What size flue pipe does your new furnace need? Today's furnaces are fairly standardized as far as connections go, but any manufacturer can make slight differences in their product.
The Right Size Flue Pipe
Discovering what size pipe to buy is simple. Measure the hole where the furnace vents. The pipe you purchase should match the inside diameter of the hole. It can be larger but must never be smaller or leaks will result. The average stove and furnace today require a 3 inch diameter pipe, but can go as large as 8 inches. A pellet stove needs only a 4 inch pipe.
Most connections today are snap lock with male and female ends. All pipe and fixtures such as elbows and caps must match. Consider buying all items from the same manufacturer for the best match.
Flues can be a big source of heat loss during winter. Insulate the opening where they pierce the housing wall or roof well. Buy the appropriate cap. A cap with grating or mesh will keep unwanted critters out of your flues. Make sure the wall thimble you purchase connects tightly to your pipe and fits snugly in the opening you have cut for it.
Remember: when installing furnace or stove pipes, everything, including the distances from walls and other flammable materials for furnace and stove pipe, is set by government regulation. Check with your local building inspector for municipal requirements.