"Mod Con" is a Modulating Condensing Boiler. Modulating means that it has a vairable fire rate. Meaning they reduce their output ot more closely match the capacity required to maintain water temp. Condenseing means that it's a high efficiency design that is 90%+ efficient. Above 85% effciency, flue gases from combustion will start to condense (think white vapor comming out a cold car tailpipe in winter). You need a different heat exchanger material when this occurs because the condensate is acidic and will destroy a cast iron heat exchanger. However, once you reach 90%+ efficiency, you can vent the flue gaes out of PVC pipe, which gives you flexibility in the installation.