Thanks for your reply although all I was looking for was some advice, as I was soon was wanting to have a new boiler fitted (not by a cowboy) and like to do my own research first. I was wanting to stick with Worcester as having used there products in the past I have always found them a very reliable product. The 37KW cdi appealed to me because of its higher flow rate for the domestic hot water, although the heating output would be way overboard. I know some boilers are range rated and after looking at the boilers specification manual, freely available on the Worcester website, I just wanted confimaton I was reading it correctly and the boiler could be range rated anywhere between 9-30 KW. While I'm on the subject could somebody also tell me what modulation means? I understand it means that the boiler will adjust its gas rate automatically depending on demand to keep its self in condensating mode. With this in mind does it matter what size boiler is fitted?