What is a furnace?
A furnace is gas unit for heating a house that it works based on air forced system. A furnace has a supply air and a return air. This comes back to the unit for heat it up again and send it back to the house. Each furnace system consist of fresh air from outside, to the return air. This has a minimum distance from the button of the furnace. There is high efficiency furnace from 92% to 97% efficiency. The big part of efficiency is the condensation, for that all the high efficiency furnaces have condensing part and it will be a floor drain or condensate pump necessary.