One thing to note, however, is there are 2 main categories of UTP(ie networking) cables: Plenum and Non-Plenum (ie 'normal') Rated.
Plenum cable is made with materials that do not give off toxic fumes when heated(and by heated, I mean heated enough to combust, since all materials give off gases when heated, in a process called pyrolysis). Thus, they can be used in HVAC ducts, since they wont spread toxic fumes if they are ignited. Non-Plenum cable DOES give off toxic fumes when heated, as do all household plastic products. Plenum-rated cable is significantly more expensive than non-plenum rated cables, so only use Plenum if you plan on running cables through HVAC ducting.