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Search “boiler not heating radiators” and you’ll find the same handful of causes repeated across dozens of sites: trapped air, low pressure, a stuck diverter valve, sludge. They’re not wrong. They’re just written as a checklist to scroll through rather than the way an engineer approaches the job, which is to start from what you’re observing and rule things out in a specific order, rather than guessing.
This guide is written from that side of the job. We carry out boiler repair London wide as a Gas Safe registered team, including Worcester Bosch boiler servicing London properties from Victorian terraces in Islington to new-build flats in Nine Elms, and this is genuinely how a call out gets worked, not a rewritten version of someone else’s list. If you’ve typed emergency boiler repair London into a search bar because your heating’s stopped and you’re already bracing for a replacement quote, read through this before you agree to anything.
Match your symptom to the likely cause
| What you’re seeing | What people usually assume | What it usually actually is | DIY or call an engineer |
| Hot water works, radiators stay cold | The boiler is dying | Diverter valve stuck in the hot water position | Call an engineer |
| Pressure keeps dropping every few days | The boiler has a leak somewhere | Expansion vessel has lost its internal charge | Call an engineer |
| One radiator cold, rest working fine | That radiator has failed | Trapped air, or sludge sitting at that radiator | Try bleeding it yourself first |
| All radiators lukewarm, uneven heat | Boiler is failing or undersized | System was never balanced correctly | Call an engineer |
| Boiler locks out only in cold weather | Boiler has broken down | Frozen condensate pipe | Try thawing it yourself first |
| Boiler cycles on and off with no pattern | Boiler is faulty | Programmer misconfigured, or a smart thermostat conflicting with it | Call an engineer |
| No heat, no hot water, older system boiler | Complete boiler failure | Circulating pump has failed | Call an engineer |
| Banging or whistling noises (kettling) | Boiler is about to break down | Limescale restricting flow, boiler overheating the restricted water | Call an engineer |
| Droning or humming sound from the boiler | Something’s badly wrong internally | Usually a failing pump, or mains pressure arriving too high | Call an engineer |
| Heating fine but performance dropping over months | Boiler is just getting old | Property extended or converted since the boiler was sized | Get a load survey |
This table alone rules out the majority of the “boiler’s broken” call outs we attend across London before a single panel comes off.
Why the noise you hear tells you something specific
Boiler noises get lumped together constantly, but each one points somewhere different, and manufacturers themselves are specific about this. Vaillant’s own guidance distinguishes drone noises (a failing pump), humming (mains water arriving at too high a pressure), and whooshing (a blocked air intake or dirty air filter) as three separate faults with three separate fixes, not variations of the same problem. Kettling, the whistling or banging sound most homeowners panic over, is specifically caused by limescale restricting water flow through the heat exchanger, forcing the boiler to heat a smaller, trapped volume of water past boiling point.
The reason this distinction matters: if you describe “a noise” to an engineer without specifying which one, you’ll get a longer diagnostic visit than necessary. Recording a short video or voice note of the actual sound before your appointment genuinely speeds up the visit, because droning, humming, whooshing, and kettling each send an engineer straight to a different component.
The causes worth understanding properly, not just naming
Diverter valve failure
This mechanical valve inside a combi boiler switches priority between your taps and your radiators. After years of use, particularly in hard water areas common across large parts of London, it can be seized in the hot water position, giving you a hot shower and a stone-cold living room. This is a recognised wear part on combi boilers generally, not evidence the whole unit is failing, and it’s a repair, not a reason to replace anything.
Expansion of vessel failure
A small pressurised tank absorbs water expansion as your system heats. Once its internal bladder loses charge, pressure drops repeatedly no matter how many times you top it up through the filling loop. We regularly meet homeowners who have re-pressurised the system three or four times over several weeks without realising the vessel itself needs attention, not the boiler.
System balance
Every radiator in a heating system is meant to allow a flow rate proportional to its size and distance from the boiler. If this was never set up correctly or has drifted after radiators were replaced over the years, you get exactly the pattern of some rooms warm and others cold that people assume means the boiler is struggling. Rebalancing is an adjustment at the radiator valves, not a boiler repair.
Sludge and debris in older pipework
London’s Victorian and Edwardian housing stock in particular tends to accumulate magnetite sludge in radiators and pipework over decades. This restricts circulation, makes the boiler work harder than it should, and can trip safety cut-outs that look identical to an internal fault. Power flush addresses it. Replacing the boiler does not, since the sludge is still sitting in the same pipes afterwards.
Boiler sizing after property changes
A boiler correctly sized for a three-bedroom house becomes undersized now as a loft conversion or rear extension adds another bathroom and several more radiators onto the same system. The symptoms, slow heating, inconsistent hot water, short cycling, look exactly like age-related failure but actually come from asking the unit to do more than it was ever designed for.
How a proper diagnostic visit works
A rushed visit starts by opening the boiler casing. A proper one starts outside it. The order we work through on a genuine call out looks like this: first, the symptom itself and when it started, since those alone rule several causes in or out immediately. Second, the system pressure and whether it’s stable or actively dropping. Third, the radiators themselves checked individually for temperature consistency, since uneven heating across the property points away from the boiler entirely. Only once those three are checked does the boiler casing come off, and even then, the diverter valve, expansion vessel connection, and control board get checked in that order before any component gets condemned as faulty.
If you’ve had a replacement recommended within minutes of an engineer arriving, without any of the above being checked first, that’s worth questioning.
When it’s genuinely time to call rather than search
Total loss of heat and hot water in cold weather, any smell of gas, a boiler that won’t reset after following the manufacturer’s instructions, or visible water leaking from the unit itself all warrant a same-day call rather than further troubleshooting. Everything in the table above can usually wait for a scheduled appointment without any risk to your system or your safety.
Get a proper diagnosis, not a guess
If your heating is doing any of the above, don’t guess it and don’t accept a replacement recommendation before the causes above have genuinely been ruled out. Call us on 07877767776 or visit www.boilerserviceslondon.co.uk and we will work through the diagnosis properly, covering Worcester Boiler Repair London, Worcester Boiler Service London, and Vaillant systems across Greater London, and we’ll tell you honestly when a repair is the right call instead of a replacement.
FAQs
It happens across most combi brands using the same basic design, but it’s a well-documented pattern on units around eight to ten years old regardless of manufacturer, which is why it comes up so often on Worcester Boiler Repair London call outs specifically.
Not necessarily. Kettling, droning, and humming often develop before the boiler control system detects anything wrong enough to trigger a code. The absence of a code doesn’t mean the absence of a developing fault, particularly with kettling caused by limescale, which worsens gradually.
This points to that specific radiator, not the boiler or the wider system. Check first whether its valve has been closed off, then bleed it to release trapped air. If it’s still cold after both checks, sludge sitting at the base of that particular radiator is the likely cause.
Basic bleeding for trapped air is safe to do yourself. Full system balancing, which involves adjusting lockshield valves against a proper flow calculation, is worth having a Gas Safe engineer handle, since getting it wrong just shifts the uneven heating problem into different rooms.
The boiler was never upgraded to match the extra radiators and bathroom added to the system, so it’s now working beyond its original design capacity. This isn’t a developing fault, it’s a mismatch that needs a proper load calculation rather than a repair.