Nothing ruins an outdoor fireplace experience faster than a fireplace that smokes back into the seating area. This is one of the most common problems with outdoor fireplaces in Broken Arrow — and in almost every case, it’s a design or construction error, not an inherent feature of outdoor fireplaces. Here’s how draft works and what proper design looks like.
How Outdoor Fireplace Draft Works
Draft is the pressure differential that pulls combustion gases up the chimney rather than out into the seating area. It’s created by the height of the chimney column (taller = more differential) and by the temperature difference between the hot flue gases and outdoor air.
Outdoor fireplaces have an inherent draft challenge: they’re open on one face (the front), which allows outdoor air pressure to work against the draft rather than with it the way an indoor fireplace benefits from house negative pressure. This means outdoor fireplaces need to be designed to tighter dimensional standards than indoor fireplaces to draft correctly.
Critical Design Dimension #1: Chimney Height
Minimum chimney height above the fireplace opening for outdoor fireplaces: 12 feet total height from firebox floor to chimney termination. This creates enough draft column to establish stable updraft in most outdoor conditions.
The chimney termination must also clear nearby obstructions:
- At least 2 feet above any structure within 10 horizontal feet
- At least 3 feet above the highest point of the fireplace opening
A fireplace built against a pergola or covered patio structure that sits above the chimney top is a smoke disaster waiting to happen — the roof blocks updraft and creates turbulence that drives smoke back into the seating area. Design the fireplace height before designing the structure height, or design the structure to clear the chimney on all sides.
Critical Design Dimension #2: The 10:1 Flue-to-Opening Ratio
The cross-sectional area of the flue must be at least 1/10 of the fireplace opening area. This is the most commonly violated rule in DIY and budget outdoor fireplace construction.
Example calculation: 36″ wide × 24″ tall opening = 864 square inches. 864 ÷ 10 = 86.4 square inches minimum flue area. An 8″ round flue is only 50 square inches — inadequate. A 10″ round flue is 78 square inches — still slightly under. A 10×13″ rectangular flue tile is 130 square inches — correct. A 13×13″ clay flue tile is 169 square inches — also correct.
Fireplaces built with undersized flues relative to the opening will smoke in almost every fire condition. This is not fixable without rebuilding the smoke chamber and replacing the flue liner.
The Smoke Chamber and Throat
Above the firebox opening, the smoke chamber should slope inward at no more than 45 degrees on each side, and the throat damper (if present) should be positioned so the opening is centered over the fire. Irregular or poorly formed smoke chambers create turbulence that causes smoke to exit at the opening instead of rising.
Oklahoma Wind Considerations
Broken Arrow’s prevailing winds can create downdraft conditions at the chimney top. A quality spark arrestor with a directional cap — designed to deflect wind and prevent downdraft — is standard on our fireplace builds. These perform better than flat screen spark arrestors in Oklahoma’s wind patterns.
We Build Fireplaces That Work
VistaScapes builds outdoor fireplaces to correct dimensional standards — correct flue sizing, correct chimney height, properly formed smoke chambers, and wind-deflecting chimney caps. Call 918-779-1317 to discuss your outdoor fireplace project.
