I changed from an RS boiler with cast iron heat exchanger and constant pilot light to a decent condenser with weather comp. The old system had reasonable controls - TRVs everywhere except the living room where a programmable stat was situated. Boiler had a pump over-run added. Stored hot water. Over a year, I'm using 20% less gas. My brother did a similar change - except that his system has no TRVs and only a basic stat, and the new one the same with no weather comp. He reports more like 25%.