It is the single most common heating question we get asked, usually in the first cold week of the year: the bottom of the radiator is warm, the top is stone cold, and the room never really heats up.
What it usually means
Air. Heating systems gradually collect air, and because air rises it gathers at the top of the radiator, where it takes up the space hot water should be filling. The radiator is not broken — part of it simply has nothing hot in it.
This is why bleeding a radiator often fixes it. It is also why the problem tends to come back if it keeps happening: air getting in repeatedly usually means something else is going on, and that is worth looking at properly rather than bleeding the same radiator every month.
When it means something else
If the pattern is reversed — warm at the top, cold at the bottom — that is a different problem entirely. That usually points to sludge: a build-up of debris settling in the bottom of the radiator and blocking flow. Bleeding will not touch it, and it tends to affect the whole system rather than one radiator.
If only the radiators furthest from the boiler are cold, the system may simply need balancing so the flow is shared properly rather than all going to the nearest rooms.
The thing to watch
After bleeding radiators, check your boiler pressure. Letting air out lowers the pressure in the system, and if it drops too far the boiler may refuse to fire. Plenty of people bleed every radiator in the house and then find the heating will not come on at all.
If the pressure keeps dropping on its own, that is not something to keep topping up and ignoring — it usually means water is escaping somewhere.
When to call
If bleeding fixes it, grand. If it comes straight back, if several radiators are cold at the bottom, or if the pressure will not hold, get it looked at. Get in touch and we will tell you honestly whether it needs a visit.