Originally Posted by
Maxt
It's considered poor practice to vent over a sidewalk, it's not necessarily against code in certain instances, but I avoid doing that for the concrete degradation and the ice build up that occurs.
Two things to look for on your install, the flue exhaust vent should usually be a minimum of 5 feet, some manufacturers say 8 feet with a minimum of 12" horizontal run. Also the vent should slope backwards towards the furnace, about 1/4" per foot so the condensate drains from the furnace instead of dripping from the flue pipe.