Squealing pipes and boiler turning on when cold is run?!

Hi
I've just replaced my water main running to the upstairs bathroom.
A couple of issues have occured after doing this-
The boiler which is new will turn on but not fire up when any cold tap is run at speed. Running very slowly doesn't turn the boiler on. I don't know how replacing a few pipes has caused this. Somewhere I read about pressure and non return valves but it never done this before.
When any cold water is used there is a high pitched squealing. It basically sounds like it's coming from whatever appliance is using the water, be it the taps, washing machine or toilets.
Again, an issue since I turned the water off to replace the cold main. Was thinking is may be the supply tap but I'm not sure.
Many thanks!

Sounds like you have air trapped somewhere that's causing the boiler to fire. Run all outlets hot and cold til no air comes out and if the boiler was fitted after removing a hot water cylinder make sure that there isn't any air trapped where the old feed cane from the cylinder

And check the dynamic pressure cos you might need to fit a pressure reducing valve!