I have a high efficient furnace. There is a drain line from the furnace where clear water comes out into the drain. Is it true that this water is pure and good for drinking?

No! The clear water coming from the furnace is very acidic and not safe to drink or allow your animals to drink. In fact the acid level in this water is so high it eats through copper fittings in just a few years. The water is a byproduct of the combustion process that takes place in the flue. With new high efficient furnaces we have discovered we can extract more heat from the combustion process by giving the hot gases a longer passage to go through. As the gases cool down they reach a point where the moisture in the gas precipitates out in the form of the water you see. This condensate should be drained into the floor drain where it can be safely drained away.