Water Hammer - after pipe change

[Justice League]

Had two pipes changed - one cold and one hot. The cold had a leak and the warm was punctured and about to leak so both were replaced - about 50cm sections in each pipe.

After that I started getting water hammer - banging of pipes. I've tried lowering the water pressure - that helps a bit but there is still a thudder when the taps are closed.

I read into this - am I correct in thinking that there needs to be air in the pipes to give the water some room to move once a tap has been closed?

How do I put air back into the pipes?

 

[Phil]

You don't want air in the pipes you need to purge it out, if you get water hammer after turning the water off you need to open every cold outlet. I had it this year and I tried everything and it only stopped when I triggered the ballvalve on the F+E tank.