Cold radiator and draining the system

[Sickboy]

I have a small mid-terrace house that was built in 1995. There are 2 radiators downstairs and 3 upstairs.

The biggest radiator in the house, downstairs in the lounge, only gets hot on the very top and down both sides, about 3 inches from the edges. The rest of the radiator is stone cold.

I have tried turning all the other radiators off at the main valve and this radiator gets hotter at the top but the rest still stays cold so I don't think it is a balancing issue.

I have tried bleeding this radiator and get only water out but the water is almost completely black. Am I right in assuming the radiator is full of crud and needs flushing through?

If the water is black should I drain the whole system rather than just this radiator?

The system uses a boiler in the kitchen and a hot water tank in the bathroom with header tanks in the loft. Should I just turn off the valves in the header tank and drain everything from the lowest rad?

The system also uses pipes made from grey plastic. Does this change anything? Should I use a chemical to flush the system through or will this damage the plastic pipes?

 

[gas4you]

Chemical won't damage the plastic pipes.

A system flush is what is required, but this one rad may need to be removed on its own, taken outside and flushed with a garden hose.

This should keep you warm over xmas.