cold rads downstairs,again!!!!!

[crapheatingsystem]

i give up,tried balancing,closing off all the upstairs rads,then all downstairs become hot(so don't think its blockage)turn on 1 rad upstairs and all downstairs go cold within minutes,tried balancing,had trvs off pump running at speed 3 or 2.very hot water gushes out down stairs rads(indicates pumps ok to me) but reattach to rad it goes cold???
any one help please.had 7 different plumbers look at this problem,i know a bit about plumbing so not a complete novice.please suggestions please.
ps done the thermometer checks as well.

 

[D_Hailsham]

Is it possible to trace all the pipe runs and create a "circuit diagram" showing how it all plumbs together, including rad outputs and pipe sizes?

It still sounds like a balancing problem and knowing how it all fits together will make balancing easier. See How to Balance a CH System

 

[Axel]

Bypass valve? Open too much? In wrong place?