Cat 1 appliances are mid efficient non positive vent pressure - non condensing. Cat 3 are mid efficient positive vent pressure, non condensing. If an appliance can run either option (e.g Viessmann Vitorond 200), what benefits are there to running a cat 3 over a cat 1? The only one I now know of is you can run smaller vent sizes on Cat 3 obviously because the positive vent pressure helps push the flue gas out instead of relying on draft to move it. The cat 3 may be slightly safter as it has to be a fully sealed vent where as a Cat 1 in a lot of cases requires some form of draft control (e.g. draft hood, barometric damper) which could spill in some situations. Any other benefits to Cat 3?
Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber