That’s an excellent question that gets right to the heart of chimney efficiency and safety.

The short answer is: You generally don’t put a single 90° elbow on a chimney flue because it severely compromises the draft and creates a major maintenance and fire hazard.

Here is the detailed explanation for the prohibition:

1. Drastic Reduction in Draft

The primary function of a chimney is to create a strong, consistent draft (or negative pressure). This draft pulls the smoke, heat, and combustion gases up and out of your home.

• Turbulence and Friction Loss: A 90° turn is a sharp, abrupt angle. When hot gases hit this corner, they stop flowing smoothly and are forced to change direction instantly. This causes massive turbulence and friction loss, which significantly slows the flow and destroys the draft.
• Reduced Effective Height: Experts estimate that a single 90° elbow can reduce your chimney’s effective operating height by anywhere from $3$ to $5$ feet (or about $1$ meter). If your chimney already has a marginal draft, a 90° bend can stop it from working altogether, causing smoke to spill back into the room.

2. Creosote and Soot Accumulation

The sharp turn is a major trap for the byproducts of combustion:

• Soot Collection: The turbulent flow makes the elbow a prime spot for soot and ash to settle and build up quickly.
• Creosote Condensation: The slowing of the hot gases in the elbow increases the chance of them cooling down. When gases cool, the moisture and unburnt wood volatiles condense on the pipe walls, forming a tar-like substance called creosote.
• Fire Hazard: Creosote is highly flammable. A thick layer of it inside a 90° bend is a significant risk for a dangerous chimney fire.

The Proper Solution: Two 45° Elbows

When you need to offset the chimney to go around a structural obstruction (like a joist or rafter), the correct solution is to use two 45° elbows connected by a straight piece of pipe (this is called an offset).

This configuration allows the hot gases to change direction in two gentle stages, maintaining much smoother flow, minimizing turbulence, and reducing the area where soot and creosote can accumulate.

Final Note on Exceptions: While a single, sharp 90° bend is almost always avoided on the main chimney stack, you may see a 90° T-Connector at the base of a vertical chimney. This component is designed to connect the horizontal appliance pipe to the vertical stack, and it includes a removable cap at the bottom to serve as a cleanout port for collected debris. This is a very different function than using a 90° elbow mid-flue to change direction.