Cold at bottom of rads = sludge. One pipe hot, one pipe cold = blockage. Drain, cut, vacuum for a DIY fix if you’re handy, but realistically, book a powerflush and fit a magnetic filter. Your boiler will thank you. Disclaimer: I am not a plumber. This is my experience as a homeowner. Always consult a Gas Safe engineer (UK) or licensed HVAC tech (US) before cutting into your heating system.
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. blocked central heating pipes
I learned the hard way that central heating pipes don’t block with ice—they block with magnetite sludge (black iron oxide). Over time, oxygen in the water corrodes steel radiators and iron pipes. This creates a black, muddy sludge that circulates. It settles in low spots, narrow pipe bends, and—most commonly—inside the return pipe where water slows down. Cold at bottom of rads = sludge