Direct vent (dual pipe) termination

What is going on

On a direct vent install, what I always check first is that the intake and the vent end up in the same pressure zone. If they do not, the pressure switch will trip on windy days and you get a no-heat call that is hard to pin down. Direct vent can exit through a side wall or the roof, or run in an unused chimney with at least 12 in. above the chimney top. Take care sealing every penetration, because moisture and air leaks cause problems later.

Likely causes

• Intake and vent in different pressure zones, causing pressure switch trips in wind
• Major obstacle between the two terminations
• Unsealed wall penetration leaking air and water
• Gap in the chimney left unsealed

Check in this order

1. Terminate both pipes in the same pressure zone: same side of roof, and no major obstacles between them.
2. For a wall penetration, use a coupling on each side of the wall joined by a short pipe, sized to the wall thickness plus socket depth.
3. Seal the wall penetration with silicone caulk.
4. Seal the chimney gap with weather-tight, corrosion-resistant flashing.
5. Check all joints for leaks after assembly.
6. Run the furnace and confirm the pressure switch stays closed during heat.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)