My furnace and water heater vent through the roof via a masonry chimney. The cover seems to have blown off the top, so I now get small amounts of water down through the vent. Question is: Since it's not a fireplace chimney, do I need an actual chimney cap, or is a roof vent useable for this application? The chimney is 18" X 18" and I'm having a hard time finding one that size and even the smaller ones I've found are pretty pricey. I'm just not sure if there are airflow issues that could be created by using a vent. The last thing I need is a bunch of carbon monoxide in the house.