Why Is My Radiator Not Heating Up? Causes, Fixes and When to Call a Plumber (London, 2026)

Last updated: September 2026

When the heating goes back on for the winter, a radiator that stays stubbornly cold is one of the most common things London homeowners notice first. In a city full of older terraces, converted flats and systems that have been added to over decades, a cold radiator is rarely a mystery once you know what the cold pattern is telling you. This guide explains what each symptom means, which fixes are safe to do yourself, what a plumber should charge to sort the rest, and the point at which the job becomes work only a Gas Safe registered engineer may legally touch.

TL;DR

A radiator that is cold at the top usually has trapped air, and bleeding it takes a few minutes. Cold at the bottom points to sludge, which needs a professional flush. If every radiator is cold, check the boiler, timer and pressure. Bleeding is safe to do yourself; boiler and gas work is for a Gas Safe registered engineer.

Read the cold pattern first

Before you touch anything, feel the radiator with the heating on. Where it is cold tells you what is wrong, and it is the fastest way to know whether this is a two-minute job or a call to a professional.

• Cold at the top, warm at the bottom: trapped air. Bleed it (safe to do yourself).
• Cold at the bottom, warm at the top: sludge build-up. Needs a professional flush.
• One radiator stone cold: a valve problem, usually the thermostatic valve or the lockshield.
• Every radiator cold: look at the boiler, the timer or the system pressure, not the radiators.
• Warm on one floor, cold on another: commonly a balancing or circulating-pump issue.

The rest of this guide takes each of these in turn.

Cold at the top but warm at the bottom: trapped air

This is the classic and most easily fixed fault. Air finds its way into a central-heating system over time and, because it is lighter than water, it rises and collects at the top of the radiator. That pocket of air stops hot water filling the upper section, so the top stays cool while the bottom feels warm [1].

Which? describes bleeding a radiator to release that air as "usually a simple job" that "can quickly fix a radiator that's too cold" [1]. It is genuinely homeowner-safe, needs only a radiator key and a cloth, and is covered step by step below. If your radiators regularly go cold at the top, Which? notes that air tends to build up again over a few years, so occasional bleeding is normal maintenance rather than a sign of a deeper fault [1].

Cold at the bottom but warm at the top: sludge

The opposite pattern points to the opposite problem. When a radiator is warm across the top but cold along the bottom, the likely cause is sludge, a build-up of iron oxide and debris that settles at the base of the radiator and blocks the flow of hot water through the lower section.

Which? Trusted Traders lists "cold radiators with only hot strips at the top" as one of the clear signs a system needs a power flush, alongside dirty, discoloured water when a radiator is bled and poor circulation compared with how the system used to perform [2]. A power flush uses a pumping machine to push water at high velocity through the boiler, pipework and radiators to clear the sludge, and it finishes with an inhibitor dose to slow future build-up [2].

This is not a do-it-yourself fix. Which? is explicit: "This is not something to attempt yourself, particularly if your engineer also needs to open up your boiler or examine other faults in your central-heating system" [2]. It also warns that a power flush cannot repair broken parts or fix pressure loss caused by a leak, so it is a cure for sludge, not for everything [2]. If the cold-at-the-bottom pattern is what you are seeing, budget for a professional visit (costs are further down this guide).

One radiator cold, or every radiator cold?

Whether the problem is one radiator or all of them narrows the cause considerably.

If one radiator is completely cold, check that both of its valves are open. Most radiators have a thermostatic radiator valve (the TRV, with a numbered dial) at one end and a lockshield valve (under a plastic cap) at the other. Vaillant's guidance is that if both valves are open and the radiator is still stone cold, a valve may have become blocked by sludge or limescale and needs professional attention; a radiator that will not heat even after that may simply be reaching the end of its life and need replacing [10]. Our guide to what it costs to replace a radiator in London covers that scenario.

A TRV can also stick. Inside the valve is a small pin that can seize in the closed position, most often after a summer of sitting untouched. Many stuck pins can be freed by gently working them, but if the pin will not move, do not force it, because a snapped valve turns a small job into a bigger one. That is the point to call a plumber.

If every radiator is cold, the radiators are almost certainly fine and the fault sits with the boiler or the controls. Check the obvious things first: is the programmer or timer actually calling for heating rather than just hot water, and is the room thermostat turned up? Then look at the boiler's pressure gauge. A reading that has dropped below roughly 1 bar points to low pressure that needs topping up [10]. If the boiler is showing a fault code or will not fire, that is boiler territory and must go to a Gas Safe registered engineer [7].

Warm on one floor, cold on another

If one part of the home heats nicely while another stays lukewarm, the most common explanations are an unbalanced system or a struggling circulating pump. Balancing shares the flow of hot water correctly across every radiator; without it, the radiators nearest the boiler heat quickly and draw off most of the heat, leaving those further along the circuit cool. Heating expert Martyn Bridges, quoted by Homebuilding & Renovating, puts it plainly: unbalanced radiators "can cause a number of issues such as cold spots in your home with uneven heating" [13]. In many homes this shows up as one whole floor running cooler than the other.

Balancing is a job a confident homeowner can do with a radiator thermometer, but it is fiddly, and Homebuilding & Renovating notes a typical professional cost of around £100 to £150 if you would rather have it done properly [13]. A pump that is failing or air-locked, by contrast, is a replacement job for a professional, priced on inspection.

How to bleed a radiator safely, step by step

Bleeding is the one fix on this list that most people can do themselves. Following Which?'s guidance [1]:

1. Turn the heating off and let the radiators cool for 20 to 30 minutes. Bleeding a hot radiator risks scalding.
2. Find the bleed valve, a small square-socket valve at the top corner of the radiator, and hold a container and cloth beneath it to catch drips.
3. Insert the radiator key and turn it slowly anticlockwise, no more than about half a turn. You should hear a hiss as the trapped air escapes.
4. Close the valve as soon as a steady trickle of water appears instead of a hiss. The air has cleared.
5. Work through the radiators one at a time, starting with the one furthest from the boiler and lowest in the house, then moving inward and upward.
6. Check the boiler pressure afterwards. Bleeding often lowers the system pressure.

Which? gives a target pressure band of 1.0 to 2.0 bar, often marked in green on the gauge [1]. Treat that as a guide rather than a rule: many combi boilers are happiest at around 1.0 to 1.5 bar when cold, so check your own boiler's manual and the markings on its gauge before topping up.

Low boiler pressure, and topping it up

Low pressure is a frequent reason a whole system underperforms, and bleeding radiators is itself a common cause of a pressure drop [1]. Topping up is usually done through the filling loop, a braided flexible hose or a filling key that connects the mains cold supply to the heating circuit. Switch the boiler off and let it cool, open the valve slowly to let water in a little at a time while watching the gauge, and close it as soon as you reach the correct pressure. Do not overfill.

One warning worth heeding: if the pressure keeps dropping back within a few days of topping up, that usually points to a leak somewhere in the system rather than trapped air, and a repeat top-up will not fix it. Look for damp patches around pipes, joints and radiator valves, and get a plumber to trace it.

In winter, a frozen condensate pipe can stop everything

During a cold snap, a whole system can shut down at once because of the boiler's condensate pipe. This is the plastic pipe that carries waste water away from a condensing boiler; where it runs outside or through an unheated space it can freeze, blocking the pipe and forcing the boiler into lockout, often with a specific fault code [11].

Baxi's guidance is that you can safely thaw the exposed outside section with "a hot water bottle or a microwaveable heating pack ... or a cloth soaked in hot water," and adds that you "can also pour hot, but NOT boiling, water onto the blockage," warning that spilt water may freeze and make the ground slippery [11]. Never use boiling water straight from the kettle, and never cut, dismantle or alter the pipe or any part of the boiler yourself. Baxi is clear on the limit: "If it still doesn't work, you should call a Gas Safe engineer to come and look at it for you" [11]. Our guide to frozen and burst pipes in London covers the wider winter picture.

When it is a do-it-yourself job, and when to call a professional

The dividing line is simple and it is a legal one. Bleeding a radiator, checking a valve is open, gently freeing a stuck TRV pin and re-pressurising a sealed system through its filling loop are all general heating tasks you can do yourself.

Anything that involves opening up the boiler, its gas connection, combustion chamber or flue, or diagnosing a boiler fault, is gas work. By law, "no person shall carry out any work in relation to a gas fitting ... unless he is competent to do so" [7], and the Health and Safety Executive is explicit that gas work must be done by a Gas Safe registered engineer and that a registered engineer must not sign off work done by someone who is not registered [6]. A power flush, a pump replacement and any work inside the boiler casing all sit on the professional side of that line.

Before any gas or boiler work starts, ask to see the engineer's Gas Safe ID card and check the categories of work listed on the back, or verify them using the Gas Safe Register's online "check the register" tool. Our guide to finding a trusted tradesperson in London walks through the checks worth doing before you let anyone start.

If you rent, a cold radiator is very likely your landlord's problem to fix. Gov.uk lists heating and hot water among the repairs landlords are "always responsible" for [8], and Citizens Advice confirms these duties "can't be removed by anything your tenancy agreement says" and that a landlord "isn't allowed to pass on the cost" to you [9]. The catch is that the landlord only has to act once they know, so report the fault to your landlord or agent promptly and in writing, and keep a copy [9].

What it costs to fix a cold radiator in London (2026)

Most cold-radiator visits are a single scheduled call-out, and the common jobs (a bleed, a balance, freeing a stuck valve or clearing an air lock) are usually an hour or less of labour. London scheduled plumbing labour commonly runs around £40 to £85 an hour, and Which? notes that prices "are likely to be at the upper end of the scale if you live in London or the South East" [3]. Our plumber hourly rate guide for London has the full breakdown, and if the radiator has failed out of hours, see our emergency plumber cost guide.

Before you book, a fair written quote should itemise:

• Labour, with an hourly rate or a fixed price for the job.
• VAT, stated clearly (some quotes are shown before VAT).
• Any parts, such as a replacement TRV, pump or radiator.
• For a power flush, whether inhibitor and chemical neutralising are included.
• Testing and rebalancing after the work, so the system is left running correctly.
• The call-out or minimum charge, and whether it is separate from the first hour.

For the bigger jobs, the following are useful 2026 markers, all excluding VAT. A power flush commonly runs about £300 to £400 for a two-bed home with six to eight radiators, £400 to £550 for a three-bed with eight to ten radiators, and £550 to £700 or more for a larger home, according to Homebuilding & Renovating (September 2025) [12]. London jobs tend to sit at the top of those bands, and a large or heavily silted system can run higher, so always get a written quote. Fitting a magnetic filter to catch sludge in future costs around £100 to £150 [12]. Replacing a single radiator is a separate job we cover in the radiator replacement cost guide.

How to stop it happening again

A little maintenance keeps radiators heating evenly and protects the boiler.

• Book an annual service. Which? advises: "You should get your boiler serviced every year by a Gas Safe-registered engineer" [4], and warns that skipping the annual service is one of the most common ways to invalidate a boiler warranty [5]. Our boiler service cost guide explains what a service should include and what it costs in London.
• Keep the system clean. An inhibitor dose slows corrosion, and a magnetic filter captures sludge before it settles in your radiators. Ask your engineer to check the inhibitor at each annual service.
• Bleed occasionally. Air builds up over a few years, so a quick bleed every so often is normal [1].
• Balance once, then leave it. Balancing is usually a one-off job after installation or a major change; you only need to revisit it if uneven heating returns.

Frequently asked questions

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

That pattern means trapped air has collected at the top of the radiator, blocking hot water from filling the upper section. It is the most common radiator fault and the easiest to fix. Bleeding the radiator with a radiator key releases the air and restores full heat, and it is a safe job to do yourself.

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

Cold along the bottom with a warm top usually means sludge, a build-up of iron oxide that settles at the base of the radiator. Which? Trusted Traders lists this as a sign a system needs a power flush, which is a professional job and not something to attempt yourself. A heating engineer flushes the system and adds an inhibitor.

How do I bleed a radiator?

Turn the heating off and let the radiators cool, then hold a cloth under the bleed valve at the top corner, insert a radiator key and turn it slowly anticlockwise about half a turn until you hear air hiss out. Close it as soon as water trickles out. Afterwards, check the boiler pressure and top up if needed.

Why is only one radiator not working when the rest are fine?

A single cold radiator usually points to a valve, not the boiler. Check that both the thermostatic valve and the lockshield valve are open. A thermostatic valve pin can stick after the summer and sometimes frees with gentle movement. If the radiator stays cold with both valves open, a valve may be blocked with sludge and needs a plumber.

Do I need a Gas Safe engineer to fix a cold radiator?

Not for everything. Bleeding a radiator, checking a valve and re-pressurising the system are safe to do yourself. But any work on the boiler itself, its gas supply or flue, is gas work that by law must be done by a Gas Safe registered engineer. Always check the engineer's Gas Safe ID card before that work begins.

Find a trusted local plumber

Whether you need a quick bleed, a stuck valve freed, or a full power flush to clear years of sludge, Loacally connects you with local tradespeople across London, each reviewed by hand before their profile goes live. Find a trusted local plumber near you and get your heating working properly before the cold sets in.

Sources

1. Which?. "How to bleed a radiator: step-by-step guide." Updated 16 September 2025. https://www.which.co.uk/reviews/boilers/article/boiler-maintenance/how-to-bleed-your-radiators-aqa4I6N1JF5T (accessed September 2026)
2. Which? Trusted Traders. "Quick guide: central heating powerflushing." Updated November 2025. https://trustedtraders.which.co.uk/articles/quick-guide-central-heating-powerflushing/ (accessed September 2026)
3. Which?. "How much do plumbers cost?" Survey November 2025, updated 1 May 2026. https://www.which.co.uk/reviews/finding-a-tradesperson/article/how-much-do-plumbers-cost-awJxi6S98hqB (accessed September 2026)
4. Which?. "Getting the best boiler service." Dated 17 September 2025. https://www.which.co.uk/reviews/boilers/article/boiler-maintenance/getting-the-best-boiler-service-a9h945b2brSV (accessed September 2026)
5. Which?. "Common mistakes that can invalidate your boiler warranty." Dated 22 October 2025. https://www.which.co.uk/news/article/common-mistakes-that-can-invalidate-your-boiler-warranty-ah99p4M3O3VL (accessed September 2026)
6. Health and Safety Executive. "Gas safety check - who can do it?" Last updated 23 September 2024. https://www.hse.gov.uk/gas/landlords/safetycheckswhocan.htm (accessed September 2026)
7. Legislation.gov.uk. "The Gas Safety (Installation and Use) Regulations 1998, Regulation 3." https://www.legislation.gov.uk/uksi/1998/2451/regulation/3 (accessed September 2026)
8. Gov.uk. "Private renting: Repairs." https://www.gov.uk/private-renting/repairs (accessed September 2026)
9. Citizens Advice. "Check if your landlord has to do repairs." https://www.citizensadvice.org.uk/housing/repairs-and-housing/repairs-and-housing-conditions/whos-responsible-for-repairs/check-if-your-landlord-has-to-do-repairs/ (accessed September 2026)
10. Vaillant. "Why is my radiator not working?" https://www.vaillant.co.uk/advice/heating-tips/home-advice/why-is-my-radiator-not-working/ (accessed September 2026)
11. Baxi. "How to thaw a frozen boiler condensate pipe." https://www.baxi.co.uk/help-and-advice/how-to-guides/how-to-thaw-a-frozen-boiler-condensate-pipe (accessed September 2026)
12. Homebuilding & Renovating. "What are power flushing costs today?" Author Steve Jenkins, figures attributed to Gordon Chalk and Alan Houghton. Updated 3 September 2025. https://www.homebuilding.co.uk/advice/power-flushing-costs (accessed September 2026)
13. Homebuilding & Renovating. "How to balance radiators to create an efficient, cost-effective and cosy heating system." Author Ben Field with Steve Jenkins, quoting Martyn Bridges. Updated 19 September 2024. https://www.homebuilding.co.uk/advice/how-to-balance-radiators (accessed September 2026)