I know this depends on the heater and other variables, but in short I'm pre-planning a replacement heater and I want to use a different location, currently the furnace is horizontal in the crawlspace, I want to later use a high efficiency furnace where it can be PVC pipe vented and the furnace installed vertically, but I'd need a 6 to 8 foot horizontal run for the vent pipe. I'd have some space for some slope, but my question is if this is unreasonable and how much slope does this use typically?