Work down the list in order. The boiler is at the bottom of it, and it is almost never the answer.
One cold room in an otherwise warm house is not a boiler problem. A boiler that cannot heat the house heats none of it. Something is stopping hot water reaching that one radiator, and there are only a handful of candidates.
Cold at the top, warm at the bottom, is air, and five minutes with a radiator key clears it. Start here because it is free and it is the most common single cause.
Cold everywhere, top and bottom, while the rest of the house is hot, usually means the thermostatic valve pin has seized. Pull the head off the valve and you will see a small pin. It should spring back when pressed. If it is stuck down, the radiator is closed no matter what the dial says.
Freeing it is often a matter of tapping it gently and working it up and down with pliers. A pin that will not free needs the valve head replaced, which is a ten minute job and does not need the system drained.
If the rooms furthest from the boiler are always the cool ones, nothing is broken. The system has simply never been balanced.
Water takes the easiest route, so radiators near the boiler get hot first and can take more than their share. The fix is the lockshield valve at the other end of each radiator, closed in slightly on the near ones so flow is pushed to the far ones.
Balancing a house properly takes a couple of hours with a thermometer on the pipework and it is done once. It is the single most common thing a cold room actually needs.
Cold along the bottom with a warm top is the opposite of air: debris has settled in the bottom of the panel and water is not circulating through it. Bleeding does nothing here.
One radiator can be taken off and flushed outside. If several are the same, the system wants a power flush and an inhibitor afterwards, or it comes straight back.
Each of those makes a working radiator look like a broken one. Worth ruling out before anybody quotes for pipework.
In this article
Still cold?
Balancing a whole house is $129 and takes an afternoon. If it turns out to be a valve, we say so rather than quote you for a boiler.
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