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HEATING ADVICE

Cold radiators:
bleed, balance or flush?

Where the radiator is cold tells you which of the three it is. Get that right and you save yourself paying for the wrong one.

5.0from 213 Google reviews

Gas Safe registeredEvery engineer on the job

Approved InstallerExtended manufacturer warranties

Get your heating sorted

THE SHORT ANSWER

Cold at the top means air, bleed it. Cold at the bottom means sludge. Whole rooms cold means it needs balancing.

Those three symptoms have three different fixes and people regularly pay for the wrong one. Air collects at the top of a radiator, so a radiator hot at the bottom and cold across the top just needs bleeding, which takes a minute and a radiator key. Sludge sinks, so cold along the bottom with a hot top means debris lying in the bottom of the radiator. And if individual radiators are fine but the far end of the house never gets warm, nothing is broken: the system is unbalanced and the flow is taking the easy route.

THE TWO JOBS

Bleeding and balancing are not the same thing.

People use the words interchangeably and then wonder why bleeding did not fix a cold back bedroom. They fix different problems.

Bleeding

Five minutes, one radiator

What it fixes

Air trapped at the top of a radiator

You need

A radiator key and a cloth

Heating

Off and cooled down first

What to do

Open the top valve slowly until water appears

Afterwards

Check boiler pressure, top up if under 1 bar

If it recurs

Air is getting in. Worth investigating

Balancing

A couple of hours, whole house

What it fixes

Some rooms warm, far rooms never warm

You need

Two thermometers and patience

The principle

Water takes the easiest path unless you stop it

What to do

Adjust the lockshield valve on each radiator

The target

About 11 to 12 degrees drop across each radiator

Cost

Nothing in parts. It is all time

THIS IS WHAT SLUDGE LOOKS LIKE

Cold at the bottom, and this is why.

Those are the magnets out of a magnetic filter on a system we look after. Everything on them is magnetite: iron oxide that forms as the inside of steel radiators corrodes, then circulates until something catches it.

Without a filter it does not disappear, it settles. It sinks to the bottom of radiators, which is the cold band along the bottom, and it collects in the boiler heat exchanger, which is what makes a boiler bang and kettle. That is why a filter goes on every boiler we fit, and why we clean it at every service.

Heat rises and dirt sinks, which is why the cold patch tells you the fault. Air collects along the top. Sludge settles along the bottom. A radiator that is cool all over is usually getting less flow than the ones nearer the boiler.

THE THREE FIXES

Which one you actually need.

Cold at the top: bleed it

Turn the heating off and let it cool, put a radiator key on the valve at the top corner with a cloth underneath, turn slowly until air hisses out and water appears, then close it. Check the boiler pressure afterwards and top up if it has dropped below 1 bar. If a radiator needs bleeding repeatedly, air is getting in somewhere and that is worth investigating.

Cold at the bottom: that is sludge

Bleeding will not touch it. Debris has settled in the bottom of the radiator and is blocking flow. Sometimes a single radiator can be taken off and flushed, but if several are doing it the whole system needs a power flush and a magnetic filter fitting to stop it coming back.

Far rooms never warm: it needs balancing

Water takes the path of least resistance, so radiators nearest the boiler get more than their share. Balancing means adjusting the lockshield valve on each radiator so the flow is shared properly. It is fiddly and takes a couple of hours across a whole house, but it costs nothing in parts and it is usually the difference between a cold back bedroom and a warm one.

YOUR SYMPTOM

So which one do you need?

Cold across the top, hot underneath

Bleed it.

Air. Radiator key, heating off, open the valve at the top corner slowly until water comes out, close it, check the boiler pressure. Five minutes and it costs nothing.

Hot at the top, cold along the bottom

That is sludge.

Bleeding will do nothing. Debris has settled in the bottom of the radiator. One radiator can often be taken off and flushed. Several doing it means the system needs a proper flush and a filter fitting.

Downstairs warm, upstairs or far rooms cold

It needs balancing.

Nothing is broken. The radiators nearest the boiler are taking more than their share and the far ones are getting what is left. Balancing shares it out properly and usually costs nothing but time.

Dirty water when you bleed, and kettling

Get the system looked at.

Black water out of a bleed valve plus a boiler that bangs or kettles means the system is dirty enough to be shortening the boiler’s life. That is the case where a power flush genuinely is the right answer.

COMMON QUESTIONS

Questions we get asked.

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

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

How do I bleed a radiator?

What does balancing radiators mean?

Why are my upstairs radiators cold when downstairs is fine?

Do I need a power flush?

Radiators not doing what they should?

Tell us which radiators, and whether they are cold at the top or the bottom. That tells us whether it is a bleed, a balance or a flush before we set off.

BOILER SERVICING

GAS SAFETY (CP12)

HEATING & PLUMBING

LANDLORD PACKAGES

GOT A PLUMBING OR HEATING JOB IN BRIDGEND?

Get your heating sorted

testimonials

Real customers. Real results.

213 five star reviews from homeowners across South Wales. Every one of them left on Google, not written by us.

David Rees

August 2026

★★★★★

An outstanding Gas Engineer and a fine individual. I cannot recommend LMB Plumbing & Heating highly enough. He installed a new Gas Boiler and overhauled our 25 year old Central Heating system for a good price.

LMB REPLIED

Thank you, David — we’re delighted to hear you had such a positive experience. We're glad the new gas boiler installation and overhaul of your…

Lee Major

July 2026

★★★★★

I can’t recommend Lloyd highly enough. From the initial Contact right through to completion of the job , his customer service was outstanding. Lloyd arrived exactly on time, was polite , friendly and professional throughout. The quality of his workmanship was excellent and he clearly takes great…

LMB REPLIED

Thank you so much, Lee, for your fantastic review and for recommending Lloyd. We’re delighted he arrived on time, was professional and friendly, and…

Dafydd Pugh

July 2026

★★★★★

Excellent. Lloyd provided an excellent service - good communication, timely arrival, left the work area clean and did the work efficiently

LMB REPLIED

Thank you Dafydd

Nigel Walters

July 2026

★★★★★

LMB responded the same day to repair a gas problem I had. Other companies I contacted were unhelpful, and most of them actually declined the work. LMB were positive & engaging from the start, fixed the problem and charged a fair price. Highly recommended, ask for Lloyd.

LMB REPLIED

Appreciate the feedback Nigel, pleasure to do work for.

Scott Rivers

July 2026

★★★★★

LMB plumbing & Heating have been fantastic since first conversation / quote to boiler install. From boiler install we had him come back to fit our shower room and the standard and service is second to none. If I could give more stars I would. Class from the lads at LMB plumbing & Heating

LMB REPLIED

Thankyou Scott, pleasure fitting the boiler and heating system for you!

Samantha

July 2026

★★★★★

I have had these gentlemen come to my property on two separate occasions. Very prompt, very polite and got the job done. I even contacted them late on Sunday evening and they politely arranged for a next day appointment, came and solved the problem. Will always use and recommend!

LMB REPLIED

Thank you so much for your wonderful review, Samantha. We're thrilled our team was prompt, polite, and able to resolve your issue quickly on short…

Craig Williams

July 2026

★★★★★

Lloyd recently fitted our new boiler. It has been fitted to a very high standard and he took care of the house when working. Polite and punctual.

LMB REPLIED

Thank you, Craig, for the excellent review. We're delighted Lloyd fitted your new boiler to such a high standard and took good care of your home. We…

Rachel 01

June 2026

★★★★★

Lloyd came out and quoted a job for us, to change our bath, sink, & toilet. He has been brilliant from start to finish I’m so glad my mum recommended him. He was incredibly professional and nothing was too much trouble. Will most certainly have him back to do future work and will 100% recommend him…

LMB REPLIED

Thank you so much, Rachel, for the wonderful review and for recommending us to your mum. I’m delighted the bathroom, sink, and toilet work met your…

David Rees

August 2026

★★★★★

LMB REPLIED

Thank you, David — we’re delighted to hear you had such a positive experience. We're glad the new gas boiler installation and overhaul of your…

Lee Major

July 2026

★★★★★

LMB REPLIED

Thank you so much, Lee, for your fantastic review and for recommending Lloyd. We’re delighted he arrived on time, was professional and friendly, and…

Dafydd Pugh

July 2026

★★★★★

Excellent. Lloyd provided an excellent service - good communication, timely arrival, left the work area clean and did the work efficiently

LMB REPLIED

Thank you Dafydd

Nigel Walters

July 2026

★★★★★

LMB REPLIED

Appreciate the feedback Nigel, pleasure to do work for.

Scott Rivers

July 2026

★★★★★

LMB REPLIED

Thankyou Scott, pleasure fitting the boiler and heating system for you!

Samantha

July 2026

★★★★★

LMB REPLIED

Thank you so much for your wonderful review, Samantha. We're thrilled our team was prompt, polite, and able to resolve your issue quickly on short…

Craig Williams

July 2026

★★★★★

Lloyd recently fitted our new boiler. It has been fitted to a very high standard and he took care of the house when working. Polite and punctual.

LMB REPLIED

Thank you, Craig, for the excellent review. We're delighted Lloyd fitted your new boiler to such a high standard and took good care of your home. We…

Rachel 01

June 2026

★★★★★

LMB REPLIED

Thank you so much, Rachel, for the wonderful review and for recommending us to your mum. I’m delighted the bathroom, sink, and toilet work met your…

ADVICE

Thirty guides,written by our team

Boilers, radiators, gas certificates, and what the jobs actually cost.

Worcester, Baxi or Vaillant?

Build quality, warranties, and what each one costs fitted.

boilers
buying

How much is a new boiler in Bridgend?

From £1,900 fitted, and what moves the price up or down.

costs
bridgend

What does a boiler service cost?

£84, what gets checked, and why skipping it voids your warranty.

costs
servicing

Landlord gas safety (CP12) in Wales

The annual legal duty, and how to do it in one visit.

landlords
legal

Boiler losing pressure?

What to check yourself, and when to stop and call.

fixes
boilers

Radiators not heating up

Cold at the top, cold at the bottom, and what each means.

fixes
radiators

ADVICE

Thirty guides,written by our team

Boilers, radiators, gas certificates, and what the jobs actually cost.

Worcester, Baxi or Vaillant?

Build quality, warranties, and what each one costs fitted.

boilers
buying

How much is a new boiler in Bridgend?

From £1,900 fitted, and what moves the price up or down.

costs
bridgend

What does a boiler service cost?

£84, what gets checked, and why skipping it voids your warranty.

costs
servicing

Landlord gas safety (CP12) in Wales

The annual legal duty, and how to do it in one visit.

landlords
legal

Boiler losing pressure?

What to check yourself, and when to stop and call.

fixes
boilers

Radiators not heating up

Cold at the top, cold at the bottom, and what each means.

fixes
radiators

ADVICE

Thirty guides,written by our team

Boilers, radiators, gas certificates, and what the jobs actually cost.

Worcester, Baxi or Vaillant?

Build quality, warranties, and what each one costs fitted.

boilers
buying

How much is a new boiler in Bridgend?

From £1,900 fitted, and what moves the price up or down.

costs
bridgend

What does a boiler service cost?

£84, what gets checked, and why skipping it voids your warranty.

costs
servicing

Landlord gas safety (CP12) in Wales

The annual legal duty, and how to do it in one visit.

landlords
legal

Boiler losing pressure?

What to check yourself, and when to stop and call.

fixes
boilers

Radiators not heating up

Cold at the top, cold at the bottom, and what each means.

fixes
radiators

BEFORE YOU GO

Keep reading.

More of the same, written by the engineers who do the work.

READ THIS FIRST

What a new boiler actually costs

Real fitted prices, what pushes them up, and the three lines to check in any quote.

Read it

→

OTHER SERVICES

New boiler installation

→

Boiler servicing

→

Boiler repair

→

Gas engineer

→

WORTH A READ

Worcester vs Baxi vs Vaillant vs Ideal

→

How to reset a boiler

→

Boiler losing pressure

→

How long a boiler lasts

→

Cold radiators:bleed, balance or flush?

Cold radiators:bleed, balance or flush?

Cold radiators:bleed, balance or flush?

Cold at the top means air, bleed it. Cold at the bottom means sludge. Whole rooms cold means it needs balancing.

Cold at the top means air, bleed it. Cold at the bottom means sludge. Whole rooms cold means it needs balancing.

Cold at the top means air, bleed it. Cold at the bottom means sludge. Whole rooms cold means it needs balancing.

Bleeding and balancing are not the same thing.

Bleeding and balancing are not the same thing.

Bleeding and balancing are not the same thing.

Bleeding

Bleeding

Balancing

Balancing

Cold at the bottom, and this is why.

Cold at the bottom, and this is why.

Cold at the bottom, and this is why.

Which one you actually need.

Which one you actually need.

Which one you actually need.

Cold at the top: bleed it

Cold at the top: bleed it

Cold at the bottom: that is sludge

Cold at the bottom: that is sludge

Far rooms never warm: it needs balancing

Far rooms never warm: it needs balancing

So which one do you need?

So which one do you need?

So which one do you need?

Bleed it.

Bleed it.

Bleed it.

That is sludge.

That is sludge.

That is sludge.

It needs balancing.

It needs balancing.

It needs balancing.

Get the system looked at.

Get the system looked at.

Get the system looked at.

Questions we get asked.

Questions we get asked.

Questions we get asked.

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

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

How do I bleed a radiator?

What does balancing radiators mean?

Why are my upstairs radiators cold when downstairs is fine?

Do I need a power flush?

Radiators not doing what they should?

testimonials

Real customers. Real results.

Thirty guides,written by our team

Thirty guides,written by our team

Thirty guides,written by our team

Keep reading.

Keep reading.

Keep reading.

What a new boiler actually costs

What a new boiler actually costs

What a new boiler actually costs

OTHER SERVICES

WORTH A READ

Cold radiators:
bleed, balance or flush?

Cold radiators:
bleed, balance or flush?

Cold radiators:
bleed, balance or flush?