Why Is My Boiler Losing Pressure?

What Pressure Should Your Boiler Be?

On most modern combi and system boilers, the pressure should be around 1 to 1.5 bar when cold (heating off for a while). When the heating is running and the system is hot, it often rises to around 1.5 to 2 bar. If the pressure drops below 1 bar when cold, many boilers will display a low-pressure error and may stop running.

Common Causes of Pressure Loss

These are the most likely reasons a sealed system loses pressure over days or weeks.

• Small leak on pipework or radiators: Even a slow weep at a joint, radiator valve, or under a floorboard can drop pressure over time.
• Radiator bleed valve or valve gland leaking: A tiny drip after bleeding can be enough to cause a noticeable drop.
• Pressure relief valve (PRV) discharging: If the system pressure gets too high, the PRV releases water to protect the boiler. This can happen intermittently.
• Expansion vessel fault: If the expansion vessel has lost its air charge or the diaphragm has failed, pressure can swing too high when hot, triggering the PRV, and then appear low when cold.
• Filling loop left open or passing: A partly open or faulty filling loop can cause pressure issues and (in some cases) lead to PRV discharge.
• Recent work or radiator bleeding: Bleeding radiators releases water/air and often needs a top-up after. If you've bled frequently, repeated top-ups can mask a leak.

Step-by-Step Checks You Can Do

You can often narrow down the cause with a few safe checks. Do not remove the boiler casing or touch any gas components.

Step 1 — Note the pressure cold vs hot

Check the gauge when the heating has been off for a while, then again when the system is fully warm. Big jumps upward when hot can suggest an expansion vessel problem.

Step 2 — Check radiators and visible pipework for leaks

Look for damp patches, staining, green/white residue at joints, or corrosion around radiator valves. Check under radiators and around any exposed pipework, especially where it enters walls or floors.

Step 3 — Inspect the pressure relief discharge pipe (outside)

Many boilers have a copper pipe that exits outside. If it's wet, dripping, or shows signs of regular discharge, the PRV may have released water—often due to overpressure from an expansion vessel issue or overfilling.

Step 4 — Confirm the filling loop valves are fully closed

The filling loop should be closed after topping up. If your system has a removable filling loop, ensure it is disconnected and capped as per the manufacturer instructions.

When to Call a Gas Safe Engineer

Call an engineer if pressure drops quickly (hours to a couple of days), if you can't find any leaks, if the pressure relief pipe is discharging, or if the boiler repeatedly locks out. A Gas Safe engineer can pressure-test the system, locate hidden leaks, check/charge the expansion vessel, and replace faulty valves safely.

Frequently Asked Questions

Is it normal for boiler pressure to drop slightly?

A small change between hot and cold is normal, but a steady drop over days/weeks usually indicates water is leaving the sealed system. If you're topping up regularly, it's best to find the cause rather than repeatedly repressurising.

Can low pressure damage my boiler?

Low pressure typically makes the boiler shut down to protect itself, so damage is unlikely from low pressure alone. The bigger issue is the underlying cause (like a leak) and the inconvenience of losing heating/hot water.

Why does my pressure drop after bleeding radiators?

Bleeding releases air and a small amount of water, so the system pressure often falls afterwards. Topping up via the filling loop is normal—if it keeps dropping repeatedly, investigate for leaks.

What does a faulty expansion vessel do?

The expansion vessel absorbs the increase in pressure as water heats up. If it's faulty or needs recharging, pressure can rise too high when hot, triggering the pressure relief valve to dump water, then appear low again when cold.

How often is “too often” to top up boiler pressure?

If you need to top up more than once every few months, or you see pressure dropping week to week, it's worth getting the system checked. Regular top-ups can increase corrosion risk and may hide a leak that should be fixed.