Chimney & Damper

Part of Kiln Design

Designing chimneys and dampers for kiln draft control — the engine that drives heat through your kiln.

Why This Matters

A kiln without a chimney is a box of fire with no direction. Hot combustion gases swirl chaotically, exit through any available crack, and leave the kiln interior unevenly heated with dead zones where ware underfires. The chimney creates directed airflow — drawing fresh air in through the firebox and pulling hot gases systematically through the kiln chamber before exhausting them.

The damper controls how fast this happens. Open the damper and you increase draft, pulling more air through the firebox (hotter fire, faster temperature rise, more oxidizing atmosphere). Close it and you restrict the flow (slower burn, more reducing atmosphere, better heat retention). Together, the chimney and damper are the primary controls for kiln temperature, atmosphere, and fuel efficiency.

Understanding draft physics lets you diagnose kiln problems — cold spots, excessive fuel consumption, inability to reach target temperature, uneven firing — and fix them through chimney modifications rather than guesswork with fuel and air inlets.

Draft Physics

The Stack Effect

Hot air is less dense than cold air. A column of hot air in a chimney weighs less than the same column of cold air outside. This density difference creates pressure — cold air pushes in at the bottom (through the firebox), and hot air rises out the top (through the chimney). The result is a continuous flow of air through the kiln.

Draft strength depends on three factors:

1. Temperature difference between chimney gases and outside air — greater difference = stronger draft
2. Chimney height — taller chimney = more weight difference = stronger draft
3. Chimney cross-section — must be sized to allow the volume of gas to flow without excessive friction

Calculating Draft

A rough rule for kiln chimneys:

Chimney Height	Approximate Draft (inches of water)	Suitable Kiln Volume
4 feet (1.2 m)	0.02-0.04	Up to 5 cu ft
6 feet (1.8 m)	0.04-0.06	5-15 cu ft
8 feet (2.4 m)	0.06-0.08	15-30 cu ft
10 feet (3.0 m)	0.08-0.12	30-50 cu ft
15 feet (4.5 m)	0.12-0.18	50-100 cu ft

When in Doubt, Build Taller

An overly tall chimney produces excess draft that can be reduced with the damper. An undersized chimney produces insufficient draft that cannot be increased without rebuilding. Add 2 feet beyond what you think you need.

Cross-Section Sizing

The chimney flue must be large enough to pass the combustion gases without creating a bottleneck, but not so large that gases cool too quickly (reducing draft) or that the chimney is structurally unwieldy.

Rule of thumb: The chimney flue cross-sectional area should be 10-15% of the kiln’s firebox opening area.

For example:

• Firebox opening: 12 x 12 inches = 144 square inches
• Chimney flue: 144 x 0.12 = ~17 square inches = roughly 4 x 4 inches internal

A round flue is more efficient than a square one (less friction per unit of area), but square is easier to build with bricks.

Chimney Construction

Materials

Material	Pros	Cons	Best For
Fired brick	Durable, retains heat, easy to build	Heavy, needs mortar	Permanent kilns
Stone	Very durable, locally available	Hard to shape, heavy	Permanent kilns
Clay pipe sections	Quick assembly, good draft	Fragile, hard to make large	Small kilns
Stacked clay pots (bottomless)	Improvised, fast	Short-lived, poor seal	Temporary kilns
Sheet metal (if available)	Light, easy to modify	Corrodes, loses heat fast	Test setups only

Brick Chimney Build Sequence

1. Foundation: The chimney base needs a solid footing. If building on ground, lay a stone or concrete pad at least 4 inches larger than the chimney on all sides. If building on the kiln structure, ensure the kiln wall can support the chimney weight.

2. Connection to kiln: The flue opening between kiln and chimney is the throat. It should be the same cross-section as the chimney flue — any constriction at this point reduces draft significantly.

3. First courses: Build the chimney walls at least one brick thick (4.5 inches / 11 cm). Use refractory mortar for the first 3-4 feet where temperatures are highest, then standard lime or clay mortar above.

4. Taper (optional): Some traditional chimneys taper slightly from bottom to top. This accelerates gas velocity at the top, improving draft in light winds. A taper from 6x6 inches internal at the base to 4x4 inches at the top works well.

5. Cap: A simple stone or clay slab overhanging the chimney top by 2-3 inches on each side prevents rain from entering while allowing gas to exit freely on all four sides.

Freestanding vs. Attached

Attached chimney: Built directly against or on top of the kiln. Simpler construction, shorter flue path, less heat loss between kiln and chimney. Most small kilns use this design.

Freestanding chimney: Connected to the kiln by an underground or above-ground flue channel. Allows the chimney to be positioned for optimal draft (uphill from kiln, away from wind-blocking structures). Necessary for downdraft kilns where the flue exits the kiln floor.

For a freestanding chimney with underground flue:

1. Dig a channel from the kiln exit to the chimney base — sloping slightly upward toward the chimney
2. Line with fired brick or flat stones
3. Cover with stone slabs, then earth backfill
4. Ensure no dips or low spots where condensation can pool and block flow

Damper Design

The damper is a movable obstruction in the chimney flue that controls how much gas can pass. Opening the damper increases draft; closing it decreases draft.

Types of Dampers

Sliding plate damper: A flat plate (stone, fired clay, or iron) that slides horizontally through a slot in the chimney wall. The simplest and most reliable design.

Construction:

1. At a convenient height (3-4 feet above ground — reachable but above the hottest zone), leave a horizontal gap in two opposite chimney walls
2. The gap should accept a plate that covers the full flue opening
3. Slide the plate in to close, pull out to open
4. Intermediate positions provide partial closure

Brick damper: A loose brick placed over or partially over the chimney top. Crude but effective for simple kilns. Move it by hand (use tongs at high temperature — the chimney top is hot).

Rotating butterfly damper: A plate mounted on a central pivot pin that rotates to open or close. More complex to build but provides fine adjustment. The pivot pin must be heat-resistant (iron rod through the chimney wall).

Never Fully Close the Damper During Active Firing

A completely sealed chimney with an active fire produces carbon monoxide buildup and can cause explosive backdraft when opened. Always leave at least 10-15% opening during any phase of firing.

Damper Position in the Chimney

Place the damper at the base of the chimney, where it exits the kiln — not at the top. Reasons:

1. Hot gases above the damper maintain chimney temperature, preserving draft even when the damper is partially closed
2. A cold upper chimney with a top damper can reverse flow (cold air sinking down)
3. Base position is easier to reach during firing

If structural constraints require a higher position, place it no higher than the lower third of the chimney height.

Draft Control During Firing

Startup (Cold Kiln)

A cold chimney has no draft — there is no temperature difference to create flow. Starting requires:

1. Open the damper fully
2. Light a small fire at the chimney base (a few sheets of newspaper or a small kindling bundle inside the chimney flue opening). This pre-heats the air column and initiates draft.
3. Once the chimney fire draws well, light the main firebox
4. Draft builds progressively as kiln and chimney temperatures rise

Wind Assist

On windy days, orient the firebox opening into the wind for the first hour. Wind pressure at the air inlet supplements the weak initial draft. Once the chimney is hot and natural draft is established, wind direction matters less.

Temperature Ramp (Bisque or Glaze Firing)

Firing Phase	Damper Position	Draft Effect
Preheat (to 500°F)	100% open	Maximum air for combustion, slow heat build
Water smoking (to 450°F)	75-100% open	Needs oxygen to carry steam out
Ramp (to 1100°F)	50-75% open	Reducing draft slows ramp for quartz inversion
Full fire (1100-2300°F)	50-75% open	Balanced combustion
Soak (hold at peak)	25-50% open	Restrict draft to hold temperature
Reduction (if desired)	15-25% open	Starve oxygen for reduction atmosphere
Cooling	0-10% open	Nearly closed to retain heat

Reading the Draft

You can observe draft behavior without instruments:

• Flame direction at the firebox: Flames should lean inward, pulled toward the kiln interior. If flames lick outward, draft is insufficient — open the damper or check for chimney blockage.
• Smoke from spy hole: A puff of smoke escaping when you open the spy hole is normal. Continuous smoke pouring out means the chimney is not drawing adequately.
• Chimney top: Vigorous heat shimmer and smoke plume = strong draft. Lazy, intermittent smoke = weak draft.
• Sound: A well-drawing kiln produces a soft roar at the firebox. Silence or intermittent puffing indicates draft problems.

Troubleshooting Draft Problems

Insufficient Draft

Symptoms: Sluggish fire, smoke leaking from joints, inability to reach target temperature.

Solutions (try in order):

1. Open damper fully
2. Check for chimney blockage (bird nests, fallen brick, creosote buildup)
3. Seal air leaks in kiln walls — every crack that lets air in short-circuits the draft path
4. Add chimney height — even a temporary extension of clay pipe or sheet metal
5. Preheat the chimney more aggressively before loading fuel

Excessive Draft

Symptoms: Fuel burns too fast, temperature hard to control, excessive fuel consumption.

Solutions:

1. Close damper partially
2. Reduce firebox air inlet size (partially block with a brick)
3. If chimney is permanently too tall, partially block the flue with a permanent brick restriction

Uneven Heating

Symptoms: One side of the kiln fires hotter than the other.

Solutions:

1. Check that the flue opening is centered on the kiln — an offset flue pulls gases preferentially to one side
2. Verify the chimney is vertical — a leaning chimney creates asymmetric draw
3. Add baffles (partial internal walls) to redirect gas flow through the cold zone
4. In downdraft kilns, check that floor flue channels are clear and evenly distributed

Wind Interference

Symptoms: Draft fluctuates wildly on windy days, with occasional backdraft puffs.

Solutions:

1. Build a wind cap — a four-sided open cover extending 6-8 inches above and around the chimney top
2. Extend chimney height above the roofline of nearby structures or above the kiln body
3. Build a wind screen (stone or earth wall) on the prevailing wind side

Secondary Air Ports

Some kiln designs benefit from secondary air inlets — small holes in the kiln wall above the firebox that admit fresh air directly into the combustion zone above the fuel bed.

Purpose

Fuel releases volatile gases (wood gas) as it heats. These gases burn only if they contact fresh oxygen. In a kiln where all air enters through the firebox bottom, the volatile gases may pass through the ware chamber unburned — wasting fuel and depositing carbon soot on the ware.

Secondary air ports inject oxygen above the fuel bed, igniting these volatiles and extracting more heat from the same amount of fuel. This can increase kiln efficiency by 15-25%.

Implementation

1. Drill or leave 2-4 holes, each 1-2 inches (2.5-5 cm) diameter, in the kiln wall just above the firebox arch
2. Angle them slightly downward toward the fuel bed
3. Make them pluggable — use tapered clay plugs to close them when you want a reducing atmosphere
4. Open them during oxidation phases and whenever you see unburned smoke exiting the chimney (smoke = wasted fuel)