When my plumber cut out the affected section of 15mm copper pipe, it wasn’t empty. It was 90% full of a solid, tar-like black paste. You couldn’t push water through it with a garden hose. This didn't happen overnight—it took 15 years of no system maintenance.

Locate the blocked pipe: Use an infrared thermometer or just feel along the pipe run. You'll find a spot where the pipe goes from hot to cold in 2 inches. Method: Isolate the system, drain below the blockage, cut the pipe, and use a wet-dry vacuum on the open end. I sucked out a golf-ball sized clump of black sludge. Then, I fed a drain snake (yes, a plumbing auger) into the pipe to break up the rest. Warning: This is messy, and if you have microbore (8-10mm) pipes, you will likely puncture them.

It started subtly. The living room radiator was lukewarm at the top but ice cold at the bottom. Then, the bedroom radiator went completely cold. The final straw was the boiler pressure dropping daily and the pipes making a sound like a boiling kettle. The giveaway? When I turned the heating on, one pipe to a radiator stayed stone cold while the other was hot.