What is the HVAC vent on my roof, and does it need maintenance?
Short answer. An HVAC vent on a roof is usually the flue for a gas furnace, often shared with a water heater, or a pair of white PVC pipes for a high efficiency furnace's intake and exhaust. Some homes also have roof caps for bath or kitchen exhaust. Each one needs a proper cap or termination and well maintained flashing to keep rain out.
Identifying the vents
- Metal flue with a round cap: a natural draft gas furnace or water heater vent, often double wall B-vent.
- White PVC pipes: high efficiency condensing furnaces use plastic pipes for exhaust, and often a second for combustion air. They frequently exit through a sidewall but can go through the roof.
- Low hooded caps: bath fan or kitchen hood exhaust, with a damper inside.
- Rooftop units: uncommon on Georgia houses, but some low slope roofs have them.
Maintenance that matters
- Flashing: check that boots and storm collars are intact and sealed, since this is where most leaks occur.
- Caps: look for missing, dented, or bird nested caps after storms.
- Clearances: PVC terminations need the clearances their manufacturer specifies from the roof surface, other vents, and windows.
- Condensation: PVC exhaust pipes from condensing furnaces carry moisture, and a sagging pipe in the attic can collect water.
- Annual HVAC service: technicians should check flue connections and the condition of the terminations.
Safety
Never cover or seal an HVAC vent during roofing work. A blocked flue can push carbon monoxide into the house. Keep CO alarms working, and if a flue cap is missing or a pipe is disconnected, call a licensed HVAC professional.
During a reroof
Flue flashing, storm collars, and PVC pipe boots should be replaced with the shingles, and pipes should be checked for adequate height above the new roof surface. See roof flashing explained and pipe boot leaks.