Radiators not getting hot
Which radiators, and which part of each one, is the whole diagnosis. Cold at the top and cold at the bottom are different faults with different answers.
What it usually means
A radiator does nothing. It has no moving parts, it consumes no fuel and it makes no decisions — it is a steel box that lets hot water give up heat to a room. So a cold radiator is never really about the radiator. Either hot water is not arriving, or the water arriving is not hot, and the pattern tells you which.
Cold across the top with a warm bottom is gas trapped inside. Air enters at every top-up, and a corroding system generates hydrogen of its own, and both collect at the highest point of each panel where the water is not moving fast enough to sweep them along. Nothing is broken and bleeding fixes it, although a radiator that needs bleeding every few weeks is telling you about corrosion rather than about air.
The reverse pattern — hot along the top, stubbornly cold along the bottom edge — is the opposite substance. Corrosion products are iron oxide and considerably denser than water, so they settle out and lie along the floor of the panel while the water flows over the top of them. No amount of bleeding touches it, because there is nothing gaseous in there.
When whole radiators are cold and others in the house are fine, look at distribution rather than at either of those. A thermostatic valve that has sat closed all summer commonly seizes with its pin held down, so it never opens again in October. A lockshield at the other end may have been closed by somebody working on the floor. And on a long circuit the radiators nearest the pump take the flow most readily, which is what balancing exists to correct.
Every radiator cold at once, with hot water still working, is a different question again and not about the radiators at all: the pump, the valve that chooses between heating and hot water, the room thermostat, or the programmer. That is the one case where the answer is upstream in the plant rather than out in the house.
How to narrow it down
Work through these in order.
Is it all of them, or only some?
Walk round and feel every one with the heating up to temperature, and write down which are cold. All of them points upstream to the pump, the controls or the valve that chooses between heating and hot water. A handful points at those radiators, their valves and how the flow is shared out.
Cold at the top, or cold at the bottom?
Put a hand at the top corner and then at the bottom edge of the same panel. Cool top with a hot bottom is trapped gas and bleeding it is a ten-minute job. Hot top with a cold bottom is settled corrosion debris lying inside the panel, and bleeding will achieve exactly nothing.
Is the hot water still fine?
Hot water working while nothing warms up puts the fault on the heating side specifically — the valve that diverts between the two, the room thermostat, the programmer or the wiring between them. It also rules out most of what people worry about, because the burner and the pump are demonstrably doing their jobs.
Are the cold ones the furthest from the boiler?
Water takes the easiest route, so on an unbalanced system the near radiators run hot and the far end of the circuit gets whatever is left. Correcting it means closing down the lockshield valves on the greedy radiators slightly, not opening the cold ones further, which is the instinct and does not work.
Have the radiator valves been turned right down all summer?
A thermostatic head that has not moved in months frequently seizes with the pin underneath it pressed in. Unscrew the head and look at the small metal pin on top of the valve body: it should stand proud and spring back when pressed. One that is stuck flush is why that radiator never warms, and freeing it costs nothing.
Has anyone had the floor up, or been decorating?
It is a surprisingly common ending to this story. A valve closed to take a radiator off a wall and never reopened, or a pipe crushed under a newly laid floor, produces exactly this symptom in one or two rooms while everything else behaves perfectly.
Does the pump make any noise at all?
Put a hand on the pump body where the pipes leave the boiler, or listen close to it. A pump running normally hums and feels faintly alive. Silence with the boiler firing, or a rattle that changes when you knock it, points at a pump that has stopped or seized, which is a common part and a routine replacement.
Codes commonly shown with this
- F.75VaillantThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Get it checked
- F.24VaillantTemperature is rising faster than it should when the burner fires. That is a circulation problem caught a step earlier than an overheat.Get it checked
- S.53VaillantNot a fault. Your boiler is in a deliberate waiting period because the gap between flow and return temperatures is too wide, which points at poor circulation.Get it checked
- F.9Glow-wormF9 on an Ultracom, Flexicom, Ultrapower or CI-range boiler is Glow-worm's water pressure sensor fault — on the hxi and Flexicom "or a blocked flow filter or pump fault". It is not the low-pressure code.Get it checked
- E60AlphaThe appliance cannot confirm the pump is moving water. Left alone it becomes an overheat fault, so it is worth acting on promptly.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£0–£700
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.