In article , MNRebecca wrote:
Thanks for all the feedback, everybody. I'll look it over thoroughly.

MNRebecca wrote:
If I disconnect the pipe from
the furnace and let it draw air from the room instead (which I'm told,
by the repair guy, is harmless),

(Doug Miller) wrote:
He's right. It's less energy-efficient, because cold air is drawn into the
house to make up for it, but it won't harm anything (other than your bank
balance).

I'm a little confused by this. The intake pipe draws cold air from
outside. The air in the room is far warmer. Where/how is cold air
being drawn into the house to make up for the fact that I disconnected
the intake pipe and made the furnace use room air instead?

When the furnace uses room air instead of outside air, air has to come from
somewhere to replace it. Where do you suppose it comes from?

*All* air that the furnace uses for combustion comes from outside the house,
in the long run. The point of having an outdoor intake for the furnace is so
that the combustion air comes *directly* from outside the house, straight into
the furnace. If the furnace is burning room air instead, that creates a slight
negative pressure in the house, and cold air comes in through various cracks,
leaks, holes, etc.