Replace your old boiler with a better one
If you need to replace your boiler(s), consider a high efficiency system like a condensing boiler.
A series of smaller boilers that cater for the heating demand can be more efficient than a single larger boiler. Multiple boilers are usually set up with an intelligent sequence controller. This controller determines which ones go online to maintain correct temperatures.
Always ask your system designers for heat loss calculations and heat demands at different times of the year.