My old furnace is going away shortly. The intake/exhaust lines are being re-routed to be about 25 ft (vs. 75 ft!). They will vent through the side of the walkout basement - about 8 feet above the snow with vinyl siding all the way to the ground.

I'm aware of two types of exhausts. One looks like it's cobbled of PVC pipes and has a separate intake and separate exhaust with vertical risers. The other is a concentric arrangement.

My current 75 ft long run (with lots of elbows!) uses the two separate pipes, but they stick out about 18" above the ground and have a 45 degree downward elbow only. I get wasp nests in the lines every year. My old house had vertical risers and no bugs. ( I've heard that bugs can't build a nest if they have to fly vertically up or down to get to it - don't know this is true or not). I've tried adding modest verticals and elbows to see if that fixes the bug problem here, but the lines are too restrictive and the system gives me pressure limit errors that clear when I remove the risers.

Since I'm putting in new, which is better - PVC risers or concentric - for bugs?? I live in Rochester NY, so freezing is an issue as well.

Any suggestions??