Buck Stove Model 21 Non-Catalytic Stove Size Guide

Under specific EPA test conditions, the Model 21 delivers heat at rates ranging from approximately 11,079 to 28,901 BTU per hour. Efficiency is 76.89 percent using the CSA B415-1.10 stack loss method (HHV weighted average across all burn rate categories). Emissions rate is 4.4 g/hr. The Model 21 is EPA certified and complies with the 2020 particulate emission standard for cordwood appliances. Heating range is 800 to 1,800 square feet depending on home insulation, ceiling height, layout, and climate zone.

• Compact zone heating for living rooms, family rooms, or single-floor heating in mid-sized homes.
• Small homes and cabins (800 to 1,800 sq ft) where the Model 91's higher heat output would be excessive.
• Mobile homes and manufactured homes. The Model 21 is the only Buck Stove wood stove approved for mobile home installation with the outside air kit.
• Masonry fireplace conversion in fireplaces with minimum 23-1/8 W × 21 H × 15 D inch interior dimensions.
• Pre-fabricated fireplace insert installations in the approved zero-clearance fireplace list.
• Alcove installations with the optional close clearance shield kit, allowing the stove to fit in a wall recess 19 inches from side walls and 7 inches from the back wall.
• Households that prefer simple operation without a catalyst to inspect, monitor, or replace over time.
• Owners who want lower long-term maintenance cost than a catalytic stove provides.

• Large homes above 1,800 square feet. The Model 21 is sized for compact-to-medium heating. For 1,800 to 3,200 sq ft, choose the Buck Stove Model 91 Catalytic.
• Cooking and baking. The Model 21 is a dedicated heater without a cooktop or oven. For wood cooking, choose the Homesteader.
• Connection to chimney flue serving another appliance. The Model 21 manual prohibits shared flue or shared chimney installations.
• Connection to air distribution duct or system. Prohibited by the manual.
• Installation in sleeping rooms (mobile home installations).
• Burning trash, garbage, artificial logs, paper logs, gift wrapping, treated or painted wood, or coal. The Model 21 manual prohibits all of these fuels.
• Use with grates, andirons, or other methods of supporting the fuel. Build the wood fire directly on the firebricks.

• Cold climate (Zone 5-7): The Model 21 covers 800 to 1,200 sq ft cleanly. For 1,200 to 1,800 sq ft in cold climates, plan for medium-high to high burn settings and verify insulation supports the demand.
• Moderate climate (Zone 4): The Model 21 covers its full rated range in average-insulation homes. Most operation will be at low-to-medium burn settings.
• Mild climate (Zone 3 and below): The Model 21 covers up to its full 1,800 sq ft range comfortably. The non-catalytic design lights off quickly for short-duration shoulder-season heating.
• Cathedral ceilings or two-story open plans: The included blower significantly improves distribution; ceiling fans on low (reverse direction in winter) further help heat reach floor level.
• Mobile homes: Specific attention to outside air supply and chassis grounding requirements is required per the manual. Do not install in sleeping rooms.

The table reflects typical sizing recommendations under average insulation and 8 foot ceiling assumptions. Actual results depend on home insulation, climate zone, layout, and air sealing.

• Side wall combustible: 25 inches.
• Front floor protector: 16 inches.
• Side floor protector: 8 inches.
• Side wood trim: 17 inches.
• Top wood trim: 17 inches.
• Mantel or brackets: 25 inches above the top of the unit.
• Minimum fireplace dimensions: Height 21 inches; Width 23-1/8 inches; Depth 15 inches.
• Approved for pre-fabricated fireplaces (ZCF): Yes — see the approved list in the manual or contact your dealer.

• A — Side wall: 19 inches.
• B — Back wall: 7 inches.
• C — Ceiling height (minimum): 84 inches.
• D — Collar to wall: 8-1/2 inches.
• E — Hearth extension side: 8 inches.
• F — Hearth extension rear: 6 inches.
• G — Hearth extension front: 16 inches.
• H — Maximum alcove depth: 48 inches.

• Install in accordance with 24 CFR, Part 3280 (HUD).
• Do not install in a sleeping room.
• Pedestal Kit with outside air kit required. Cut a 4-1/4 inch diameter hole through the floor protector pad and floor; secure the outside air duct to the inside bottom of the pedestal stand.
• Bolt the pedestal stand to the mobile home floor with four 3/8 inch by 1-1/2 inch lag bolts through the four mounting holes.
• Maintain the structural integrity of the mobile home floor. Move opening and reposition the heater location if necessary.
• If home is underpinned: Outside air duct must extend through the underpinning as illustrated in the manual.

• Material: Non-combustible, minimum 3/8 inch thickness, or equivalent (use R-value calculation method documented in the manual for alternate materials).
• Extends: 16 inches from door opening in front of the stove, 8 inches from door opening on each side, 6 inches at the rear, and should be under the chimney connector for rear vent runs.

• Chimney type: 6 inch UL 103 HT factory-built chimney (Type HT, 2,100 degree Fahrenheit) or code-approved masonry chimney with a flue liner.
• Minimum chimney height: 15 feet total height.
• 2-10-3 rule: Chimney must extend at least 3 feet above the roof at point of penetration and at least 2 feet higher than any roof surface within 10 feet horizontally.
• Connector pipe options: Single wall blued or black steel (24 ga minimum), or DVL close clearance double wall pipe (significantly reduces required wall clearances).
• Connector joints: Secure each section with three sheet metal screws or rivets; crimped end faces downward.
• Single appliance flue: Do not connect this unit to a chimney flue serving another appliance.

The Model 21 does not use a catalytic combustor. Clean combustion is achieved through three stainless steel secondary air tubes installed above the firebox: two front/rear tubes with 24 holes each and one middle tube with 12 holes. Preheated combustion air is injected through these tubes into the upper firebox where it mixes with rising combustion gases at high temperature. The mix ignites and burns the remaining particulates and gases, achieving EPA-certified clean combustion without a catalyst.

This design has two practical implications: (1) no catalyst to inspect or replace over the life of the stove, reducing long-term maintenance cost; (2) secondary combustion requires sufficient firebox temperature, so the Model 21 performs best at moderate-to-high burn rates rather than very low slow burns.

For peak performance, use dry seasoned natural hardwood, loaded front-to-back in the firebox. Hardwoods such as oak, maple, ash, beech, and hickory deliver longer burn times and higher heat output than softwoods. The manual notes that softwoods like pine create more creosote, clog the chimney faster, and produce a less efficient burn.

• Best wood species: Oak, maple, ash, beech, hickory, locust, cherry.
• Seasoning time: 6 to 12 months stacked under cover with air circulation.
• Moisture content target: Below 20 percent. Wood seasoned outside uncovered may have 40 percent moisture content.
• Stack pattern: Criss-cross under a shelter to allow air flow.
• Maximum log length: 11 inches front-to-back, 18 inches side-to-side. The 1.6 cu ft firebox accommodates typical cordwood splits.

Do NOT burn:

• Trash or garbage.
• Artificial logs or paper logs.
• Gift wrapping or colored paper.
• Treated wood, painted wood, plywood, particleboard.
• Coal of any kind.
• Chemical accelerants — gasoline, gasoline-type lantern fuel, kerosene, charcoal lighter fluid, or similar liquids. These fluids give off volatile fumes and can explode.

• Build directly on firebricks. Do not use a grate, andirons, or any other method of supporting the fuel.
• Open primary air control fully when starting from a cold stove (slide the air control rod to the open position).
• Starting fire procedure: Twist two pieces of non-colored newspaper into a roll, place on the floor of the firebox. Lay several pieces of dry kindling on top, then three or four small pieces of firewood (2-3 inch diameter) on top of the kindling. Light the newspaper, close and latch the door. If draft is sluggish, hold a piece of lit newspaper near the open damper to preheat the chimney.
• Reloading: Once a coal bed is established, load with seasoned hardwood placed front-to-rear. The fueling door must remain closed during operation.
• Never over-fire. If any part of the stove or chimney connector starts to glow red, you are over-firing. Reduce the primary air setting.
• Manufacturer-set minimum low burn rate must not be altered. Federal regulations prohibit altering the burn rate or operating in a manner inconsistent with the manual.

• Automatic thermostat: When the stove gets hot enough, the thermostat activates the room air blower. Set the rheostat speed to low when burning at low, medium-low, or medium-high; set the rheostat speed to high when burning at high.
• Turn off the blower before refueling or removing ashes. Be sure to turn the blower back on when finished.
• Power cord routing: Do not run the power cord underneath the heater or in walkways or heavy traffic areas.

• The Model 21 does not include an ash pan. Ashes are removed manually when cool.
• Removal frequency: Remove when ashes get to 2 to 3 inches deep, or every few days during heavy use.
• Disposal: Place in a metal container with a tight-fitting lid on a non-combustible floor or on the ground, away from combustible materials, pending final disposal. Cinders should thoroughly cool before final disposal.
• Never remove ashes with the blower running.

Compare the Model 21 against the Homesteader and Model 91, or browse the full Buck Stove collection.