Gas fireplace venting done by the manual in Rock Lake

When the inner pipe leaks exhaust into the air side

In a coaxial vent, exhaust travels in the inner pipe and fresh air in the outer one. If the inner pipe leaks, exhaust slips into the incoming air, and the fire starts breathing its own fumes.

One maker lists this as a cause when the pilot and main burner go out together while the fireplace is running, and the fix it gives is to find the leak and replace the defective section. Our technicians look for disconnected, damaged or unseated inner joints first, since those are the ones the room never shows.

Horizontal runs with the wrong pitch

Horizontal vent pipe is not simply run flat. One direct-vent manual's troubleshooting table lists an improperly pitched horizontal run as a reason flames die out, and it tells the installer to check for an upward rise of 1/4 inch per foot and never run the pipe level or downhill.

The same manual gives different rules for different pipe types and layouts, so our crew checks which rule applies to your run before measuring. The finding goes in your estimate with the section of pipe it concerns.

A cap under a porch, deck or balcony

A cap tucked beneath a deck can fill the space underneath with exhaust. One direct-vent manual permits a termination below a veranda, porch, deck or balcony only where at least two sides under the floor are completely open, and it wants 12 inches between the cap and the underside of that floor.

Skirting, lattice and storage often close those sides after the deck is built. Our pros check how open the space really is today and explain what the manual allows.

Room to breathe in front of the cap

A sealed fireplace pulls its combustion air in at the cap, so the space in front of the cap matters. One direct-vent manual keeps a 24 inch zone in front of the termination hood free of shrubs, garden sheds, fences, decks and utility buildings.

Yards change. A hedge fills in, a shed gets delivered, a gate goes in. Our crew measures that 24 inch zone on every sidewall vent and tells you exactly what has to move.

• Bushes and hedges
• Garden sheds and utility buildings
• Fences and gates
• Decks built after the fireplace

Five feet above the draft hood

A natural-vent fireplace needs enough height for its exhaust to rise. Florida's fuel gas code, in section 503.6.6, wants a Type B gas vent to end no less than 5 feet higher than the highest draft hood or flue collar connected to it.

One B-vent fireplace manual goes further, asking for at least 8 feet of vertical vent between the top of the fireplace and the top of the cap. Our crew measures what it can and uses the stricter figure when judging whether the vent is tall enough.