Can a Boiler Lose Pressure Without a Leak?

In this guide, our Gas Safe engineers explain what might be happening, how to check safely, and when to call a professional.

If your boiler pressure keeps dropping with no visible leaks, there’s still an underlying issue.

Pressure loss doesn’t always mean water on the floor - air in the system, worn parts, or hidden faults can cause gradual drops over time.

Boiler pressure controls how hot water and heating circulate.

When it falls too low, radiators may stop working or the boiler can shut down.

If your boiler losing pressure is caused by a leak, check out our full boiler losing pressure guide!

Understanding why a boiler can lose pressure without a visible leak

How a sealed heating system manages pressure

Modern boilers use a sealed system that keeps water circulating under pressure.

When water heats up, it expands, and the expansion vessel absorbs this change to keep pressure stable.

If pressure drops, air or water is escaping somewhere, or the vessel isn’t regulating correctly.

Normal boiler pressure readings (cold vs hot)

If the gauge reads below 1 bar when cold, your boiler likely needs repressurising. You can check our guide on what your boiler pressure should be for more details on ideal readings and how to fix pressure issues safely.

Can a boiler lose pressure without leaking water?

Yes. Some boilers release water internally or through overflow pipes where it evaporates.

Air pockets, valve faults, or hidden leaks under floors can also lower pressure without visible signs.

A failed expansion vessel or relief valve can discharge water too slowly to notice.

Common reasons your boiler is losing pressure but no leak is found

1. Air trapped in the system or recently bled radiators

Bleeding radiators releases air and a small amount of water, lowering system pressure.

Trapped air can also build up naturally over time.

Fix: Repressurise the system using the filling loop until the gauge reads 1 to 1.5 bar.
Repairable? Yes, a simple fix.
Cost: Free if done yourself.

2. Faulty expansion vessel releasing excess pressure

If the expansion vessel loses charge or the diaphragm fails, it can’t absorb pressure changes.

The relief valve may then release water internally or outside through a copper pipe.

Fix: A Gas Safe engineer can recharge or replace the vessel.
Repairable? Yes, with the right tools and parts.
Cost: £250-£450 depending on your boiler.

3. Problem with the pressure relief valve or filling loop

A faulty pressure relief valve can discharge water slowly, causing pressure loss.

A filling loop left slightly open can also make pressure fluctuate.

Fix: Make sure the filling loop is fully closed. If pressure keeps dropping, have the relief valve checked.
Repairable? Yes, straightforward for an engineer.
Cost: £100-£200.

4. Corrosion or sludge build-up affecting water volume

Rust and sludge can block circulation and cause pressure imbalances.

Corrosion also weakens joints, leading to minor leaks.

Fix: A professional power flush and inhibitor treatment restore system health.
Repairable? Yes, and helps prevent future issues.
Cost: £150-£250.

5. Seasonal temperature changes and system contraction

Cold weather can make pipes contract and cause small pressure dips.

If the pressure stabilises as the system warms up, it’s usually not a fault.

Fix: Monitor the gauge and top up if needed.
Repairable? Not required unless pressure keeps dropping.
Cost: Free.

How to check for hidden causes of pressure loss

Read the pressure gauge and note how quickly it drops

Check the gauge when the system is cold, then again after 24 hours.

A steady drop means there’s a fault somewhere in the system.

Inspect visible joints, radiators, and the condensate pipe

Look for damp patches, rust, or water residue.

A dripping condensate pipe can also signal internal pressure loss.

Listen for gurgling or air movement in the system

Noises like gurgling or bubbling suggest trapped air.

Bleeding radiators can usually solve this.

Check the filling loop valves are closed

Ensure both valves on the filling loop are turned off after topping up.

If left slightly open, pressure will keep rising and falling.

Look for slow drips or damp patches near pipes or valves

Check around the boiler, skirting boards, and pipe joints for moisture or staining - signs of hidden leaks.

How to repressurise your boiler

Step-by-step: topping up a combi boiler

1. Turn off the boiler and allow it to cool.
   

2. Locate the filling loop - a flexible metal hose with two valves underneath the boiler.
   

3. Slowly open both valves until you hear water flowing.
   

4. Watch the gauge rise to 1-1.5 bar, then close both valves.
   

5. Turn the boiler back on and recheck the reading once it heats up.

   Cost: Free if done yourself.

If the pressure keeps dropping after topping up

If pressure falls again within a day or two, there’s likely an internal fault such as a vessel or valve issue.

Avoid topping up repeatedly, as it introduces air and can cause corrosion.

Call a Gas Safe engineer to diagnose the cause.

Cost: Diagnostic visit £80-£120.

When to call a Gas Safe engineer instead of DIY fixes

• Pressure drops repeatedly
  

• Damp patches near the boiler
  

• Fault codes or flashing pressure warnings
  

• Unusual hissing or gurgling noises
  

Switch off your boiler and book an inspection if you spot any of these.

When to call a gas safe engineer for ongoing pressure drops

Expansion vessel or pressure relief valve failure

If the expansion vessel fails, it can’t maintain stable pressure.

The relief valve may then discharge water automatically. Both parts should be tested and replaced if necessary.

Cost: £250-£450 depending on boiler type.

Internal component issues such as heat exchanger leaks

Cracks or corrosion in the heat exchanger can cause pressure loss inside the boiler.

This fault isn’t visible externally and needs professional assessment.

Fix: Replace the damaged part if repair isn’t viable.
Cost: £400-£800.

Professional testing to detect hidden system leaks

Engineers can pressure-test your system to confirm whether water is escaping internally or externally.

This ensures the right fix is applied without unnecessary part replacements.

Cost: £100-£200.

Preventing Pressure Loss in the Future

• Book annual servicing and system checks - A yearly boiler service helps spot corrosion, worn seals, or early signs of pressure loss.

• Keep inhibitor levels topped up - Corrosion inhibitors protect against rust and sludge build-up, keeping pressure stable.

• Monitor pressure monthly, especially in winter - Check your gauge every few weeks and top up if it drops slightly.

• Watch for early warning signs - Cold radiator spots, drips, or new noises can indicate air or developing leaks.

Final advice on dealing with boiler pressure drops with no leak

A boiler can lose pressure even without a visible leak.

Trapped air, a faulty expansion vessel, or hidden pipework issues are often to blame.

Topping up occasionally is fine, but repeated drops mean there’s a deeper problem.

Always call a Gas Safe engineer if pressure keeps falling.

*Any prices are guides only

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