I had the problem of no heat. Furness man came and found that the intake pipe was frozen . He cleared it and the heat came back on. While I was outside later, I saw the exhaust from the furness come out of it's pipe and get sucked into the intake pipe, where it froze again. Called the company again and told them what was happening, they advised me to clear the intake pipe and they would have a repair man extend the intake pipe so the exhaust would'nt get back in. Company thanked me as they had had the same problem at other locations and didn't know what was the problem was.
if they installed your furnace then hopefully they did not charge you to correct this
should be common knowledge with all furnace installers that the intake and vent should be about 2 feet from each other to keep the intake from freezing up