Microbore Return Line Restriction

[merlypops]

I hope someone out there can help me.
I have two radiators which were fitted in an extension on 8mm microbore pipe. They were never great at heating but since we changed the boiler they don't heat up at all. The original boiler was a gravity fed system but now it is pressurised.
I think I have isolated the problem to being some sort of restriction on the return flow, but I know the pipework is clear and flowing freely (I had the radiators off in case they had silted up).
The microbore is Tee'd off a 15mm line very close to the 22mm main pipe.
Is it possible that the pressure in the main return line is not allowing flow from the microbore into the main line and if so is there any way to fix it without ripping the floors up and increasing the pipe size?

 

[ajstoneservices]

Try shutting all the other rads down leaving just those two open see what happens.
The system may just need balancing.

 

[merlypops]

Thanks. I'll give that a try tomorrow and let you know how it goes.

 

[merlypops]

Done as suggested and the two offending radiators do heat up - albeit slowly but that is probably due to the microbore.

I will try balancing the system as described on here, but that may take a while as it seems that it is actually set up as two separate loops and there are 19 radiators.