Boiler losing pressure overnight: what it can mean
Quick answer
If your boiler’s pressure is noticeably lower each morning, this pattern often points to a slow leak somewhere in the system, since heating is usually off overnight so it isn’t linked to expansion. Comparing your evening and morning readings over a few days can help confirm the pattern. If pressure keeps dropping with no visible leak, contact a heating engineer.
Safety first
- If you smell gas or suspect carbon monoxide, leave the property and call 0800 111 999.
- Avoid topping up every single morning without investigating — this can mask a leak that’s getting worse.
- If you find water near electrical fittings, isolate power at the consumer unit only if it’s safe, then call a professional.
Full checklist: Safety guidance
What you may notice
- Pressure is consistently lower each morning than the night before — may suggest a slow, steady leak somewhere in the system.
- The drop is roughly the same amount each night — can support a small, consistent leak rather than something intermittent.
- Damp patches near radiator valves or pipe joints appear over the following days — this may help locate where the leak is.
- No visible leak anywhere despite consistent overnight drops — worth mentioning to an engineer, as some leaks are hidden within floors or walls.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Record the pressure last thing at night and first thing in the morning, for a few nights. This confirms whether the drop is a genuine pattern rather than a one-off reading.
- Check for visible damp or staining around radiator valves and pipe joints. Since heating is off overnight, any leak found is more likely a constant one rather than something linked to heat expansion.
- Check the outside pressure relief pipe for dripping or staining. This can indicate the pressure relief valve has discharged at some point.
- Confirm the filling loop is fully closed. A filling loop left slightly open can cause a slow, steady pressure loss.
What you can do safely
- Recording pressure at night and in the morning over a few days to confirm the pattern.
- Checking accessible radiator valves and pipe joints for visible signs of a leak.
- Making sure the filling loop is fully closed.
- Topping up only to the level in your manual if needed, while you arrange further checks.
When to call a professional
- The overnight drop continues over several nights with no visible leak found.
- You’re topping up daily just to keep the system running.
- Water is found near electrical fittings.
- The pressure relief pipe shows signs of discharge.
What to tell the engineer
- Your recorded pressure readings over the past few nights.
- Whether the drop amount is roughly consistent each night.
- Any visible leaks or damp patches found, and where.
- Whether the filling loop was open or closed when checked.
FAQs
Is overnight pressure loss different from pressure dropping during the day?
The pattern is a useful clue rather than a completely different problem. Since heating is usually off overnight, a consistent overnight drop points more clearly towards a steady leak rather than something linked to heat expansion.
How much pressure loss overnight is normal?
A small amount of natural pressure change can occur, but this varies by system, so it’s best to track your own readings over several nights and mention the pattern to an engineer rather than comparing to a fixed figure.
Can I just top up every morning instead of investigating?
This isn’t recommended as an ongoing routine, since it can hide a leak that’s slowly getting worse. If you’re topping up regularly, it’s worth getting the cause checked.
Where do slow leaks usually happen?
Common locations include radiator valves, pipe joints, and the filling loop, though some leaks can be hidden within floors or walls. A visual check of accessible areas is a reasonable first step before calling an engineer.
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