Why is my radiator cold at the bottom? (3 likely causes)

If your radiator is hot at the top but cold at the bottom, you're looking at one of three things — almost always the first.

1. Sludge build-up — black, magnetite-rich gunk sinking to the bottom of the radiator. The fix is a powerflush of the system (£495 typically), or for a single offending radiator, a chemical clean and fresh inhibitor.
2. A failing pump — water just isn't being pushed through hard enough. Listen near the boiler — a quiet, struggling pump usually means a £180–£260 swap.
3. An air lock further down the system — rarer, but possible after a recent drain-down or repair. Bleeding alone won't fix it; we'd cycle the system to clear the lock.

Bleeding the radiator helps if it's cold at the top, not the bottom — different problem entirely.