Boiler Pressure Dropping Overnight in London?
Why Your Boiler Pressure Keeps Dropping Overnight
If your boiler pressure is dropping overnight, something is usually letting water escape from the system, even if there is no obvious leak on the floor. A lot of homeowners check around the boiler, see nothing dripping, then assume the gauge is faulty. Sometimes it is. Most of the time though, the system is slowly losing pressure somewhere.
You might top it up before bed, everything seems normal, then wake up and the pressure has dropped again by morning. We see that quite a bit in older Croydon and South London heating systems, especially where there are ageing radiator valves, tiny pipe leaks under floors, or expansion vessel problems starting to develop inside the boiler.
The awkward part is that not every leak leaves a visible puddle. Small leaks can dry before you notice them, particularly around radiator valves, pipe joints, or under floorboards where nobody looks unless there is already damage. In some cases the pressure relief pipe outside drips occasionally and homeowners never realise because it only happens when the heating is running hard.
Then there are systems where the pressure drop only becomes obvious once the heating cools down overnight. Expansion vessel faults, slow PRV discharge and tiny hidden leaks often follow that pattern. Those faults can take a while to show themselves properly. One day the pressure drops every few weeks. A month later it is every night.
We see this a lot in London homes, especially older houses in Thornton Heath, Norbury and West Croydon where pipework runs under floors, behind boxing, or through awkward cupboards. A leak does not always leave a puddle where you can see it.
MPS Heating & Plumbing LTD helps with boiler pressure faults across Croydon, Thornton Heath, Addiscombe, Norbury, South Croydon, West Croydon, Selhurst and East Croydon. We have a 5.0 Google rating from 60 reviews.
If you are topping the boiler up every day, call 0203 576 6769 and we can talk through what is likely happening before you book.
Why Boiler Pressure Drops More Overnight
When the heating is on, the water in the system heats up and expands, so the gauge can look higher for a while. Overnight, once the heating goes off, that same water cools down and contracts. If there is already a small leak or a pressure issue in the boiler, that drop becomes much more obvious by morning.. A healthy sealed system should still hold pressure, but if there is a weak point, that overnight cooling often makes the drop easier to notice.
That weak point could be a passing pressure relief valve, a tired expansion vessel, a tiny radiator valve leak, air being released from the system, or pipework losing a small amount of water under the floor.
In flats near East Croydon or older homes around London Road and Mitcham Road, we often find the problem is not obvious from the boiler itself. The gauge drops, but the actual water loss may be outside through the copper discharge pipe, under floorboards, or around old radiator valves.
If the pressure fault follows a missed annual check, it may be worth booking a proper boiler service with pressure checks rather than just topping it up again and hoping it settles.
Common Causes Of Boiler Pressure Dropping Overnight
| Possible Cause | What It Usually Means |
|---|---|
| Expansion vessel fault | One of the more common ones. The pressure climbs when the heating is running, sometimes close to 3 bar, then the boiler dumps water outside through the relief pipe. By morning the system has cooled down and the pressure is low again. A lot of people only notice the topping up part and miss what happened the night before. This is also closely linked to boiler pressure rising too high during heating cycles. |
| PRV passing outside | The pressure relief valve can start letting water out slowly through the copper pipe outside the house. Usually there is no leak indoors, so homeowners assume the pressure gauge is faulty until they spot drips outside or a wet patch near the wall. |
| Radiator valve weep | Small radiator valve leaks are easy to miss. Sometimes it is not even wet by the time you check it. You just get that crusty white staining, green marks around the pipe, or a slight metallic smell near the radiator that has been there for weeks. |
| Pipework under floors | This can be the awkward one. Older heating systems in places like Croydon terraces and extended houses often have hidden joints under timber floors that start leaking very slowly over time. Not enough to flood anything, just enough for the pressure to keep dropping every few days or overnight. |
| Internal boiler leak | Sometimes the leak is inside the boiler itself, which is why nothing obvious shows around the property. We occasionally find water marks inside the casing or leaks running into the condensate section where they evaporate once the boiler heats up, so the pressure disappears without leaving a puddle on the floor. |
Boiler Losing Pressure But No Leak?
This is the bit that frustrates most people. The boiler keeps losing pressure, but there is no wet patch, no dripping radiator, and nothing obvious under the boiler.
That does not mean there is no leak. It just means the water may be escaping somewhere awkward. We have seen tiny leaks on radiator valves hidden behind curtains, PRV pipes dripping outside overnight, and pipework weeping under floorboards in older Thornton Heath and Norbury houses.
An engineer will usually look for pressure movement, staining around valves, discharge from the outside pipe, signs of sludge, air in radiators, and whether the expansion vessel is doing its job properly.
There is no point guessing. If the same drop keeps happening every morning, the system needs checking properly.
Stop Constant Topping Up
Topping up once after bleeding radiators is normal. Topping up every day is not. Fresh water brings oxygen into the system, weakens inhibitor levels and can speed up sludge and corrosion inside radiators, pumps and heat exchangers.
Check The Outside Pipe
If the copper pipe outside is damp or leaves marks on the wall or ground, the PRV may be passing. Sometimes it only lets water out when the boiler gets hot, so it can be missed during a quick glance.
Look Around Radiator Valves
Run tissue around radiator valves, especially downstairs. If there is green staining, crusty residue or a damp patch near the valve, that small weep may be enough to make the boiler pressure drop overnight.
A Common London Pressure Fault We See
A customer in a converted flat near East Croydon had been topping the boiler up every night. The pressure looked fine after filling, then dropped to nearly zero by the next morning.
There was no leak under the boiler. No puddle by the radiators. The actual issue was the external PRV pipe passing slowly, made worse by an expansion vessel that was no longer absorbing pressure properly. Once the system got hot, it dumped a bit of water outside. By morning, the gauge had dropped again.
If you are in Croydon or nearby and the pressure keeps dropping like this, call 0203 576 6769. We will help work out whether it sounds like a boiler fault, pipework issue or service-related problem.
No Guesswork
We check the pressure behaviour, expansion vessel, PRV discharge, filling loop, radiator valves, visible pipework and signs of internal leaks before saying what needs doing.
Straight Advice
If it only needs a small repair, we will say that. If it is more than a service or needs proper boiler repair work, we will explain why. No rushed checks and no pretending every pressure issue is the same.
Local Experience
Gas Safe Registered — ID 662170. We work on Worcester Bosch, Vaillant, Ideal and Baxi boilers across Croydon, Thornton Heath, Addiscombe, Norbury and nearby London areas.
What Happens Next
The cost depends on what is causing the pressure drop. A leaking radiator valve, a faulty PRV, a flat expansion vessel and hidden pipework leak are all different jobs, so we do not invent a fixed price before seeing the problem.
We will explain what we find, what can be repaired, and whether the boiler needs a service, pressure repair or further leak investigation before any work goes ahead.
What Customers Have Said
“The boiler kept losing pressure and I could not find a leak anywhere. MPS found the issue quickly and explained it properly before doing the repair.”
— Sarah, Croydon“Really helpful. They checked the boiler, radiators and outside pipe instead of just topping it up and leaving. Straightforward advice.”
— Michael, Thornton Heath“Pressure was dropping every morning in our flat. They found the fault and got the heating running again without making it complicated.”
— Priya, East CroydonNeed Help With Boiler Pressure Dropping Overnight?
If your boiler pressure keeps dropping overnight and you are constantly topping the system back up, it is usually worth getting it checked properly before it turns into a bigger repair. We cover Croydon, Thornton Heath, Addiscombe, Norbury, South Croydon, West Croydon, Selhurst and East Croydon.
Most people call after the same pattern repeats for a few days. Pressure drops overnight, heating still sort of works, then eventually the boiler starts locking out or needing topped up every morning before work. Sometimes it is a small valve leak. Sometimes the boiler has been quietly losing pressure for months and finally gets bad enough to notice.
When you call, we will normally ask a few simple things first. How low the pressure is falling, how often you are refilling it, whether the pressure rises when the heating comes on, and if you have spotted any drips outside near the copper discharge pipe. Those details usually tell you quite a lot before anyone even arrives.
Boiler Pressure Dropping Overnight FAQs
Why does my boiler pressure only drop overnight?
It often becomes more noticeable overnight because the heating cools down and the water inside the system contracts. If there is a weak expansion vessel, a small leak, a passing PRV or hidden pipework issue, the pressure can drop by morning.
Can a boiler lose pressure without a visible leak?
Yes. Water can escape through the outside PRV pipe, under floors, around radiator valves, inside the boiler, or through very small weeps that dry before you see them.
Is it safe to keep topping up my boiler?
Doing it once may be fine, especially after bleeding radiators. But if you are topping it up every day or every few days, stop treating it as normal. Constant fresh water can dilute inhibitor, bring oxygen into the system and make corrosion worse.
Could the expansion vessel cause pressure to drop overnight?
Yes. A faulty or flat expansion vessel can make the pressure rise too high when the heating is on, push water out through the PRV, then leave the boiler low on pressure once everything cools down.
Do I need a boiler repair or just a boiler service?
Not always easy to tell at first. If the pressure has dropped once after bleeding a radiator or topping the system up, a normal service and pressure check might be enough. But if you are refilling the boiler every few days, or waking up to low pressure most mornings, there is usually something else going on in the system.
A proper diagnosis is normally the starting point. We regularly find things like failing expansion vessels, PRVs dripping outside, slow radiator valve leaks, or small pipework leaks under floors that a standard service alone would not fix. Sometimes the service uncovers the problem anyway. Other times the boiler has already moved past servicing territory and into repair work.
Do you cover boiler pressure problems in Croydon?
Yes. MPS Heating & Plumbing LTD covers Croydon, Thornton Heath, Addiscombe, Norbury, South Croydon, West Croydon, Selhurst, East Croydon and nearby London areas.