Years ago, I got a call to a house with no heat.
The problem turned out to be that mice had
come in the exhaust vent tube. They got into
the furnace compartment, unable to escape.
They turned thier attention to chewing wires,
and that stopped the furnace from running.
The dead mouse smell was overpowering.
Really awful.

Christopher A. Young
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I found it very
curious that the install manual for my Rheem condensing
furnace says that you can't put ANYTHING over the
2" vent pipes that extend outside. Apparently the concern
is the effect George described, ie snow/ice freezing up
over any screening. The obvious downside is that there
is a free and open route into either the exhaust or intake.
The latter would allow a rodent into the unit compartment
with the wiring, gas valve, etc. It would seem to me the
risk of something bad happening, including blocking the
vents without some kind of screen, is at least as great
as the snow problem.