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Radiators Not Heating Up But Boiler's On? Here's Why
Your boiler is humming away, the hot water runs hot, yet the radiators stay stubbornly cold. The fix usually depends on one thing: are all your radiators cold, or just one? This guide walks you through a quick self-diagnosis, the likely cause, indicative 2026 UK costs to fix it, and whether boiler cover might have helped with the bill. It's general information, not advice on your specific boiler.
Quick answer
If one radiator is cold, the usual cause is trapped air (bleed it) or a stuck thermostatic valve. If all radiators are cold while the boiler still fires, the most common causes are low boiler pressure (below 1 bar), a frozen condensate pipe in winter, a seized circulation pump, or a stuck diverter valve on a combi.
Anything involving gas, the burner, the flue, the gas valve, combustion or the boiler's internal components is a job for a Gas Safe registered engineer. Bleeding, balancing and topping up pressure within range are safe DIY jobs. If you smell gas, call the National Gas Emergency line on 0800 111 999.
First, work out WHICH radiators are cold
This is the single most useful thing you can do, and it takes seconds. The cause of cold radiators tends to split by pattern, so before you touch a valve or a filling loop, walk round the house and feel each radiator top and bottom.
Then match what you find to one of the groups below.
The general rule: if every radiator is cold, the problem is usually at the boiler or in the primary circuit (pressure, pump, diverter, frozen pipe). If only one or two are cold while the rest are hot, the problem is usually local to those radiators (air, valves or sludge). This split rules out most causes quickly.
Cold at the top, warm at the bottom = trapped air
If a radiator is warm low down but cold across the top, air is usually trapped inside. The air rises and sits at the top, blocking hot water from filling that section.
The fix is to bleed it. Turn the heating on for 15-20 minutes to build pressure, switch it off, let radiators cool for about an hour, then bleed from the radiator nearest the boiler outward. Never bleed with the system running.
Our full walkthrough on how to bleed a radiator covers the key, the cloth and what to do if water sprays out. After bleeding, always re-check your pressure gauge — bleeding releases water and can drop the pressure below 1 bar.
Cold at the bottom, warm at the top = sludge
The opposite pattern often points to sludge. Over years, magnetite (black iron-oxide silt) can settle in the bottom of radiators, creating a cold dead zone while the top still warms.
Tell-tale signs are cold patches low down, radiators that take ages to warm, and dirty black water when you bleed. A power flush clears it out.
As a rough guide, budget from around £350-£500 for a typical 8-10 radiator home, though heavily contaminated, larger or London systems can run well over £800 — see our breakdown of power flush cost. These are indicative 2026 ranges; always get a quote. To help stop it recurring, you can ask the engineer to fit a magnetic filter (commonly from around £80-£150 fitted), which catches magnetite before it settles.
One radiator stone cold = a stuck or closed valve
If a single radiator is cold top to bottom while its neighbours are hot, a common culprit is a valve. Each radiator typically has a thermostatic radiator valve (TRV) at one end and a lockshield at the other.
First check the TRV is turned up. Then check the pin underneath isn't stuck. With the system off, unscrew the TRV head and you'll see a small spring-loaded pin — if it's seized down, the valve stays shut.
- You can often free a stuck pin by gently pressing it up and down (protect it with a cloth and use light pressure — forcing it risks damage). It should spring back on its own.
- If the pin won't move or the valve is corroded, the TRV may need replacing — parts are commonly £10-£50, or roughly £80-£150 fitted by an engineer (indicative 2026).
- Check the lockshield (the capped valve) hasn't been wound fully closed.
All radiators cold = low boiler pressure
If nothing is heating, look at the pressure gauge on the boiler. When cold, it should usually sit between 1.0 and 1.5 bar. Below 1 bar and the system tends to struggle; drop towards 0.5 bar and many boilers go into lockout and won't run the heating at all.
Topping up is generally a safe DIY job using the filling loop — the small silver braided hose under the boiler. Open both valves slowly until the gauge reaches about 1.2 bar, then close them. Check your boiler's own manual, as filling-loop arrangements vary.
Our guide on how to top up low boiler pressure shows the filling-loop steps. If pressure keeps dropping every few days, there may be a leak or a failed expansion vessel — that needs an engineer.
All radiators cold = a failing circulation pump
The circulation pump pushes hot water round the radiators. If it seizes, the boiler may still fire and make hot water but nothing circulates, so every radiator stays cold.
Symptoms can include the boiler getting hot while the pipes stay cold, no gentle hum from the pump, or grinding and vibration. Sometimes a seized pump can be freed temporarily by loosening the bleed screw, but a failing pump usually needs replacing.
Expect roughly £150-£450 fitted depending on pump brand, isolation valves and your area (London and the South East tend to be at the higher end) — full detail in our circulation pump replacement cost guide. A central-heating pump sits on the water (wet) side, so it is typically a job for a qualified heating engineer rather than DIY; it is not usually classed as notifiable gas work, but leave it to a competent professional.
Hot water fine but no heating = a stuck diverter valve
On a combi boiler, a diverter valve switches flow between heating and hot water. If it sticks in the hot-water position, your taps may run hot but the radiators get nothing.
A classic sign is plenty of hot water at the taps yet cold radiators with the heating set to on. There is no safe DIY fix — the valve is inside the boiler, so this is a job for a qualified, manufacturer-competent engineer.
A faulty diverter valve commonly costs around £250-£500 to replace (indicative 2026). If you have hot water but no heat, our guide on hot water but no heating covers the other things to rule out first.
Winter only: a frozen condensate pipe or lockout
Here's a seasonal cause that's easy to miss. Modern condensing boilers drain acidic condensate through a plastic pipe, often run outside. In a cold snap that pipe can freeze, the condensate backs up, and the boiler shuts down — sometimes after firing briefly, which is why it can feel like "the boiler is on" but heat fails.
You may see an error code (commonly EA on Worcester, F1 on Glow-worm/Ferroli, L2/F2 on Ideal — codes vary by make) and hear gurgling. Thawing the pipe with a jug of warm — not boiling — water over the frozen section, starting at the end furthest from the boiler, often gets it going again, then reset the boiler.
Our guide to a frozen condensate pipe has the full method and how to lag the pipe so it's less likely to recur. If the boiler stays in lockout after thawing, or you can't reach the pipe safely, that's an engineer call.
Thermostat, programmer or smart-controls not calling for heat
Sometimes the boiler runs (for hot water) but the heating never fires because nothing is telling it to. Check the basics before assuming a fault.
- Is the room thermostat set above the current room temperature?
- Is the programmer or timer actually scheduled to be ON right now?
- On a smart stat (Nest, Hive), has a schedule, holiday mode or flat battery left it on frost-protect?
A dead thermostat battery or a wiring fault can both stop the call for heat. If the controls look right but heating still won't fire, an engineer can test the wiring and PCB.
Downstairs cold, upstairs hot = the system may need balancing
If radiators nearest the boiler run hot but distant ones stay cool, the system may be unbalanced — hot water takes the path of least resistance and short-circuits the far rads.
Balancing uses the lockshield valves to slow the flow through nearer radiators so the far ones get their share. A common method: measure the temperature difference across each radiator (flow vs return) and aim for roughly an 11-12°C drop, adjusting lockshields a little at a time.
It's fiddly but free, and it's generally a safe DIY job. Do it after bleeding and after confirming pressure is correct.
The step-by-step DIY fix order
Work through the safe jobs in this sequence before calling anyone:
- Bleed any radiator that's cold at the top.
- Check pressure and top up to about 1.0-1.5 bar cold (bleeding can drop it).
- Check valves — TRVs turned up, pins free, lockshields open.
- Balance the system if some rads run hotter than others.
- Power flush (engineer) only if cold-at-the-bottom sludge persists after the above.
If you've done all the safe steps and radiators are still cold, the fault is more likely the pump, diverter, PCB or a leak — and that's where a registered engineer comes in.
Indicative 2026 UK fix costs — and might boiler cover have paid?
Here's what each fix typically costs in 2026, and whether a boiler-and-central-heating cover plan might have covered it. Whether something is covered depends entirely on the individual plan's terms, limits and exclusions — these columns are general patterns only, not a guarantee. Prices are indicative UK ranges last checked in 2026; always confirm with the engineer or on the provider's own page.
| Fault | Indicative 2026 cost | DIY or engineer? | Often covered? |
|---|---|---|---|
| Bleed a radiator (trapped air) | £0 (DIY) | DIY | n/a |
| Top up low pressure | £0 (DIY) | DIY | n/a |
| Balance the system | £0 DIY, or ~£80-£150 if engineer | Either | Varies by plan |
| Replace a stuck TRV | £10-£50 part; ~£80-£150 fitted | Engineer (fitted) | Varies by plan |
| Thaw frozen condensate pipe | £0 DIY, or call-out if iced solid | DIY first | Varies by plan |
| Power flush (sludge) | ~£350-£500+ typical | Engineer | Usually excluded (maintenance) |
| Circulation pump replacement | ~£150-£450 fitted | Engineer | Often (breakdown plans) |
| Diverter valve replacement (combi) | ~£250-£500 | Engineer | Often (breakdown plans) |
| Magnetic filter fitted | ~£80-£150 | Engineer | Sometimes (as upgrade) |
| Engineer diagnosis / call-out | ~£60-£120 (emergency higher) | Engineer | Often included |
A seized pump or diverter valve can cost more than a year of cover, depending on the plan. For more figures, see our 2026 boiler repair costs guide.
When to call a Gas Safe engineer
Stop and call a registered engineer the moment the job touches gas, the burner, the flue, the gas valve, combustion, the PCB or the boiler's sealed internals. By law, work on gas appliances must be carried out by a Gas Safe registered engineer. Wet-side jobs such as a circulation pump are heating/plumbing work for a qualified engineer, but still not DIY.
A diagnosis or call-out commonly runs around £60-£120, often with the first hour included; emergency and out-of-hours visits cost more. Always check the engineer's Gas Safe ID card for any gas work.
Safety first: if you smell gas, hear hissing, or feel unwell, leave the property and call the National Gas Emergency line on 0800 111 999 immediately. Do not touch electrical switches or the boiler.
Could boiler cover have paid for this?
If your radiators went cold because of a seized pump (~£150-£450), a stuck diverter (~£250-£500) or an engineer call-out, a boiler-and-central-heating cover plan might have covered most or all of it, depending on the plan — often for a monthly premium that can be less than a single repair. Cover terms, limits, exclusions and any excess vary a lot, so always check the specifics.
It's worth weighing up for yourself. Cover can suit people who want predictable costs and a fast call-out; a savings buffer can suit those happy to pay as they go. Maintenance jobs like a power flush are commonly excluded either way. This is general information, not a recommendation about what's right for you.
We compare a selected panel of providers — not the whole market — so you can see what's included before you buy. Start with our roundup of how to compare boiler cover, and always confirm cover limits, exclusions and excess on the provider's own page.
Disclosure: BoilerCoverUK may earn a commission if you take out a plan through our links, at no extra cost to you. We compare a selected panel of providers, not the whole market. Some plans are FCA-regulated insurance; others are service or care plans, which are not insurance — we tell you which is which on each comparison. This article is general information, not financial, insurance or gas-safety advice.
Why is my boiler on but my radiators are cold?
It usually depends which radiators are cold. If just one is cold, the common causes are trapped air (bleed it) or a stuck valve. If all of them are cold while the boiler fires, look at low pressure (below 1 bar), a seized circulation pump, a stuck diverter valve on a combi, or — in winter — a frozen condensate pipe causing lockout. Hot water working but no heat at all on a combi often points to the diverter valve. This is general guidance; a Gas Safe engineer can diagnose your specific boiler.
Should I bleed radiators with the heating on or off?
Off. Run the heating for 15-20 minutes to build pressure, then switch it off and let the radiators cool for about an hour before bleeding. Bleeding with the system running can spray hot water and may not release the air properly. Start with the radiator nearest the boiler and work outward, and re-check your boiler pressure afterwards as bleeding lowers it.
Can low pressure stop radiators heating up?
Yes, it commonly does. When cold, a boiler should usually read 1.0-1.5 bar. Below 1 bar the system struggles to circulate, and around 0.5 bar many boilers go into lockout and won't run the heating at all. Topping up via the filling loop to about 1.2 bar is generally a safe DIY fix. If pressure keeps falling, there may be a leak or a failed expansion vessel, which needs an engineer.
Is a power flush worth it?
If your radiators are cold at the bottom, you get black water when bleeding, or the system is slow to warm, sludge is a likely cause and a power flush (commonly from around £350-£500, more for larger or London systems) is often worthwhile. To help stop it recurring, you can have a magnetic filter fitted (often around £80-£150). A power flush is generally classed as maintenance, so most cover plans exclude it — check the plan's terms.
Why do I have hot water but no heating?
On a combi boiler this often means a stuck diverter valve — jammed in the hot-water position so flow never reaches the radiators. Replacement commonly costs around £250-£500 and is a job for a qualified engineer (the valve is inside the boiler). Before assuming the valve, check the thermostat is calling for heat, the programmer is set to ON, and the pressure is in range.
My condensate pipe is frozen — can I fix it myself?
Often, yes. Pour warm (not boiling) water over the frozen outside section of the plastic pipe, starting at the end furthest from the boiler, then reset the boiler. Lagging the pipe afterwards helps prevent it freezing again. If the boiler stays in lockout after thawing or you can't reach the pipe safely, call an engineer.
Compare boiler cover the easy way
Compare boiler & central heating cover from a selected panel of UK providers and find a plan that fits your boiler and budget. Information, not advice — we show a chosen panel, not the whole market.
Compare boiler coverThis article is general information, not financial or gas-safety advice. We compare a selected panel of providers, not the whole market, and may earn a commission if you buy through our links. Always have gas appliances checked and repaired by a Gas Safe registered engineer; in a gas emergency call 0800 111 999. Prices are indicative UK guides for 2026 — confirm current prices on the provider's own site.