I have a small furnace room (oil boiler, with indirect hot water tank, and oil Tank). I've been told that I need to get fresh air into the room for proper burner efficiency, etc. I've run and piped through to the outside and placed the other end of the vent approximately 12 inch from the floor close to the boiler.

I noticed that my expensive air conditioned air is just flowing out of the house through the vent. I was thinking about installing a similar vent cap to a dryer vent or bathroom exhaust fan that would allow air in (when needed) but minimize air going out. Basically reversing the dryer vent cap. Unfortunately I can't find anything.

Does anyone know what is typically done for this? Is there such a vent cap? Any ideas?