combustion air intake for NG furnace

Peter Stock

Recently purchased 50 year old home - cold region just north of Toronto, Canada. New high-eff NGas furnace installed just prior to our purchase.

Air for combustion is drawn directly from the room the furnace is contained in, and vented directly outside through the basement wall in a 3 inch PVC pipe. Fresh outside air is drawn into the cold air return side of the duct system through a duct in the crawlspace of our roof - presumably the thinking is that the air to eventually be combusted will be at room temperature before being drawn into that part of the furnace.

Ideally I would like to draw air directly from outside to feed the combustion chamber, allowing the removal of significant ductwork - however, temperatures here can often be well below freezing for much of the winter and directly drawn air could be quite cold.

Is there a minimum or optimum combustion air temperature required for most NG furnaces? Our furnace is a Rheem Classic 90 Plus.

Thankx