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The stovepipe on my free-standing gas fireplace has a ring of quarter-sized vent holes at the base of the pipe, right where the fireplace meets the stovepipe. I've always wondered about the purpose of these holes.

I understand that it is important air is drafted through the fireplace and the fire to feed the combustion, but why would one want to allow room air to mix with the flue exhaust?

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    Can you tell if it is single or double wall pipe? – mikes Sep 12 '17 at 10:02
  • It's single walled. There is an annular cone/lip behind the holes to, I presume, help guide the stove fumes past the vents and continue up the pipe, but otherwise it's all open to the room air. – pscl Sep 12 '17 at 15:57
  • Double wall pipe was required for the flue for my tankless water heater. Air does not circulate in the inner space between the walls. There are some modern flue arrangements (only for tankless heaters AFAIK) with concentric piping for the combustion air and the exhaust fumes. – Jim Stewart Sep 12 '17 at 18:34
  • Thanks for the comments. The stovepipe I'm referring to is directly exposed in the living area which I believe is allowed to be single walled. There is a box at the ceiling where it transitions to a double walled chimney as it enters the attic and roof. – pscl Sep 13 '17 at 8:47
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I always assumed this allows extra air to be drawn into the flue to insure oxidation of any CO to CO2. These holes would also by bringing in more air increase the back pressure and so lower the air draw at the burner into the optimum range.

I know the flue on the old gas tank water heaters was fitted with a "draft hood" designed to take in extra air at the top of the tank / bottom of the flue. It seems to be that if not designed correctly this opening could allow escape of combustion gasses out these holes, but I guess the openings generally work as intended.

This video asserts that the purpose of the vent hood is to prevent downdrafts from wind blowing over and into the vent on the roof from extinguishing the gas flame.

My 13-year-old Bosch tankless water heater (natural gas fired) does not have visible openings at the bottom of the flue. It is a "first generation" model with a standing pilot light and without a power vent.

My 26-year-old Carrier natural gas furnace likewise does not have these holes but it has a power vent on the exhaust flue.

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If it's a double-walled flue I imagine it's to allow some air to move through the gap between the walls for cooling. A closed cavity would heat up fairly quickly, eliminating much of the safety benefit of the double-wall arrangement. Chimney effect (convection) would create a slight draft up and out. Chances are some such ports are hidden inside the outer shell of other fireplaces.

This is mostly speculation. I'm not a chimney expert.

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