BoilerFaultCodesRadiator not heating up? Start by counting the cold ones.
This is the question that decides everything. Walk round the house with the heating on for ten minutes and count. One cold radiator is a problem with that radiator, and you can usually sort it yourself. All of them cold is a problem with the boiler, pump or controls, and it is a different job entirely.
Stuck valve, trapped air or balancing. Often free to fix.
Check pressure first, then the programmer.
Just one radiator is cold
Work through these in order. The first two account for most cases and cost nothing.
A stuck valve pin
Safe to tryTake the plastic head off the thermostatic valve (the one with numbers on it, usually bottom left). Underneath is a small metal pin. It should spring back when you press it. If it is stuck down, the valve is shut and no water is getting in. Free it by gently working it up and down with pliers, or tap it lightly. This is the single most common cause of one cold radiator, and it happens over summer when the valve sits unused for months.
Trapped air at the top
Safe to tryCold at the top and warm at the bottom means air. Turn the heating off, let it cool, then open the bleed valve at the top corner with a radiator key until the hissing stops and water appears. Close it, and check your boiler pressure afterwards because bleeding drops it.
Full bleeding guide →Still cold after bleeding
Safe to tryIf you bled it, got no air out, and it is still cold, the water is not reaching the radiator at all. Go back to the valve pin above. If both valves are open and the pin moves freely, it is a balancing problem or a blockage.
Cold at the bottom, hot at the top
Engineer jobThat is sludge sitting in the bottom of the radiator, not air, and bleeding will not shift it. It needs flushing.
The furthest radiator from the boiler
Engineer jobIf the cold one is the last on the circuit or upstairs at the far end, the system is unbalanced. Water is taking the easy route through the nearer radiators. An engineer balances it by partially closing the lockshield valves on the radiators that heat up fastest, which pushes flow to the far end.
A new radiator that has never worked
If it was fitted recently and has never got hot, the usual causes are a valve left shut after installation, the lockshield never opened, or air that was never bled out. Check both valves are open before anything else. A towel rail that stays cold in summer is often normal, since many are wired to the heating circuit and only run when the heating does.
Every radiator is cold
Now you are looking at the boiler and the controls rather than the radiators. Check in this order.
Boiler pressure
Look at the gauge on the front of the boiler. Cold, it should sit around 1 to 1.5 bar. Ideal states the pressure for a Logic Heat should be "between 1 & 1.5bar on the system pressure gauge", and Worcester tells you to "fill the system to a pressure up to 1.5 bar". Too low and the boiler will refuse to fire: an Ideal Logic Combi "will not operate if the pressure has reduced to less than 0.3 bar".
How to repressurise →A fault code on the display
If there is a code or a flashing light, that tells you exactly what the boiler thinks is wrong. Look it up rather than guessing.
Look up your fault code →The programmer and thermostat
Obvious, and worth ruling out before you pay anyone. Check the timer is on rather than sitting between scheduled periods, the room thermostat is set above current room temperature, and the batteries in a wireless thermostat are not flat. Flat thermostat batteries are a common autumn callout.
Hot water works but heating does not
If the taps run hot and the radiators stay cold, the boiler is fine. The fault is in what directs the heat, usually a diverter valve on a combi or a motorised valve on a system boiler.
Read the diverter valve guide →The pump
If the boiler fires but nothing circulates, the pump may have seized. On an older system you can sometimes hear it running. This is an engineer job.
What it costs if you need an engineer
| Job | Typical cost |
|---|---|
| Replace a thermostatic radiator valve | £90 to £150 |
| Balance the whole system | £100 to £200 |
| Single radiator flush | £80 to £150 |
| Full power flush | £350 to £700 |
| Replace a circulation pump | £200 to £350 |
Ranges are typical UK prices including parts and labour. Rates vary by region and by how much of the system needs draining to do the work.
Stop and call an engineer
Everything above stops at the radiator and its valves. Anything involving the boiler casing, the gas supply or the flue is for a Gas Safe registered engineer only. If you can smell gas, leave the house and ring the National Gas Emergency line on 0800 111 999.
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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