Why Does My Boiler Lose Pressure Overnight?

Short answer

Boiler pressure can drop overnight due to cooling water, small air releases, or minor system leaks that become more noticeable when the heating is off. In many cases this is normal, but repeated or significant pressure loss should be checked.

Why boiler pressure drops when the heating is off

Boiler pressure changes as water heats up and cools down.

Overnight, when the heating is usually off:

• water inside the system cools and contracts
• pressure naturally reduces slightly
• air can collect at high points in the system

This can make the pressure gauge appear lower in the morning than it was the night before.

What level of pressure drop is normal

Small changes in pressure are expected.

Usually normal:

• pressure drops slightly (for example from 1.5 to 1.2 bar)
• pressure rises again when heating is on
• the boiler operates normally

Not normal:

• pressure drops below the minimum recommended level
• pressure continues to fall every night
• the boiler regularly needs topping up

A steady downward trend is more important than a single drop.

Common reasons for overnight pressure loss

Several factors can cause pressure to fall overnight.

Cooling system water

As water cools, it takes up less space, reducing pressure slightly.

Trapped air being released

Air moving through radiators can lower pressure after bleeding or heating cycles.

Small leaks

Tiny leaks may not be obvious but can reduce pressure slowly over time.

Faulty expansion vessel

If the expansion vessel is not working properly, pressure changes become more noticeable.

Is it safe to ignore a boiler that loses pressure overnight?

Sometimes, yes — but patterns matter.

Usually safe if:

• pressure drop is small
• pressure stabilises during the day
• the boiler works normally

Not safe to ignore if:

• pressure drops every night
• the boiler frequently shuts down
• you need to top up pressure often
• visible leaks appear

Regular pressure loss should always be investigated.

What to check first (in order)

Before assuming a fault, check these steps:

1. Check the pressure gauge in the morning and evening
   Look for a pattern rather than a one-off change.
2. Bleed any noisy radiators
   Trapped air can affect pressure readings.
3. Look for signs of leaks
   Check around radiators, valves, and pipe joints.
4. Check the filling loop is closed
   An open or faulty filling loop can affect pressure.
5. Monitor over several days
   Consistent loss is more important than single drops.

Why this is common in UK homes

Boiler pressure changes overnight are common in UK properties because:

• sealed central heating systems are widely used
• heating is often turned off overnight
• older systems react more to temperature change
• small pressure variations are normal in colder weather

These factors make overnight pressure changes common, especially in winter.

When to get help

You should consider professional advice if:

• pressure drops below the minimum level repeatedly
• the boiler loses pressure daily
• leaks are visible
• the boiler frequently shuts down

An engineer can identify leaks or expansion vessel issues.

Final reassurance

A small drop in boiler pressure overnight is often normal and related to cooling water and air movement. As long as the pressure stabilises and the boiler works properly, it is rarely a serious problem. Persistent pressure loss, however, should always be checked.

Internal links to add

• Radiator Gurgling Sounds: Causes and Fixes
• Radiator Still Noisy After Bleeding: What It Means
• Should I Worry About a Noisy Radiator?