Factory-Built Chimneys

Factory-Built Chimneys

Factory-built chimneys are pre-engineered, prefabricated metal chimney systems designed, tested, and listed by nationally recognized testing laboratories (UL, ULC, Intertek) for safe venting of fuel-burning appliances. These systems provide lighter weight, faster installation, and lower cost alternatives to masonry chimneys while meeting stringent safety and performance standards.

Classifications and Standards

Class A Chimneys (UL 103)

Definition: Factory-built chimneys tested and listed to UL 103 standard for residential-type and building heating appliances.

Temperature rating:

• Continuous: 1,000°F flue gas temperature
• Intermittent (10 minutes): 1,400°F
• Test condition: 1,700°F for 1 hour without damage

Approved fuels:

• Gas (natural, propane)
• Oil (distillate)
• Solid fuel (wood, coal, pellets)
• Multi-fuel capability (check specific listing)

Construction:

• Double-wall or triple-wall insulated design
• Inner wall: Stainless steel (typically 304 or 316)
• Outer wall: Galvanized steel or stainless steel
• Insulation: Mineral wool, ceramic fiber, or vermiculite
• Thickness: 1 in. minimum insulation for most applications

Applications:

• Residential fireplaces, wood stoves, pellet stoves
• Furnaces and boilers (gas, oil, solid fuel)
• Replacement for deteriorated masonry chimneys
• New construction installations

HT Chimneys (High-Temperature)

Definition: Factory-built chimneys listed for high-temperature residential-type appliances per UL 103HT.

Temperature rating:

• Continuous: 1,000°F minimum
• Test condition: 2,100°F for 1 hour without damage

Applications:

• Residential solid fuel appliances (wood stoves, fireplaces)
• Commercial solid fuel applications
• Higher temperature requirements than standard Class A

Construction:

• Heavier insulation than standard Class A
• Reinforced joints and supports
• Higher-grade stainless steel inner wall

Medium-Heat Appliance Chimneys

UL 959 listing: Commercial and industrial medium-heat appliances

Temperature range: 1,000°F continuous flue temperature

Applications:

• Commercial kitchen exhaust
• Industrial process equipment
• Commercial heating appliances

Construction Details

Wall Assembly

Triple-wall construction (most common):

1. Inner liner: Stainless steel, 22-24 gauge

   • Direct contact with flue gases
   • Corrosion and temperature resistant

2. Insulation layer: 1-2 in. thickness

   • Mineral wool: 2,000°F+ temperature rating
   • Ceramic fiber blanket: Lightweight, high temperature
   • Maintains flue gas temperature
   • Reduces outer wall temperature

3. Outer wall: Galvanized or stainless steel, 24-26 gauge

   • Structural support
   • Weather protection
   • Low exterior surface temperature

Double-wall construction (less common):

• Inner liner and outer wall only
• Air space instead of solid insulation
• Lower cost, lower performance
• Specific applications only

Joint Systems

Twist-lock connection:

• Male and female interlocking sections
• Quarter-turn to lock
• Positive mechanical connection
• Fast, simple installation

Locking band:

• Stainless steel band clamps over joint
• Secures connection against wind, vibration
• Required at each joint per installation instructions

Condensate-tight joints:

• Special gaskets or sealants for condensing applications
• Prevents acidic condensate leakage
• Required for Category IV appliances

Section Lengths

Standard lengths:

• 6 in., 12 in., 18 in., 24 in., 36 in., 48 in.
• Allows custom total height
• Mix-and-match to achieve required length

Starting sections:

• Adapter to connect appliance outlet
• May include damper, cleanout tee, or support
• Listed for specific chimney brand

Finishing sections:

• Storm collar
• Flashing
• Chimney cap

Clearances and Fire Safety

Clearance to Combustibles

Listed clearance: Stamped on chimney, typically:

• 2 in. clearance: Most common for residential Class A
• 1 in. clearance: Some high-performance models
• 6 in. clearance: Older or commercial models

Zero-clearance NOT applicable:

• “Zero-clearance” refers to fireplaces, not chimneys
• All chimneys require listed clearance to combustibles
• Maintain clearance throughout height

Enclosed chases:

• Combustible chase construction permitted
• Maintain listed clearance to all chase surfaces
• Ventilate chase at top and bottom
• Insulate chase in cold climates to reduce exterior condensation

Penetrations

Ceiling/floor penetrations:

• Listed ceiling support box or radiation shield
• Firestop spacer maintains clearance
• Load transfer from chimney to structure

Roof penetration:

• Adjustable roof flashing (0/12 to 12/12 pitch)
• Storm collar prevents water entry
• Clearance to roof deck: 2 in. typical

Attic/concealed space:

• Radiation shield every 8 ft of concealed run
• Prevents overheating of surrounding materials
• Fire-rated access for inspection

Structural Support

Support Requirements

Maximum unsupported height: Varies by manufacturer, typically:

• Roof-mounted: 5 ft above roof support
• Wall-mounted: Support bracket every 6-8 ft
• Cathedral ceiling: May require intermediate supports

Support methods:

1. Ceiling support box:

   • Transfers chimney weight to ceiling joists
   • Maintains clearance to combustibles
   • Encloses chimney at ceiling penetration

2. Roof support:

   • Adjustable flashing with support collar
   • For installations starting at roof level
   • Limited height above roof per manufacturer

3. Wall bands:

   • Secure chimney to exterior wall
   • Prevent lateral movement from wind
   • Required every 6-8 ft of exposed height

4. Offset supports:

   • Special fittings for horizontal runs
   • Transfer weight to wall or structure
   • Limit horizontal run per manufacturer

Seismic and Wind Considerations

High-wind areas:

• Additional wall bands
• Guy wires for very tall installations
• Reduce unsupported height above roof

Seismic zones:

• Flexible connections where chimney penetrates structure
• Allow independent movement
• Prevent binding during earthquake

Installation Requirements

Height Requirements

Same as masonry chimneys per NFPA 211:

Minimum height:

• 3 ft above roof penetration point
• 2 ft above any roof surface within 10 ft horizontally

Effective height:

• Minimum 15 ft for adequate draft (typical)
• Follow appliance manufacturer’s requirements
• Check local codes for specific requirements

Termination

Chimney cap:

• Listed cap for specific chimney brand/model
• Prevents rain, debris, animal entry
• Spark arrestor for solid fuel (mesh ≤ ½ in.)
• Must not restrict airflow (4× flue area minimum)

Clearance from building:

• Maintain listed clearance from building surfaces
• Additional clearance from windows, air intakes per code
• Typical: 3 ft from window or door opening

Connector Pipes

Single-wall connector:

• Connects appliance to chimney base
• Material: 24-gauge galvanized or stainless steel
• Clearance to combustibles: 18 in. (6 in. with shielding)
• Maximum length: 75% of chimney height, or 10 ft maximum
• Slope: ¼ in./ft upward to chimney

Listed vent connector:

• Type B vent or special connector
• Reduced clearances per listing
• Used where space constraints prevent single-wall clearance

Sizing and Performance

Chimney Diameter Selection

Match appliance outlet:

• Most common: Use same diameter as appliance outlet
• Reduces turbulence, improves draft
• Simplifies connection

Sizing up:

• Increase one size for very tall chimneys (>30 ft)
• Very cold climate exterior installations
• Increase draft to overcome additional resistance

Sizing down:

• Not recommended
• Increases velocity, pressure drop
• May violate manufacturer’s requirements

Common sizes:

• 5 in., 6 in., 7 in., 8 in., 10 in., 12 in., 14 in., 16 in., 18 in. diameter

Draft Performance

Factory-built chimneys generally provide better draft than masonry:

Advantages:

• Insulation maintains flue gas temperature
• Smooth inner liner (low friction)
• Round cross-section (optimal flow)
• Tight joints (minimal air leakage)

Performance factors:

• Exterior installations: Insulation maintains draft in cold weather
• Tall chimneys: Greater natural draft
• Tight construction: Less parasitic air

Material Compatibility

Stainless Steel Grades

Selection criteria:

• Fuel type and sulfur content
• Appliance efficiency (condensing or non-condensing)
• Environment (coastal, industrial)

Insulation Materials

Mineral wool:

• Temperature rating: 2,000°F+
• Non-combustible, won’t settle
• Good compressive strength

Ceramic fiber:

• Temperature rating: 2,300°F+
• Lightweight, excellent insulation
• Higher cost than mineral wool

Vermiculite:

• Expanded mica mineral
• Pourable, fills voids
• Lower temperature rating than wool

Common Installation Errors

Mixing brands:

• Each manufacturer’s system is unique
• Components not interchangeable
• Voids listing, creates safety hazard
• Solution: Use all components from same manufacturer

Inadequate support:

• Exceeding unsupported height
• Chimney weight on appliance
• Solution: Follow manufacturer’s support requirements exactly

Clearance violations:

• Framing, insulation too close to chimney
• Heat damage, fire risk
• Solution: Verify clearances before closing walls

Improper termination:

• Too short above roof
• Subject to downdrafts
• Solution: Apply 3 ft / 2-10 rule

Horizontal connector too long:

• Excessive resistance
• Condensation, creosote
• Solution: Limit to code maximum (typically 75% of vertical height)

Advantages vs. Masonry

Factory-built advantages:

• Lighter weight (no foundation required for wall-mounted)
• Faster installation (hours vs. days)
• Lower material cost
• Better draft (insulated, smooth liner)
• Easier to install in existing buildings
• Variety of diameters available

Masonry advantages:

• Longer service life (50+ years vs. 15-30 years)
• Traditional aesthetic
• Higher heat capacity (thermal mass)
• Can be relined multiple times
• No proprietary parts

Inspection and Maintenance

Annual inspection:

• Check joints for separation
• Verify cap and termination condition
• Look for rust, corrosion, damage
• Ensure clearances maintained (renovations may have compromised)

Cleaning:

• Same frequency as masonry chimneys
• Gas: Every 3-5 years or as needed
• Oil: Annually
• Solid fuel: When creosote exceeds ⅛ in.

Replacement indicators:

• Inner liner perforation or severe corrosion
• Insulation deterioration or settling
• Joint separation or damage
• Manufacturer’s recommended service life exceeded (typically 15-30 years)

Professional installation:

• NFI-certified installers recommended
• Ensure proper listing and code compliance
• Manufacturer’s warranty often requires professional installation

Factory-built chimneys provide safe, efficient, and cost-effective venting solutions when properly selected, installed, and maintained according to manufacturer’s instructions and applicable codes.