Boiler Losing Pressure But No Leaks? Hidden Causes Explained by Heating Experts

Can a Boiler Lose Pressure Without Any Leaks?

Yes — a boiler losing pressure but no leaks is far more common than most homeowners realise.

Modern boilers are sealed systems. When pressure drops without visible leaks, it usually means:

• Water is being released through a safety outlet
• Air is escaping internally
• Pressure readings are inaccurate
• System balance is failing during heating cycles

Ignoring repeated pressure loss can lead to boiler shutdowns, cold radiators, and expensive repairs.

Understanding Normal Boiler Pressure

Before diagnosing faults, it helps to know what “normal” looks like.

Boiler State	Pressure Range	Meaning
Cold / Idle	1.0 – 1.5 bar	Normal
Heating On	1.5 – 2.0 bar	Normal
Too Low	Below 1.0 bar	Boiler may fail
Too High	Above 2.5–3.0 bar	Safety valve activates

If your boiler losing pressure but no leaks causesreadings to fall below 1.0 bar, performance and safety are compromised.

Main Reasons a Boiler Loses Pressure With No Visible Leaks

Below are the most common hidden causes we diagnose during call-outs.

1. Faulty Pressure Relief Valve (PRV)

A worn pressure relief valve is one of the top causes of a boiler losing pressure but no leaks indoors.

The PRV protects your system by releasing water if pressure becomes too high. Over time, it may:

• Fail to reseal
• Drip intermittently
• Release water through an external discharge pipe

Because this pipe exits outside, homeowners rarely notice water loss.

Expert solution:
An engineer checks the discharge pipe and replaces the PRV if faulty.

2. Expansion Vessel Failure

The expansion vessel controls pressure changes when water heats and cools. If it fails, pressure spikes during heating and drops sharply when cooling.

This cycle causes:

• Repeated PRV discharge
• Pressure loss with no internal leaks
• Frequent need to repressurise

A failed vessel is a classic reason for a boiler losing pressure but no leaks.

Engineer fix:
Testing air charge and re-pressurising — or replacing the vessel if the internal diaphragm has failed.

3. Leaking Schrader Valve

The Schrader valve sits on the expansion vessel and allows air pressure adjustments.

If it leaks air:

• The vessel can’t absorb pressure changes
• Water is expelled via the PRV
• Pressure drops repeatedly

This fault is invisible unless tested professionally.

Important sign:
If water exits the valve instead of air, the vessel must be replaced.

4. Air Trapped in the System or Recent Radiator Bleeding

Bleeding radiators releases trapped air—but also removes water volume.

If the system isn’t topped up correctly afterward, the result looks like a boiler losing pressure but no leaks.

Signs include:

• Gurgling radiators
• Uneven heating
• Pressure drop after bleeding

Correct fix:
Bleed fully, then repressurise to 1.0–1.5 bar.

5. Sludge or Corrosion Inside Pipework

Internal corrosion creates sludge that restricts flow and stresses the boiler.

Common symptoms:

• Cold radiator bottoms
• Dirty bleed water
• Noisy boiler
• Constant pressure loss

This hidden resistance makes pressure unstable even without leaks.

Professional solution:
Power flush + inhibitor to prevent future corrosion.

6. Faulty Boiler Pressure Gauge

Sometimes the system pressure is fine—but the gauge lies.

A faulty gauge can:

• Stick
• Jump suddenly
• Show pressure dropping incorrectly

This can mislead homeowners into thinking the boiler losing pressure but no leaks.

Engineer check:
Compare gauge readings with actual system pressure.

Safe DIY Checks You Can Do

Homeowners can safely try the following basic checks only:

✔ Check pressure gauge movement
✔ Inspect visible pipework and radiators
✔ Bleed radiators if needed
✔ Top up pressure using the filling loop

 Never open boiler casing or replace components yourself.

Why Boiler Pressure Keeps Dropping Repeatedly

If your boiler losing pressure but no leaks keeps happening, it usually means:

• A safety component is failing
• Pressure imbalance during heating
• Internal discharge you can’t see

Repeated topping-up is a warning sign — not a solution.

When to Call a Gas Safe Engineer

Contact OSO Group Engineers if:

• Pressure drops weekly or daily
• Boiler shuts down repeatedly
• You hear discharge outside
• Pressure rises then crashes during heating

Delaying repairs increases the risk of PRV damage, PCB faults, and full system failure.

Why Choose OSO Group Engineers?

✔ Gas Safe registered
✔ Boiler pressure diagnostics specialists
✔ Honest, fixed-price repairs
✔ Long-term fault prevention
✔ UK-based rapid response

We don’t just top up pressure — we find the real cause.

Preventing Future Pressure Loss

• Annual boiler servicing
• Expansion vessel testing
• System inhibitor use
• Early fault diagnosis

Preventive care saves money and avoids winter breakdowns.

FAQ