Cat 6 is usually 22 or 24 gauge, the thermostat wire is 18 gauge. The larger the conductor, the smaller the number. Pair up 2 conductors of the Cat 6 for a single wire of the thermostat. If it is a long run from T-stat to furnace.
Let's Get It Fixed!
HomeX virtual experts will assess your issue (for free!) and can resolve simple problems.