Boiler Losing Pressure? Causes, Fixes & Hidden Leak Detection

What Does It Mean When a Boiler Loses Pressure?

A boiler that’s losing pressure consistently is showing signs of a system fault or internal leak. This usually means that water is escaping from the heating system, or there is a failure in internal components such as the expansion vessel or pressure release valve.

The most common symptom is a pressure gauge dropping below 1 bar — often followed by the boiler shutting off or displaying an error code like F22, F75, or E119.

What Causes Boiler Pressure Loss?

Here are the most common causes of pressure loss in modern central heating systems:

1. Hidden Leak in the Heating System

Even micro leaks behind walls or under floors can cause gradual pressure loss. These are often invisible but reduce system volume over time.

2. Faulty Expansion Vessel

When the expansion vessel membrane fails, the system cannot maintain stable pressure, especially after heating up.

3. Pressure Relief Valve (PRV) Discharge

If the PRV is stuck or faulty, water can be discharged outside via the overflow pipe — often without the homeowner noticing.

4. Recently Bled Radiators

After bleeding air from radiators, the system needs topping up. Forgetting to do so can result in low pressure and eventual boiler shutdown.

5. Faulty Filling Loop or Loose Connections

A leaking or partially open filling loop can result in pressure instability, or intermittent drops without obvious signs of leakage.

Symptoms of a Boiler Losing Pressure

If your boiler is losing pressure, you may notice:

Constant pressure drop on the gauge (e.g., from 1.5 to 0.5 bar)
Cold radiators despite the boiler being “on”
Need to manually top up water every few days
Gurgling or tapping sounds in pipes or radiators
Error codes such as F22 (Vaillant), F1 (Ideal), or E1 (Baxi)

How to Fix Boiler Pressure Loss (Safely)

Step 1: Repressurise the Boiler

Use the filling loop to raise pressure to around 1.5 bar. Most systems operate best between 1.0 and 1.5 bar.

Step 2: Bleed Radiators (If Recently Refilled)

Trapped air can skew pressure readings and prevent proper heating. Bleed radiators carefully with a key.

Step 3: Monitor for Pressure Drop

If pressure continues to fall after refilling, you likely have a leak or component fault.

Boiler Losing Pressure? It Might Be a Leak You Can’t See

If your boiler loses pressure again after refilling, you may have a hidden leak in:

Pipework beneath floors or concrete
Joints behind walls
Radiators or towel rails
Boiler seals or internal fittings

At ADI Leak Detection, we use non-invasive leak tracing technology:

🎧 Acoustic sensors
🌡️ Thermal imaging
🧪 Hydrogen tracer gas

These allow us to find hidden water leaks without causing damage to your home.

ADI Leak Detection is accredited with The Chartered Institute of Plumbing and Heating Engineering.

ADI Leak Detection is registered with The Water Industry Approved Plumbers’ Scheme.

ADI Leak Detection is a Watersafe Accredited contractor.

Our Reviews & Accreditation’s

Frequently Asked Questions About Boiler Pressure Loss

What should boiler pressure be?

Ideal pressure is between 1.0 and 1.5 bar when cold. If it drops below 1.0, the system may stop working.

Is boiler pressure loss always caused by a leak?

Not always. While leaks are the most common cause, failed components like the expansion vessel or PRV can also result in pressure drops.

How do I know if my boiler is leaking?

Look for:

Constant pressure drops
PRV overflow pipe dripping outside
Stains near radiators or pipes
Water under boiler unit

If nothing is visible, a hidden leak may be the issue.

Can I fix boiler pressure loss myself?

You can repressurise the boiler and bleed radiators, but if the pressure keeps dropping, it’s time to call a professional — especially to avoid long-term damage from undetected leaks.

Is boiler pressure loss dangerous?

Yes, it can lead to:

System airlocks
Poor heating performance
Boiler damage
Water damage if a leak is severe

Why does my boiler keep losing pressure overnight?

Slow leaks or thermal contraction may cause night-time pressure drops. This usually indicates a micro leak or failed expansion vessel.

Can weather affect boiler pressure?

Extreme cold can cause pipe contraction or even bursts — both of which may cause pressure loss. Frozen condensate pipes are also common in winter.

Should I call a plumber or a leak detection expert?

If you can’t see the leak, call a leak detection specialist. We use specialist equipment to detect leaks that plumbers can’t find without cutting into walls or floors.