I would think the warmer air would keep it from freezing over.@Bud
I have not tested yet. This was my new house build so I implemented the system just to be safe. I also have read that if you have a straight pipe, largest diameter possible, towards the conditioned (warmer) walls then convection will cause enough ventilation without the use of a constantly running - energy using fan.
I have 4" of pea gravel and a 10mil poly barrier below my concrete. I can see how the moisture from rain is probably not a problem. It would be non-concern if I did have a fan causing negative pressure. I really hope to not need one. I do know that an animal may be on the roof or checking out the pipe cause problems with it. I'd buy that varmit guard but it seems overpriced online there and just because of the concern that the moist air freezes on the grill part and can cause blockage in the winter.