Really need help with a central heating problem (old, but sturdy back boiler open vented system in a flat - only 4 rads): The 2 rads furthest from the boiler/pump take ages to warm up and then they only get lukewarm. The 2 nearest rads are boiling.
Thus half the flat is freezing.

Not sure what it is. Took off one of the cold rads, but no sludge at all. Also run off some water from the in valve of the rad, also totally clean. Bled all rads - no trapped air at all.

Not sure if its an airlock, or some other blockage - the system is quite noisy with a sort of intermittent slithery/gurgling sound. Or maybe the 3 way manual valve which is seized in what looks like the mid (both) position, is really favoring the hot water. Can't shift it at all.

I'm really not sure what to do next ?

Steve, December 2014

Bilko,

thanks.
I should have said that, I've already tried balancing. Made no difference.

Also, tried completely turning of the 1st hot rad, no real difference.
Tried turning off both hot rads, made a marginal difference.

I'm considering draining down the system, and refill. Can't do any harm? Don't what else to do?

Steve, December 2014

Try shutting off the hot radiators and fully opening the cool ones and see what happens - sometimes all the available heat gets used up by the radiators closest to the boiler and the water's cold by the time it reaches the ones at the end.

I think the exercise is called balancing - the outlet valve - the lock shield valve - is closed and the main control valve opened fully - the lock shield valve gets opened a bit at a time until a satisfactory temperature is found.Daily control then reverts to the main valve.

Bilko, December 2014