John, …. Thanks for the reply. After hearing most install spec’s require the slope back to the furnace, … it still makes no sense to me. I understand the slope up to the terminating point, as that applies to any gas combusting furnace. It’s enough the furnace has to deal with its own condensate, but to deal with rain water as well ?? With heavy enough rains, that could be quite a bit of water. That flue pipe could fill up rather quickly!! I’ve always seen them horizontal. Good idea, …. I’ll certainly look it up online to see what Carrier has to say about it.

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Robert
Roberts Home Inspection Service