I'm attempting to replace my existing back boiler and indirect tank with a condensing boiler. Unfortunately, I'm having a lot of trouble working out just how big a boiler I need. My house is a 1900 terrace with three bedrooms, six radiators, and one bathroom. So far, the estimates of what I need vary from 30kW (from the plumber who's going to fit the boiler) to 15kW (from B&Q)... which doesn't really help me much. It does occur to me that given that a really powerful electric shower is 10kW, a 30kW boiler could well be overkill for me. But I do need it able to produce enough hot water for a shower *and* run the CH at the same time. Any suggestions?