cold radiator problem continues

Hello all. I have a new (2 months old) Trinity combi condensing boiler (gas), with 3 zones. One zone includes two loops to four radiators on the second floor, and one loop to 2 radiators on the first floor. The boiler is in the basement. All of the old pipes were cut at the basement wall, and new plastic tubing runs from the boiler to the copper pipes to the radiators.

The problem is that one radiator on the first floor does not get hot. When I bleed it, it gets hot and then cools once the bleeder valve is closed. The supply and return pipes for this loop are hot as they leave and return to the boiler. These two rads were previously on a separate zone, controlled by a thermostat that is no longer in use. The problem is not with the radiator. The plumber was sure it was, but he removed it, and replaced it with copper tubing from supply to return, and it all remains cold.

From reading this forum, I am thinking a possible airlock (where? - plumber says no), sludge, (bleed water runs clear), balancing (if this were the case, would the return pipe at the boiler be hot?), and last, could there be a zone control valve somewhere behind the wall that is stuck in the closed position that was once controlled by the old thermostat?

Am I on the right track? Any ideas on the most likely problem?
Thanks in advance for your help.

Ed

Turn the all rads off apart from the problem one, if it starts heating up and working ok then you can at least disgard a blockage\crushed pipe etc, if it does work start opening other rads one by one (15mins apart) and see the effect on the problem rad if it immediatly cools when opening one more rad then there is probably a restriction. If it has good flow and heat up until the point befoire the last couple of rads are opened then may just need balancing.


hope this helps

Thanks. In this case where there are only 2 radiators in the loop, do I have to turn off all the radiators in the zone, or just the one working rad in the loop?
Again, thanks for your reply.

Ed[/quote]