Sensor pipe on Baxi Bermuda inset 2

Hi
I've just bought a Baxi Bermuda inset 2 boiler/gas fire which is going to go into a friends house. As part of experience for college I'm going to convert the system to fully pumped and fortunately he has a corgi registered neighbour who will connect up. I have two questions.
1)The old system is gravity HW and uses 28mm flow and return pipework to the cylinder. I'm going to use these pipes as a "shared" flow and the cylinder return to boiler. If I use a 22mm myson aerjec and reduce the flow pipe near the cylinder to 22mm will I have any problems with the aerjec? ie- because there would possibly more flow to it than if the whole pipe was in 22mm causing an inbalance. I know you can get an aejec in 28mm but the 22mm one goes better with the rest of the pipework I'm going to put in. (I've installed an aerjec before)
2) Looking at the boiler the sensor pipe has a hole in it about half way up it. It's a neat whole, looks like it's factory done, is it supposed to be there, if so why?!
This is a second hand boiler, but if I can't do an apprenticeship how the hell else do I learn without crazy schemes like this one?!
Thanks to any replies
Jon
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Found out today from Baxi that the hole is an air release on the sensor pipe.
Jon
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