Radiator Not Heating — Diagnosis & Repair

A radiator that's cold at the top but hot at the bottom usually means trapped air — a bleed job. Cold at the bottom but hot at the top usually points to sludge or debris settled inside it — a different job entirely, sometimes needing a power flush. One radiator cold while the rest of the house is fine is often a stuck or closed valve, or a balancing issue. All four look similar from the sofa; they're not the same fix.

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Where it's a single stubborn radiator, our engineers check the obvious first — valve position, whether it's been bled recently, whether the lockshield valve has been altered — before assuming anything more involved. Where it's several radiators across the system, or radiators that have been getting progressively less effective over time, that pattern usually points at system-wide sludge, which is a different conversation from a single bleed valve.

Radiators that never worked correctly since a boiler swap or a new radiator being added are often a balancing problem — the system needs the flow through each radiator set correctly relative to the others, not just parts replaced.

A single-radiator bleed or valve check is a small job; sludge assessment or system balancing across multiple radiators is quoted once the scope is confirmed on site.

What you get

Correctly distinguishes trapped air, sludge, a stuck valve, and a balancing fault — the four usual causes.
Radiator bleeding and valve repair for single cold-radiator faults.
Sludge and debris assessment when multiple radiators are affected, with power-flush advice where relevant.
System balancing when radiators have never heated evenly since an installation or change.
Explains the cause in plain terms before recommending a fix.
Works alongside boiler and pressure diagnosis when radiator symptoms point at a wider system issue.

How it works

A method, not a guess

Which radiators, what pattern

One radiator or several, cold at the top, bottom, or all over, and whether it changed suddenly or has been getting worse — the pattern narrows the cause before the visit.

On-site check

The engineer checks valves, bleeds where trapped air is suspected, and assesses for sludge or debris where the pattern suggests it — rather than assuming the same fix for every cold radiator.

Confirm the cause

If it's more than a bleed — a stuck valve, heavy sludge, or a balancing issue across several radiators — you're told what's actually wrong and what fixing it properly involves.

Fix and check flow

Once approved, the engineer carries out the repair, bleed, or balancing adjustment and confirms the radiator is heating evenly, top to bottom, before finishing.

Questions

Radiator Not Heating FAQs for London homeowners

Why is my radiator cold at the top but hot at the bottom?

That's the classic sign of trapped air, and it's usually fixed with a bleed — releasing the air lets water fill the radiator fully. It's one of the quickest and cheapest radiator faults to resolve.

Why is my radiator cold at the bottom but hot at the top?

This points the other way — towards sludge or debris settled at the bottom of the radiator, restricting water flow through that section. Bleeding won't fix it; it usually needs a flush, sometimes just of that radiator, sometimes system-wide if several radiators show the same pattern.

One radiator never gets as hot as the others — why?

Often a balancing issue — the flow through that radiator relative to the rest of the system needs adjusting at the lockshield valve. It can also be a partially closed or stuck valve. Both are diagnosable on site without guesswork.

Do I need a power flush, or is that always oversold?

It genuinely helps when sludge is affecting multiple radiators or heating efficiency system-wide, but it isn't the answer to every cold radiator — a single radiator with trapped air just needs bleeding. We'll tell you which situation you're actually in before recommending a flush.

Can a cold radiator be a sign of a bigger boiler problem?

Occasionally — if every radiator is lukewarm rather than properly hot, that can point at low system pressure or a pump issue rather than the radiators themselves. If it's isolated to one or two radiators, it's far more likely a local fault at the radiator, not the boiler.

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