Tip number 104: Condense your heat

Condenser boilers are the most efficient, with 88% of fuel converted
into heat compared to 72% with conventional boilers. However, the
water vapours they let off can be irritating so make sure you site
your flue away from your neighbors' windows and patios. Consider
choosing a condenser if you need to replace an existing gas or oil
central heating system.

Taken from the book '1001 little ways to save our planet' by Esme Floyd.