Today I had the flow and return pipes for my extension (on a separate zone) rerouted through my loft. It seems that I have an air lock in the return pipe as it isn't getting warm, even though the flow pipe is. My plumber had advised me that this shouldn't be a problem as I have a system boiler (and hence is pressurised). However, it now appears that there is an air lock and he is now considering whether air vents in the pipes are the answer. Do you think this is the case and should this have been done in the first place?