Just had a new cylinder installed and moved next to the boiler.When i turn just the hot water on it heats up and after a bit the rads upstairs get hot, then when i switch the c/h on the over flow from the header tank for the c/h in the loft overflows. PLEASE HELP!!!. :( :(
Get the guys back who fitted your cylinder. It sounds like the upstairs rads are heating by gravity up the return from the cylinder, also they may have turned the pump speed up causing your system to "pump over", or the ball valve in your expansion tank needs looked at. Either way, if it wasn't doing all this before the cylinder replacement, get them back to sort it out.