurising and call an engineer — adding water repeatedly to a leaking system accelerates corrosion inside the pipework.
Need an engineer? Find a local Gas Safe engineer in Birmingham, Manchester, London, and across the UK — get a free quote here.
What You Should Not Do Yourself
The filling loop and bleed screws are the extent of safe DIY on a sealed heating system. Several things that look simple are illegal or dangerous to attempt yourself:
Do not touch gas pipework. Under the Gas Safety (Installation and Use) Regulations 1998, only a Gas Safe registered engineer may carry out work on gas fittings and appliances. This is a criminal offence if breached, not just a guideline.
Do not open the boiler casing to inspect or adjust internal components such as the expansion vessel, heat exchanger, or pressure relief valve. Beyond the legal issue, incorrectly reassembled sealed systems can fail dangerously.
Do not replace the expansion vessel yourself. The system must be safely drained and the replacement vessel pre-charged to the correct pressure before installation — an incorrectly installed vessel will simply fail again.
Insurance and compliance implications. Unauthorised gas or sealed system work voids most home insurance policies and buildings insurance. It also creates liability issues if the property is rented.
To confirm any engineer you hire is legitimate, search the Gas Safe Register (GasSafeRegister.co.uk) by licence number — every registered engineer carries a Gas Safe ID card showing which appliance types they are qualified to work on.
When to Call a Gas Safe Engineer
Call an engineer if any of the following apply:
- Pressure drops again within 24–48 hours of repressurising, with no obvious external leak
- You can see damp, rust staining, or water on or around the boiler casing
- The pressure relief discharge pipe (the small pipe exiting through an outside wall, usually near the boiler) is dripping or running continuously
- The boiler keeps locking out even after you repressurise it
- You cannot locate the filling loop or repressurise successfully
- The pressure gauge is cracked, stuck, or you cannot read it
Gas Safe engineers in Leeds, Sheffield, and Bristol can typically carry out a pressure diagnosis and repair in a single visit.
Get a same-day quote from a vetted local Gas Safe engineer.
How Much Does It Cost to Fix?
| Fix | Typical Cost |
|---|---|
| Repressurising yourself | Free |
| Engineer callout + diagnosis | £80–£120 |
| Tightening/replacing a radiator valve | £80–£150 (including callout) |
| Replacing the expansion vessel | £150–£300 (parts + labour) |
| Heat exchanger repair | £200–£500 depending on severity and boiler model |
| New boiler (if heat exchanger is beyond repair) | £1,500–£3,000 installed |
If you have a boiler cover plan through a provider such as British Gas HomeCare, Homeserve, or a similar scheme, recurring pressure loss caused by an internal fault is typically covered — check your policy before paying for a callout.
Frequently Asked Questions
What is the correct pressure for a boiler?
Most combi and system boilers should operate between 1 and 1.5 bar when cold, rising to around 1.5–2 bar when hot. Check the manufacturer’s label on the boiler casing for the exact recommended range. If the needle falls below 0.5 bar, the boiler will likely lock out.
Why does my boiler keep losing pressure?
The two most common causes are a small water leak somewhere in the system — often at a radiator valve, pipe joint, or the boiler itself — and a faulty expansion vessel. A one-off pressure drop after bleeding radiators is normal. If pressure drops repeatedly with no obvious leak, call a Gas Safe engineer to inspect the expansion vessel and heat exchanger.
Can I repressurise my boiler myself?
Yes. Repressurising via the filling loop is a task any homeowner can do safely. Locate the filling loop (usually a braided silver hose or a keyless valve under the boiler), open it slowly until the gauge reads around 1.2 bar, then close it. Full step-by-step instructions are in your boiler manual. Do not overpressurise — stop at 1.5 bar maximum.
Will my boiler explode if the pressure is too low?
No. Low pressure causes the boiler to shut down automatically as a safety measure — it will not explode. The boiler’s pressure sensor triggers a lockout before any damage can occur. Repressurise using the filling loop and restart the boiler. If pressure keeps dropping, the issue is a leak or a faulty expansion vessel — call a Gas Safe engineer.
Can I fix a boiler pressure leak myself?
It depends on where the leak is. Tightening a loose radiator valve bleed screw is something you can do yourself. Any work on gas pipework, the heat exchanger, or sealed components of the boiler must be carried out by a Gas Safe registered engineer — it is illegal to do this work yourself, and doing so voids your home insurance.
How much does it cost to fix a boiler losing pressure?
If repressurising via the filling loop solves the problem, there is no cost. If an engineer is needed, a standard callout and diagnosis typically costs £80–£120. Replacing an expansion vessel costs £150–£300 including labour. Fixing a heat exchanger leak can range from £200 to £500 depending on severity and boiler model.