We have a problem with exhaust coming back out through the draft hood on top of our LP gas water heater. It is pretty cold outside about 15 F degrees. We have single wall pipe on the interior and double wall pipe on the exterior, this pipe runs outside the house and itís about 4 or so feet above the edge of the roof/gutters and it is capped. Piping is up to code.
The pipe inside, above the water heater also has condensation on it and feels kind of cold. Our basement it around 60-65 degrees. There is no blockage in the vent pipe. We also have a CO2 detector which the alarm HAS NOT gone off due to this. I do realize the danger with the CO2 and the tank is on vacation mode. This has happened before when it is really cold. Once it warms up outside the issue resolves itself.

Any advice on how to fix and what caused this to happen would be appreciated. Thank you.