Boiler Losing Pressure Overnight?

Is It Normal for a Boiler to Lose Pressure Overnight?

No — a healthy boiler should not lose pressure regularly, whether overnight or during the day.

A small drop over many months can be normal, but if:

• You’re topping up weekly (or more)
• Pressure drops while the heating is off
• Pressure falls with no visible leaks

…then there’s likely an underlying fault.

If you’re topping up your boiler in the evening, only to wake up and find the pressure has dropped again, you’re not alone. This is one of the most common boiler issues we see across South Wales — and it’s usually a sign that something isn’t quite right.

The good news? In many cases, the cause is diagnosable and fixable once you know what to look for.

Engineer Insight:

“When pressure drops overnight but looks fine during the day, it often points to a small leak, expansion vessel issue, or pressure relief valve passing water after heating cycles.”

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Why Overnight Pressure Loss Happens (What the Timing Often Tells Us)

Overnight, your heating system cools down. As water cools it contracts slightly, and pipework/valves also change temperature. That’s why a pressure drop that’s most noticeable in the morning often points to:

• a small leak that shows more as the system cools
• a component that struggles to hold pressure after a heating cycle
• pressure being discharged (sometimes outside) and only noticed later

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Common Reasons Boilers Lose Pressure Overnight

1) A Small Hidden Leak

Tiny leaks can evaporate before you ever see water. Common places include:

• Radiator valves (especially TRVs)
• Pipe joints under floors or behind units
• Internal boiler components

Why it can feel “overnight”: as the system cools, joints and seals can relax and small weeps can become more noticeable.

2) Faulty Expansion Vessel (EV)

The expansion vessel absorbs pressure changes when water heats and cools.

If it fails, you may see:

• Pressure rising when the heating is on
• Pressure dropping as the system cools (often overnight)
• Frequent pressure fluctuations across a day

3) Pressure Relief Valve (PRV) Passing Water

A worn PRV may release water during pressure spikes. Check the small copper pipe outside:

• Drips or staining can indicate pressure is being dumped
• Loss often shows after heating cycles — then the gauge looks low the next morning

4) Air in the System (Often a Secondary Symptom)

Air pockets can:

• Cause pressure imbalance
• Trigger false low-pressure readings
• Appear after bleeding radiators or topping up

This is often a symptom rather than the root cause — especially if you’re topping up frequently.

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Is It Dangerous If Boiler Pressure Keeps Dropping?

Low pressure itself isn’t usually dangerous, but the cause can be. Left unresolved, it can lead to:

• Boiler shutdowns
• Internal corrosion
• Heat exchanger damage
• Costly breakdowns in winter

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Should I Keep Topping the Boiler Up?

Occasional topping up is fine — frequent topping up is not.

Repeatedly adding fresh water:

• Introduces oxygen
• Accelerates corrosion
• Masks the real fault

If pressure keeps dropping, it’s time for a proper diagnosis.

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When to Call a Gas Safe Engineer

You should get the boiler checked if:

• Pressure drops repeatedly
• You see water outside the property (especially from the PRV discharge pipe)
• The boiler locks out
• You’re unsure where the pressure is going

A qualified engineer can pressure-test the system and identify the root cause safely.

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Key Takeaways

• A healthy sealed heating system should not regularly lose pressure, whether overnight or during the day.

• Small pressure drops over many months can be normal — frequent top-ups are not.

• Overnight pressure loss often points to:

  • A small hidden leak

  • A failing expansion vessel

  • A pressure relief valve (PRV) passing water

  • Air in the system (usually a secondary symptom)

• Topping up repeatedly introduces oxygen, which can accelerate corrosion and mask the real issue.

• If pressure drops only after heating cycles, the expansion vessel or PRV is a common cause.

• Water outside near the copper discharge pipe usually indicates PRV discharge.

• Low pressure itself is rarely dangerous, but ignoring it can lead to breakdowns, corrosion, or heat-exchanger damage.

• If you’re topping up more than occasionally, or can’t see where the pressure is going, it’s time for a Gas Safe inspection and pressure test.

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