Hi, wondering if someone can help!!

I bought a new shower head as the current one had a crack in it, so I took if of and put on the new one and when I switched the shower on, water came pouring out of the back of the shower unit! I then took the new shower head off and put the old one back on but the same thing happened! Does anyone know what I've done! Its a rented flat and I've just moved in.

Also when I put on the hot water taps in the kitchen and bathroom the pipes make a really loud noise, think this means there is air stuck in the pipes? how do I fix this?

Thanks

Linkback: https://forum.plumbers.nz/q-and-a-baths-and-showers/64/shower-problem/797/