Hello.
I was wondering if anyone has a solution for this problem.

I recently installed a power-vented natural gas water heater to replace a 20-year-old electric.

The installation was done by me and inspected by both a gas fitter and the local gas company. All is good…

However, there is a significant amount of cold outside air coming in the 2″ (system 636) exhaust from the power vent, and its enough to spin the fan on the power vent backwards. (when not running of course)

The vent goes straight up from the water heater about 2′ then a 90Deg. turn about 2′ to another 90 deg turn about 2′ out the wall and there is a 90Deg on the outside that points down and there is a bug screen (supplied with the heater) glued into that final 90.

My question is, is there a flapper, damper product I could use instead of the last 90 to prevent cold air from entering the vent. OR should I try adding another couple 90 deg bends outside or a T?

Having that cold air come in and down through the hot water tank can not be efficient.

Thank You