A furnace is gas unit for heating a house that it works based on forced air system.
A furnace has a supply air and a return air. The cold air comes back in to the unit for heat it up again and push it to the house by a blower motor. Each furnace system consist of fresh air from outside, to the return air. This has a minimum distance from the bottom 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.