I am looking to change my old wall mounted gas boiler, but I am not sure of what size (output) my new boiler should be in btu's --my home is a 2 bedroom semi-detatched bungalow, with 6 radiators ---3 double and three single, two of the singles are fairly small)------- standard gravity system with power shower---would I be right in saying a 30,000 btu boiler would suffice?