Boiler Losing Pressure In Croydon?
Here’s what usually causes it, what you can safely check, and when it needs an engineer.
If your boiler is losing pressure and you keep having to top it up, or your boiler keeps losing pressure every few days, something is normally happening inside the heating system. Sometimes there is no visible leak at all, which is why many homeowners assume the boiler is losing pressure but no leak can actually be found.
We see this a lot in Croydon and South London homes, especially older houses in Thornton Heath, flats near East Croydon, and properties where pipework runs under floors or behind boxed-in walls.
MPS Heating & Plumbing LTD works across Croydon, Addiscombe, Norbury, South Croydon, West Croydon and Selhurst, diagnosing pressure faults on Worcester Bosch, Vaillant, Ideal and Baxi boilers.
MPS Heating & Plumbing LTD has a 5.0 Google rating from 62 reviews.
If you are in Croydon and the pressure keeps dropping, call 0203 576 6769 and we can talk through what is happening before you book anything in. If the issue has already left you with no heating or hot water, see our emergency boiler service in Croydon page.
Repeated pressure loss means something needs checking
Hidden leaks, PRV discharge and expansion-vessel faults can all cause the pressure to keep falling.
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If you are wondering why your boiler is losing pressure, the answer usually comes down to either water escaping somewhere in the system or pressure control components not behaving properly.
Most modern combi boilers and sealed heating systems need pressure to push water around the radiators and pipework. If a combi boiler is losing pressure repeatedly, the boiler may stop firing, show an error code, or leave you with no hot water while the heating still works. When the pressure drops too low, the boiler may stop firing, show an error code, or keep needing to be reset.
Normal pressure is usually around 1 to 1.5 bar when the system is cold. It can rise when the heating is on because hot water expands. A small movement is normal. A regular drop back down to zero, or close to zero, is not normal and usually means the boiler is losing pressure somewhere within the sealed system.
In London homes, we often find the issue is not the boiler itself at first glance. It can be a small leak under timber floors, an old radiator valve weeping slowly, a pressure relief valve letting by outside, or an expansion vessel that has stopped doing its job properly.
If the problem started after a missed annual check, it may be worth looking at our whether servicing can help prevent pressure-related breakdowns so the pressure issue is not treated as guesswork.
Common Causes Of A Boiler Losing Pressure
| Possible Cause | What It Usually Means |
|---|---|
| Small leak on the system | A radiator valve, pipe joint, towel rail, or hidden fitting may be letting out a tiny amount of water. It may dry before you notice it. |
| Expansion vessel fault | The vessel may have lost its air charge or failed internally. Pressure can rise when hot, then drop again once the boiler cools. |
| PRV pipe dripping outside | The pressure relief valve may be passing water out through the copper pipe outside. This is easy to miss if the pipe exits low down or round the side wall. |
| Recent radiator bleeding | Bleeding radiators releases air, but it can also lower system pressure. One top-up after bleeding can be normal. Constant top-ups are not. |
| Internal boiler component leak | Seals, heat exchanger areas, or internal connections can leak inside the boiler casing. This needs a Gas Safe engineer, not DIY opening. |
The Pressure Can Drop Even When You Cannot See A Leak
This is the bit that catches people out. A homeowner tops the boiler up, the heating works for a day or two, then the gauge is down again.
They look around the boiler cupboard, see no water, and assume the boiler is just “playing up”.
This is why a combi boiler can keep losing pressure but show no obvious leaks inside the property.
In older Thornton Heath and Norbury houses, pipework can run under floorboards with old compression joints hidden away. In flats near East Croydon, pipework may disappear behind boxed-in sections or into awkward cupboards where a small leak is not visible.
An engineer will usually look at the pressure pattern first. Does it drop overnight? Does it rise too high when heating is on? Is there water from the outside PRV pipe? That pattern tells you more than guessing.
It is not always dramatic. Sometimes the fault is boring, slow, and hidden. But it still needs finding before it causes bigger damage.
When Topping Up Is Okay
If you have just bled radiators or the system has had work done, one top-up may be normal. Set it around the correct cold pressure and watch what happens over the next day or two.
When Topping Up Is A Warning
If you are topping it up every few days, every week, or the boiler constantly loses pressure after heating has been on, there is likely a fault that needs diagnosing properly.
If the boiler has been acting differently for a while, our signs your boiler needs servicing guide explains some of the early patterns we commonly see.
Why It Can Damage The System
Constantly adding fresh water can bring more oxygen and minerals into the heating system. Over time that can make corrosion, sludge, and radiator problems worse.
A Common Croydon Callout We See
A customer in South Croydon had a combi boiler losing pressure every other morning. No puddle near the boiler. No water around the radiators. The heating still worked once they topped it up, so they left it for a few weeks.
When checked properly, the outside PRV pipe was staining the wall slightly. The pressure was rising too high when the heating got hot, then discharging outside and dropping again when it cooled. That pointed toward the expansion vessel and pressure relief side, not a random boiler reset issue.
If you are seeing that kind of pattern in Croydon, call 0203 576 6769. It is better to check it before it turns into a no heating call on a cold evening.
No Guesswork
We look at the pressure gauge, heating behaviour, radiator valves, visible pipework, PRV discharge pipe, and the boiler condition before saying what is likely wrong.
If the issue points toward overdue maintenance, our guide on what happens during a boiler service explains the checks involved.
Straight Advice
If it only needs a small fix, we will say so. If it needs further leak tracing, plumbing work, or boiler repair work, we explain why before carrying out work. For wider leaks, pipework or water issues, you can also see our plumbers in Croydon page.
Local Experience
MPS Heating & Plumbing LTD is Gas Safe Registered — ID 662170, working across Croydon homes on Worcester Bosch, Vaillant, Ideal and Baxi boilers. If you’re dealing with recurring manufacturer-specific pressure faults, see our Vaillant Boiler Repairs Croydon and Glow-worm Boiler Repairs Croydon pages.
What Happens Next
There is no honest fixed price without seeing what is causing the pressure drop. A simple radiator valve issue is different from a hidden leak under flooring, and an expansion vessel fault is different again.
We check the likely causes first, explain what we find, and quote before repair work goes ahead. No rushed checks. No pretending a service fixes every pressure fault.
If the boiler has simply gone too long without maintenance, our guide on how often a boiler should be serviced may help going forward.
Real Google Reviews From MPS Customers
“Ansar is a true professional that takes pride in his work. His service is excellent, efficient and the site left clean and tidy. After completing 20+ jobs for my management business, I can highly recommend him.”
— James Hutton“Straight-forward, friendly and professional boiler service recommended via a work colleague. I would onward recommend.”
— Shane H“Great work! Really quick response and turnaround to identify and fix the problem. Followed up to make sure there were no…”
— Michael Arko-AdjeiBoiler Pressure Keeps Dropping?
If you are in Croydon, East Croydon, West Croydon, Norbury, Selhurst, Thornton Heath or Addiscombe, we can help diagnose why the boiler keeps losing pressure.
When you call, we will ask how often it drops, what the gauge reads cold and hot, whether there is any water near the boiler, and whether the outside copper pipe is dripping.
Boiler Losing Pressure FAQs
Why does my boiler keep losing pressure?
A boiler usually keeps losing pressure because water is escaping somewhere in the heating system, the expansion vessel is not controlling pressure properly, or the PRV is discharging water outside. The pattern of the pressure drop helps narrow it down. If the pressure also rises too high when heating is on, see our guide on boiler pressure being too high.
Can a boiler lose pressure with no visible leak?
Yes. The leak can be under floorboards, behind walls, around a radiator valve, inside the boiler casing, or outside through the pressure relief pipe. A small leak may dry before you spot it.
What pressure should my boiler be on?
Most sealed systems sit around 1 to 1.5 bar when cold. It may rise when the heating is running. If it drops close to zero repeatedly, that is not normal and should be checked.
Is it safe to keep topping up boiler pressure?
Doing it once after bleeding radiators may be fine. Constantly topping it up is not a fix. It can hide the real fault and may add more oxygen and minerals into the system, which can make sludge and corrosion worse over time.
When should I call a Gas Safe engineer?
Call an engineer if the pressure keeps dropping, if the boiler shows fault codes, if you see water inside or under the boiler, if the outside PRV pipe is dripping, or if you need the boiler casing opened. Gas-related boiler checks should not be DIY.