adding new radiators

Just had an extension built and added 4 new radiators. Plumbed them all in but they are not heating up. I have a single looped system in the house all ready and joined the rads onto that system. The pipes going into the first rad is hot and so is the second rad. Not getting any heat to the third or fourth or the one in the old part of the house that I joined them to. I have a worcester 28si combi boiler so it is big enough to run all the rads. Any help PLEASE!!!!!!!!

Turn all other rads off except for one of the suspect ones and see if that one warms up. Open all the others one by one
Could be the pump needs turning up one, or you need to balance your system.
Have you fed them each off the 22 mm f+ rs.
Finally, is the loop you are referring to a one pipe system.

thanks for your reply. I am not sure whether I have joined the pipes right or not. I have a 15mm feed all through out the house, so used 15mm pipes to join them together. The rad in the old part of the house that i have joined them to was the last rad in the system. Does this make a difference? We have no pump to turn up either.

You have plumbed them all off the last rad, which is why it will not work as you are feeding them off too small a feed.

Find your 22 mm feeds (probably under floorboards upstairs) and extend them so that you can take branches to each rad from them.

If too hard, you can feed 2 rads off one branch, but flow is better straight from 22 mm to each rad.