825 is way too small. doesn't matter if you only have 4 out of 7 rads on at any one time, your plumber needs to calculate for the house and not just for you. 825 is suitable for a 1 bed/1 bath flat, you need the 830 or even better the 835. I prefer the ecotoc plus, its a bigger boiler and has SS heat exchanger and has been around for years and reliable. ecofit pure is relatively new but I fit them as they have a rear flue outlet which is very handy and they fit in a kitchen cupboard very nicely. They have a cast aluminium heat exchanger but as long as the system is flushed properly and chemically treated you shouldn't have any problems.