Should expansion pipes be flat or sloping?

[RigidRaider]

The expansion pipe from my cylinder crosses about 8' of attic horizontally before going up into the F&E tank. It slopes upwards at about 5 degrees, which is good for encouraging trapped air to escape but I'm surprised at how far along it gets warm when the boiler starts up and the tank contents expand. I read somewhere that a sloping pipe encourages convection within the pipe and the flow of warm water upwards, replaced by cold water flowing downwards.

Any thoughts on this from more experienced plumbers?

 

[John506]

I suppose it wouldn't really matter as the expanded water will still rise up and over, but personally I would have put and offset bend on it just to keep it vertical.