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.
I’m writing this while my house is finally warm again. For the last three weeks, I lived through the silent hell of a blocked central heating system. If your radiators are cold at the bottom, your boiler is kettling (making rumbling noises), or some rooms just refuse to heat up, read this. You likely have a sludge blockage. blocked central heating pipes
Before calling a plumber, check your system type. If you have microbore (small plastic or copper pipes like spaghetti), you are high risk. Those block permanently and often require full repiping. If you have standard 22mm/15mm copper, a powerflush will save you. Locate the blocked pipe: Use an infrared thermometer