In article , MNRebecca wrote:
Another question. Is it possible the builder/installer (furnace went
in during a major house renovation) made the intake pipe face east
(instead of down) to achieve a 180 degree variance with the exhaust
pipe? They're only a few inches apart.

Very likely that's the reason.

If I add some pipe and change
the intake direction from east to straight down, it'll only be 90
degrees different from the exhaust pipe opening. Have I just greatly
increased my likelihood of carbon monoxide poisoning (from the intake
pipe sucking in exhaust)

Slightly increased, perhaps, but:
(a) if you don't already have a CO detector in the house, you should anyway;
(b) it's not rocket science to add elbows and extensions to the pipes to
increase the separation; and...

or doesn't it matter much since it's only
going to get burned up in the furnace anyway?

(c) most of it is going to get burned up in the furnace anyway.

By the way, I double-checked.
1) They really do face east (intake) and west (exhaust). I generally
think of the wind as coming primarily from the NW in my area, but I
guess it does come from E or SE about half the time.

You could check with the National Weather Service; they keep records of that
sort of thing.

2) The pipes are about 1 foot above the ground, not 2.5. Maybe the
intake doesn't face down for fear of a drift forming beneath it.

I'm sure that's the reason. It shouldn't be too hard to extend the pipe
upward, though, then put two elbows on it so the opening faces down but is
much higher above the ground.