What is cold venting and how does it work?

What I think you may be describing is "power venting" where a blower is used to force the byproducts of a highly efficient furnace or water heater or other devices through PVC pipes and out the side of a building. There is not enough heat to cause a rise in the effulent up a chimney. The gas is has to be forced up or out the side of the house.

Usually outside combustion air is used in this system too.