Fix it · practical guide
Radiators cold at the top, bottom or middle: what each pattern means
In one line: Cold tops want bleeding, cold bottoms want flushing, one cold radiator wants a valve check. The pattern-by-pattern diagnosis before you spend anything.
Radiators fail in patterns, and each pattern points at a different five-pound fix. Read the pattern before touching a wrench - it is the difference between a ten-minute bleed and an unnecessary power-flush quote.
Cold at the top, warm at the bottom
The classic: hydrogen and air have collected at the top of the radiator, and hot water cannot fill it. The fix is a two-minute bleed with a radiator key - the full walkthrough is in the bleeding guide (key, towel, half-turn, hiss-then-drip, and check the boiler pressure afterwards, because bleeding lowers it). If the top goes cold again within weeks, either the system keeps taking in air or it is due the deeper clean below.
Cold at the bottom, warm at the top
The opposite pattern means sludge: years of iron oxide sediment have settled into the bottom of the radiator, and hot water is flowing over a mud bank. Bleeding does nothing here. The homeowner-scale move is a radiator flush (isolate, remove, hose out in the garden - heavy but doable) or, for a whole house of sludgy radiators, a professional power flush. Blackish water when you bleed is the sludge signature; an inhibitor top-up at the service prevents its return.
One radiator cold all over
First check its valves: a thermostatic valve that has not moved in months can stick closed - remove the head and free the pin with a gentle push. Both valves open and still cold: the radiator has stopped being invited to the party (a balancing question - the upstairs rooms pinching the flow), and gentle valve adjustments around the house even it out.
Several radiators cold, or the whole system sulking
That is the pattern that leads back to the boiler itself - check the pressure gauge first, and if pressure is fine and the pump and timer behave, the repair conversation has begun. Slot the whole walk-through into the October winter routine - every radiator tested before the first cold week is the cheap version of this job. And for the money side of the same system, the thermostat maths and the draught work cut the bill the radiators generate.
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General information, not professional advice. Homes differ — if a job is beyond your confidence or the guide’s boundary, that is what tradespeople are for.