A radiator that won't heat properly is usually telling you something specific. Where the cold patch sits, whether it's one radiator or the whole house, and whether your hot water still works all point to different causes. Several you can fix yourself in ten minutes with a radiator key.
This guide takes each symptom in turn: what it means, what's safe to try, and when to call an engineer. If there's no heating and no hot water at all, start with our boiler not working checklist instead.
The short answer
- Cold at the top: trapped air. Bleed the radiator, then check boiler pressure.
- Cold at the bottom or patchy in the middle: sludge. The system needs cleaning, then a magnetic filter and inhibitor.
- One radiator cold all over: usually a stuck TRV pin or a closed lockshield valve.
- Upstairs or far radiators lukewarm: the system probably needs balancing.
- All radiators cold but hot water works: check the controls; if they're right, it's an engineer job.
- Our repair visit is £85 fixed for the visit and diagnosis, with the repair priced before any work starts.
What your radiator is telling you
Run the heating for 20 minutes, then carefully feel each radiator (they can be hot enough to burn) and match what you find.
- Cold at the top, warm at the bottom. Likely cause: Trapped air. What you can do: Bleed it, check pressure. When to call us: Needs bleeding every few weeks.
- Cold at the bottom or patchy in the middle. Likely cause: Sludge (magnetite). What you can do: Nothing lasting. When to call us: Book a visit for cleaning.
- One radiator cold, others fine. Likely cause: Stuck TRV pin, closed lockshield, air. What you can do: Free the pin, open the lockshield, bleed. When to call us: Pin won't move or valve weeps.
- Far or upstairs radiators lukewarm. Likely cause: Unbalanced system, air, weak pump. What you can do: Bleed, then balance. When to call us: Still uneven after balancing.
- All cold, hot water fine. Likely cause: Controls, diverter valve, zone valve, pump. What you can do: Check timer, thermostat, pressure. When to call us: Settings right, still no heat.
- Hot with the heating off. Likely cause: Valve stuck open. What you can do: Check the programmer. When to call us: Always.
Cold at the top: how to bleed a radiator
Air rises to the top of a radiator and keeps hot water out, often with a gurgle. You need a radiator key (or a flat screwdriver for some newer bleed screws) and an old cloth.
- Put the heating on and note which radiators are cold at the top.
- Turn the heating off and let them cool. The water can be hot enough to scald.
- Hold the cloth under the bleed valve (the small fitting at one top corner) and fit the key.
- Turn it slowly anticlockwise, about a quarter turn. Air will hiss out.
- When water comes out steadily, turn the key clockwise to close it. Snug, not forced.
- Repeat on the other cold-topped radiators.
- Check the boiler pressure gauge. If it's dropped below about 1 bar, top up through the filling loop to around 1 to 1.5 bar (cold), following your boiler manual. No gauge and a tank in the loft? The system tops itself up.
- Turn the heating back on and check each radiator heats right to the top.
Bleeding the same radiator every few weeks isn't normal. Air is getting in, or a leak is being topped up, and every top-up brings in oxygen that feeds corrosion. Black water when you bleed means sludge.
Cold at the bottom: sludge, power flushing and filters
Most radiators are steel, and the water inside slowly corrodes them. The result is magnetite, a black sludge that settles along the bottom. Hot water flows over it, so the top heats and the bottom stays cold. Black water when bleeding, slow-heating radiators and a noisier boiler are other signs. Sludge also clogs boilers and pumps, so it causes breakdowns, not just cold radiators.
Do I need a power flush?
Not always. The code of practice, BS 7593:2019, says an existing system should be cleaned by a recognised method before a filter and inhibitor go in. Power flushing is one method, alongside mains-pressure and gravity cleaning. In Scotland, the Domestic Building Services Compliance Guide also expects a system to be thoroughly cleaned and flushed when a new boiler is fitted.
- Chemical clean. How it works: Cleaner circulated by the system's own pump, then flushed out. Best for: Light to moderate sludge, circulation still reasonable.
- Power flush. How it works: Separate high-flow pump and chemicals, radiator by radiator. Best for: Heavy sludge, several cold radiators, or a chemical clean hasn't worked.
- Single radiator flush. How it works: Radiator taken off and flushed out. Best for: One badly blocked radiator.
A power flush is more thorough but can take most of a day. On an old, corroded system the strong flow can expose weak joints or thin radiators, and no flush repairs a failed pump or a valve already damaged by sludge. We look at the system before recommending one.
Keeping it clean: filter and inhibitor
- Magnetic system filter: fitted on the return to the boiler, it catches magnetite before it reaches the boiler. BS 7593 recommends a permanent filter after cleaning, cleaned at each annual service.
- Inhibitor: a chemical that slows corrosion. Levels should be checked yearly, with a re-dose suggested every five years unless a lab test shows it's still working.
Microbore pipework and Glasgow's soft water
Some older installations use microbore: thin 8mm or 10mm pipes instead of the usual 15mm. Narrow pipes choke more easily, so a cold microbore radiator may have a blocked pipe rather than a blocked radiator. It can be cleaned, but carefully, and occasionally a section is better replaced.
Glasgow's water comes mainly from Loch Katrine, and most Scottish water is classed as soft, so limescale is a minor issue here. That doesn't protect your radiators: sludge is corrosion, not scale. Water treatment manufacturers note that soft, low-alkalinity water can be more corrosive, and recommend inhibitor in soft-water areas too. Many tenement and ex-council systems have been altered and drained down over the years, so if you're replacing a tenement boiler, plan for the system to be cleaned first.
One radiator cold, the rest fine
If one radiator is cold top to bottom, the problem is almost always at that radiator.
Free a stuck TRV pin
Check the thermostatic valve (the numbered head) is turned up. If it is, the pin underneath may have stuck after a summer off.
- Turn the head to maximum, then unscrew it at the collar or nut at its base.
- A working pin stands proud and springs back when pressed. A stuck one sits low.
- Ease it up and down gently with pliers, using small movements, not force. A light tap on the valve body (not the pin) can help.
- Once it moves freely, refit the head and turn the heating on.
Stop if the pin won't budge or the valve weeps. Forcing it can snap the pin, and replacing a valve is a job for us. A pin that keeps sticking means a worn valve.
Open the lockshield, then bleed
The valve at the other end, under a plain cap, is the lockshield. It's often left closed after decorating. Remove the cap and turn the spindle anticlockwise with a lockshield key or spanner. Still cold? Bleed it. If both valves are open and it won't heat, sludge may be blocking the valve or pipe, which needs an engineer.
Upstairs radiators cold: how to balance radiators
Radiators nearest the boiler grab most of the flow; those at the end of the run, often upstairs, get what's left. Bleed everything and check pressure first, as air collecting upstairs is a common cause. Then balance. You need a lockshield key or spanner and ideally a clip-on or infrared thermometer.
- With the heating cool, turn every TRV to maximum and open every lockshield fully.
- Turn the heating on and note the order radiators warm up in.
- Let everything cool, then switch back on.
- On the first radiator to warm, close the lockshield, then open it a quarter turn.
- Measure the pipe temperature at each end. Open the lockshield gradually until the pipes are about 12°C apart.
- Repeat down your list. The furthest radiators will need their lockshields open more.
Around 12°C is the usual DIY guide; on a condensing boiler an engineer may aim nearer 20°C, which helps it run efficiently. If far radiators still won't heat, suspect sludge, an airlock in the pipework or a weak pump.
All radiators cold but the hot water works
The boiler is firing, so check the controls: programmer set to heating (not hot water only) with the right time, room thermostat turned above room temperature, smart thermostat batteries and connection, and pressure at about 1 to 1.5 bar. If all that's fine, it's likely one of these engineer jobs:
- Diverter valve (combis): stuck in the hot water position, so no heat reaches the radiators.
- Motorised zone valve (system and regular boilers): its motor fails and the heating circuit stays shut.
- Pump: on some older systems the cylinder heats by gravity but radiators need the pump.
Never open the boiler casing: that's for a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. Headaches, dizziness, nausea or breathlessness that ease outdoors can mean carbon monoxide: get outside and call the same number.
Radiators hot when the heating's off
If the programmer isn't calling for heat, a valve is letting water through. On a combi, a diverter valve stuck part-way warms the radiators whenever you run a hot tap. On system and regular boilers, it's usually a zone valve not closing or, on older systems, gravity circulation through a failed check valve. Both are engineer repairs.
How Titan Heat can help
If you've bled, checked and balanced and the radiators still aren't right, book a repair visit. It's £85 fixed for the visit and diagnosis, and you get a fixed repair price before any work starts. Tell us which radiators are affected and how.
We replace seized valves, fix pump and diverter faults, balance awkward systems and clean sludged ones, finishing with a magnetic filter and inhibitor. Chemical clean and power flush prices: From £799.
Our work carries a 1-year parts and labour guarantee. A boiler service is £89 and a good time to check the filter and inhibitor. See our heating and plumbing services and boiler repair costs in Glasgow. We cover Glasgow, Lanarkshire, Renfrewshire, East Renfrewshire, and East and West Dunbartonshire.
Frequently asked questions
Why is my radiator cold at the bottom but hot at the top?
Sludge from internal corrosion has settled along the bottom. Bleeding won't help: the system needs cleaning, then inhibitor and ideally a magnetic filter.
Why is my radiator cold at the top?
Trapped air. Turn the heating off, let the radiator cool, bleed it until water appears, then check and top up your boiler pressure.
Can I bleed a radiator with the heating on?
Better not. Manufacturers such as Worcester Bosch advise turning the heating off and letting radiators cool first, as the water can scald.
How do I know if I need a power flush?
Several radiators cold at the bottom, black water when bleeding and poor circulation suggest heavy sludge. Lighter sludge can often be cleared with a chemical clean. Either way, finish with inhibitor and a filter.
Does a magnetic filter get rid of sludge?
It catches sludge as it circulates and protects the boiler, but won't clear sludge already packed into radiators. That's why the system is cleaned first.
How much does it cost to fix a radiator that won't heat?
Bleeding, freeing a TRV pin and balancing cost nothing if you do them yourself. Our repair visit is £85 fixed for the visit and diagnosis, with the repair priced before any work starts.
