Finished Fitting a new combi to a property on Monday and no matter what I tried I couldn't get any heat to the drops. With all the rads bar the drops turned off the return to the boiler was red hot but none of the rads were getting hot. This led me to believe that there must be some sort of bypass on the system. I Lifted a few boards and checked the return and it was stone cold as far as the bathroom. The only place there could be a problem was under the tiled bathroom floor.
I returned today with a borescope and struck lucky on the third hole I drilled in the ceiling. I found this.
Bypass by
Mike Jackson1, on Flickr
No wonder the drops wouldn't heat up.