Follow the manuf directions. Each has their own design and therefore their own specifications. On my preferred brand they ask that it be graded back to the unit. I do that except on the piece exiting the building which I grade to the outside. I do not want rain water coming back to the unit.

Agreed on the instructions, just curious about the "why" part. I confirmed with Rheem that the exhaust side could actually be mounted with the slope in either direction and it should still drain properly if sloped back towards the condenser, but it would consume the (built-in) neutralizing agent much faster making it potentially not last the warranty period. On the input side, they do not have an integral drain, so any humidity either has to condense in the pipe and drain outwards or get vaporized in the combustion if if drains inwards (an unsupported configuration). I haven't confirmed it, but suspect the Noritz burner can either handle more moisture in the combustion chamber or has an internal drain. Regardless, good response from the manufacturer.

Dan