Wood and Pellet Stoves

Wood and Pellet Stove Fundamentals

Wood and pellet stoves provide renewable biomass heating for residential applications, converting solid fuel to useful heat through controlled combustion. Modern EPA-certified units achieve 63-85% efficiency while meeting strict emission standards: 2.0 g/hr particulates for catalytic stoves, 2.5 g/hr for non-catalytic designs.

Heat output ranges from 10,000 to 100,000 BTU/hr depending on fuel loading, air supply, and combustion technology. Unlike central heating systems, wood and pellet stoves require active management including fuel loading, ash removal, and combustion control to maintain performance and safety.

Wood Stove Types and Technologies

EPA Certification Requirements

Emission Limits (NSPS 2015)

• Catalytic stoves: ≤2.0 g/hr particulate matter
• Non-catalytic stoves: ≤2.5 g/hr particulate matter
• Testing: Washington State test method (crib wood, actual use conditions)
• Certification label required on all units manufactured after 1988

Uncertified pre-EPA stoves emit 15-30 g/hr, producing excessive creosote, reducing efficiency, and impacting air quality.

Catalytic Combustor Stoves

Catalytic Combustion Principle Ceramic honeycomb coated with platinum or palladium catalyzes combustion of smoke and volatiles at temperatures as low as 500-600°F instead of typical 1,000-1,200°F required for secondary combustion.

Catalyst Specifications

• Material: Cordierite ceramic substrate with noble metal coating
• Cell density: 25-100 cells per square inch
• Operating temperature: 500-1,800°F (1,000-1,600°F optimal)
• Dimensions: 6-12 inches diameter or equivalent area
• Life expectancy: 12,000-20,000 hours (6-10 seasons)

Heat Release and Efficiency Catalytic stoves provide:

• Extended burn times: 8-12 hours on single load
• Lower visible emissions during low-burn operation
• Efficiency: 72-85% (HHV basis)
• Steadier heat output through longer, slower combustion

Combustion Process Smoke gases bypass catalyst during startup (bypass damper open). Once stove reaches 500°F, close bypass to route gases through catalyst:

$$\text{Volatiles} + \text{O}_2 \xrightarrow{\text{catalyst}} \text{CO}_2 + \text{H}_2\text{O} + \text{heat}$$

Catalyst reduces activation energy, enabling complete combustion at lower temperature.

Maintenance Requirements

• Monthly inspection for ash blockage or coating
• Gentle brushing (soft bristle) to remove surface ash
• Annual professional cleaning or replacement
• Thermal shock avoidance (no cold air on hot catalyst)
• Monitor for degradation: increased smoke, reduced performance

Non-Catalytic Stoves

Secondary Combustion Technology Preheated air injected above fuel load ignites smoke and volatiles in controlled pattern. Baffles direct gases through high-temperature zone for complete combustion.

Design Features

Air Injection Tubes Stainless steel tubes with holes positioned above firebox:

• Preheat air temperature: 400-800°F
• Injection velocity creates turbulent mixing
• Multiple tubes create combustion curtain
• Holes: 1/8 to 3/16 inch diameter, spaced 2-4 inches

Refractory Baffles Insulated firebox walls and baffle plates:

• Material: Firebrick, ceramic fiber, or vermiculite board
• Function: Reflects heat, maintains high combustion temperature
• Temperature: 1,400-2,000°F in primary combustion zone

Performance Characteristics

• Efficiency: 63-78% (HHV basis)
• Burn time: 4-8 hours per load
• Particulate emissions: 1.5-2.5 g/hr (EPA-certified models)
• Lower initial cost and maintenance than catalytic

Combustion Air Control Single or multiple air controls regulate:

• Primary air: Under fuel bed, controls burn rate
• Secondary air: Above fuel, provides oxygen for volatile combustion

Proper adjustment critical for efficiency and emission control.

Advanced Hybrid Designs

Combination catalyst and secondary air injection:

• Benefits of both technologies
• Efficiency: 78-85%
• Extended burn times with low emissions
• More complex operation and maintenance

Wood Fuel Characteristics

Moisture Content

Critical Parameter Wet wood reduces efficiency and increases creosote formation. Moisture content by weight:

$$MC = \frac{m_{water}}{m_{dry}} \times 100%$$

Performance Impact

Energy Loss from Moisture Heat of vaporization: 970 BTU/lb water For 30% moisture content wood:

• 2.5 lb water per 10 lb load
• Energy loss: 2,425 BTU just for evaporation
• Reduces net heat output 12-18%

Seasoning Requirements

• Split wood to 3-6 inch thickness
• Stack with air circulation
• Cover top, leave sides exposed
• Season 6-12 months for softwoods
• Season 12-24 months for dense hardwoods
• Target moisture content: 15-20%

Moisture meters provide accurate field measurement (pin-type or microwave).

Species and Heat Content

Energy Density Varies by species density and resin content:

Cord = 128 ft³ stacked wood (4 ft × 4 ft × 8 ft)

Hardwood vs. Softwood

• Hardwoods: Higher density, longer burn time, more coals
• Softwoods: Faster ignition, quicker heat, less coaling
• Best practice: Mix for startup (softwood) and extended burn (hardwood)

Particulate and Creosote Formation

Incomplete Combustion Products Smoke contains unburned volatiles:

• Tar droplets and condensable organic compounds
• Carbon particles (soot)
• Water vapor
• CO and CO₂

Creosote Stages

1. First-degree: Brown or black crusty flakes, easy removal
2. Second-degree: Black tar-like coating, requires mechanical scraping
3. Third-degree: Hardened glazed coating, highly flammable, difficult removal

Formation Conditions

• Flue gas temperature <250°F (condensation temperature)
• Excess moisture in fuel
• Restricted airflow (smoldering combustion)
• Oversized flue cross-section (low velocity, temperature loss)

Prevention: Burn seasoned wood, maintain hot fires, properly sized chimney.

Pellet Stove Operation

Pellet Fuel Specifications

Manufacturing Process Sawdust, wood chips, or agricultural biomass compressed under high pressure without binders. Lignin in wood acts as natural binding agent.

Pellet Standards (PFI)

• Premium grade: <1% ash, <8% moisture, 8,500 BTU/lb minimum
• Standard grade: <3% ash, <8% moisture, 8,000 BTU/lb minimum
• Dimensions: 1/4 to 5/16 inch diameter, 1/2 to 1-1/2 inch length
• Bulk density: 40-45 lb/ft³

Advantages Over Cordwood

• Consistent moisture content (4-8%)
• Uniform size enables automated feeding
• Higher bulk density (easier storage and transport)
• Lower emissions through optimized combustion
• No seasoning required

Auger Feed Systems

Automated Fuel Delivery Auger screw conveys pellets from hopper to burn pot:

• Motor type: Gear motor or direct drive
• Feed rate: 1-6 lb/hr depending on heat demand
• Cycle control: Thermostat or programmable controller
• Reverse capability: Clears jams

Feed Rate Calculation For desired heat output:

$$\dot{m}{pellets} = \frac{Q{output}}{\eta \cdot HV_{pellets}}$$

Where:

• $\dot{m}_{pellets}$ = pellet consumption rate (lb/hr)
• $Q_{output}$ = desired heat output (BTU/hr)
• $\eta$ = combustion efficiency (0.75-0.85)
• $HV_{pellets}$ = heating value (8,500 BTU/lb typical)

30,000 BTU/hr output at 80% efficiency: $\dot{m}_{pellets} = 30,000 / (0.8 × 8,500) = 4.4$ lb/hr

Combustion Air Management

Blower Systems

Combustion Air Blower Delivers primary air to burn pot:

• Airflow: 50-150 CFM
• Variable speed control matches fuel feed rate
• Creates slightly negative pressure in burn pot
• Ensures complete combustion with minimal excess air

Convection Blower Distributes heat to room:

• Airflow: 80-200 CFM
• Temperature-activated or continuous operation
• Noise level: 35-50 dBA

Air-Fuel Ratio Control Modern pellet stoves modulate both fuel and air:

• Low fire: 25-40% of maximum input
• High fire: 100% rated input
• Automatic adjustment maintains optimal combustion
• Oxygen sensor feedback on premium models

Ash Removal and Maintenance

Ash Production Premium pellets: 0.5-1% ash by weight Standard pellets: 1-3% ash by weight

For 4 lb/hr consumption over 8-hour burn: Premium: 0.16-0.32 lb ash per day Standard: 0.32-0.96 lb ash per day

Maintenance Schedule

• Daily: Check burn pot, remove clinkers
• Weekly: Empty ash pan
• Monthly: Clean heat exchanger, inspect door gasket
• Annually: Professional cleaning of exhaust system

Clinker Formation High-ash pellets or contaminated fuel creates fused ash deposits:

• Blocks airflow in burn pot
• Reduces combustion efficiency
• Requires mechanical removal
• Prevented by premium fuel and proper air settings

Combustion Air Requirements

Outdoor Air Supply

Code Requirements NFPA 211 recommends outside combustion air for:

• All solid fuel appliances in tight construction
• Homes with mechanical ventilation systems
• Buildings with multiple exhaust devices

Sizing Outdoor Air Duct Minimum free area:

$$A_{duct} = \frac{Q_{input}}{K}$$

Where:

• $A_{duct}$ = duct free area (in²)
• $Q_{input}$ = maximum input rate (BTU/hr)
• $K$ = constant (4,000 for directly connected duct)

For 75,000 BTU/hr stove: $A_{duct} = 75,000/4,000 = 18.75$ in² Use 5-inch round duct (19.6 in² area)

Installation Requirements

• Connect to rear or bottom air inlet on stove
• Terminate 12 inches minimum above grade
• Screen or louver to prevent debris entry
• Sealed connection prevents air leakage

Building Depressurization

Pressure Effects Exhaust fans, fireplaces, and leaky ductwork create negative pressure:

$$\Delta P_{building} = \sum Q_{exhaust} - \sum Q_{supply}$$

Negative pressure can cause:

• Backdrafting of combustion products
• Reduced stove performance
• Spillage of smoke during door opening

Makeup Air Requirements

• Measure building pressure with manometer during worst-case operation
• If pressure drops below -3 Pa, provide makeup air
• Size makeup air 10-15% larger than exhaust airflow
• Motorized or passive damper options

Installation Clearances and Floor Protection

Clearances to Combustibles

NFPA 211 Standard Clearances (unprotected surfaces)

Listed Clearances Manufacturer testing may demonstrate reduced clearances:

• Typical listed rear: 12-18 inches
• Typical listed side: 12-16 inches
• Follow listing label specifications exactly

Clearance Reduction Methods

Wall Shields Ventilated air space between shield and combustible wall:

$$C_r = C \times \frac{F}{100}$$

Where:

• $C_r$ = reduced clearance
• $C$ = standard clearance
• $F$ = reduction factor (typically 33-66%)

NFPA 211 Approved Shields

• Sheet metal (24 gauge minimum) with 1-inch air space: 66% reduction
• Mineral fiber board with sheet metal, 1-inch air space: 50% reduction
• Brick with 1-inch air space: 50% reduction

For 36-inch standard clearance with 50% reduction: $C_r = 36 × 0.50 = 18$ inches minimum

Spacer Requirements Non-combustible spacers maintain air gap:

• Minimum 1-inch air space critical
• Use ceramic or metal spacers
• Open top and bottom for airflow

Floor Protection

Purpose

• Protects combustible flooring from radiant heat
• Catches embers during loading
• Prevents ember ignition of floor during ash removal

Material Requirements Non-combustible materials:

• Tile over cement board
• Stone or brick (1/2 inch minimum)
• Steel or iron plate
• Type R ember protection board
• Tempered glass (3/8 inch minimum)

Sizing Requirements

• Front: 16-18 inches beyond door opening
• Sides: 8 inches beyond stove footprint
• Rear: Flush with back of unit (unless rear-loading door)

R-Value Requirements Some listings specify thermal resistance:

• Type R = 1.0 or greater
• Achieved with cement board under tile
• Or double layer of 1/2 inch cement board

Chimney and Venting Systems

Chimney Sizing

Cross-Sectional Area Chimney must match stove flue collar size or specific listing:

• Undersized: Poor draft, smoking, creosote
• Oversized: Cool flue gases, low velocity, creosote

Height Requirements Minimum height for adequate draft:

• 12-15 feet for most installations
• 20-25 feet for very low draft locations
• Measure from stove flue collar to cap

Draft Calculation Theoretical draft pressure:

$$\Delta P = 0.0254 H \left(\frac{1}{T_o} - \frac{1}{T_f}\right)$$

Where:

• $\Delta P$ = draft pressure (inches w.c.)
• $H$ = chimney height (feet)
• $T_o$ = outdoor temperature (°R = °F + 460)
• $T_f$ = flue gas temperature (°R)

For 20-foot chimney, 0°F outdoor, 400°F flue gas: $\Delta P = 0.0254 × 20 × (1/460 - 1/860) = 0.0262$ inches w.c.

Target draft: 0.02-0.06 inches w.c.

Chimney Materials

Class A Chimney Listed factory-built double or triple wall:

• Tested to 2,100°F for 10 minutes plus 1,000°F continuous
• Zero clearance to combustibles (per listing)
• Stainless steel inner liner
• Use only same manufacturer’s components throughout

Masonry Chimney with Liner Clay tile or stainless steel liner required:

• Tile liner: 5/8 inch wall thickness minimum
• Stainless steel: 316Ti or 304L alloy, 0.015-0.018 inch wall
• Connect through thimble with listed connector
• Liner size matches stove collar

Stovepipe Single-wall connector from stove to chimney:

• 24-gauge steel minimum
• Maximum length: 10 feet (some jurisdictions)
• Maximum horizontal run: 75% of vertical chimney height
• Slope upward 1/4 inch per foot minimum
• 18-inch clearance to combustibles

Chimney Termination

Height Above Roof 3-2-10 rule:

• 3 feet above roof penetration
• 2 feet above any roof surface within 10 feet horizontally
• Top of chimney higher than ridge if within 10 feet

Cap Requirements

• Listed chimney cap required
• Screen: 1/2 to 3/4 inch mesh (prevents bird entry, retains large sparks)
• Rain cover maintains downdraft protection
• Clearance between cap and flue exit for proper draft

Efficiency Ratings and Heat Output

Efficiency Measurement

HHV vs. LHV Basis Wood stove efficiency typically reported on HHV (higher heating value) basis:

• HHV includes latent heat of water vapor
• LHV excludes latent heat (vapor not condensed)
• Wood HHV approximately 8,600 BTU/lb (dry basis, mixed species)

$$\eta_{HHV} = \frac{Q_{delivered}}{m_{fuel} \cdot HHV}$$

EPA-certified stoves:

• Catalytic: 72-85% (HHV)
• Non-catalytic: 63-78% (HHV)
• Pellet stoves: 70-83% (HHV)

Heat Output Control

Wood Stove Air Settings

• High burn: Maximum air, 50,000-100,000 BTU/hr
• Medium burn: Partial air, 25,000-50,000 BTU/hr
• Low burn: Minimum air, 10,000-25,000 BTU/hr

Output varies with fuel load size, wood species, and moisture content.

Pellet Stove Modulation Automatic fuel and air adjustment:

• Low setting: 8,000-15,000 BTU/hr
• Medium setting: 15,000-30,000 BTU/hr
• High setting: 30,000-50,000 BTU/hr

Programmable thermostats enable automatic operation and setback.

Safety Considerations

Creosote Fires

Ignition Risk Third-degree creosote ignites at 451°F:

• Flame temperatures: 2,000-2,800°F
• Duration: Minutes to hours
• Sound: Loud roaring, rumbling
• Visible: Flames or dense smoke from cap

Prevention

• Burn only seasoned wood (<20% moisture)
• Maintain hot, clean burns
• Annual chimney inspection
• Cleaning when creosote exceeds 1/8 inch thickness

Response

1. Close all air controls (starve oxygen)
2. Alert household, evacuate if necessary
3. Call fire department immediately
4. Monitor from safe distance
5. Do NOT use water (steam explosion risk in chimney)

Carbon Monoxide

Sources

• Backdrafting from building depressurization
• Leaking stovepipe or chimney connections
• Door gasket failure
• Improper installation

Protection

• Install CO detector on each floor
• Place within 15 feet of stove
• Test monthly
• Replace per manufacturer schedule (typically 5-7 years)

Operating Best Practices

Startup and Kindling

Top-Down Fire Reduces smoke and creosote during startup:

1. Layer large splits on bottom (3-5 pieces)
2. Add medium splits (criss-cross pattern)
3. Place kindling on top
4. Add newspaper or fire starter on very top
5. Light from top, burns downward

Advantages: Less smoke, better draft establishment, easier ignition.

Refueling Procedure

Safe Loading

1. Allow fire to burn to coal bed
2. Open air control fully
3. Wait 10-15 seconds for draft to increase
4. Crack door slowly (prevents smoke spillage)
5. Load wood to rear, leave front open for airflow
6. Close door, adjust air control to desired burn rate

Load size affects burn duration:

• Small load (1-2 splits): 1-2 hours
• Medium load (3-5 splits): 3-5 hours
• Full load: 6-10 hours (catalytic stoves)

Ash Management

Ash Depth Maintain 1-2 inch bed:

• Insulates floor of firebox
• Protects air inlet from warping
• Supports coal bed for sustained burning

Remove when depth exceeds 3-4 inches.

Safe Disposal

• Use metal container with tight lid
• Store outdoors on non-combustible surface
• Keep 10 feet from buildings
• Wait minimum 3-5 days before final disposal
• Ashes can remain hot for days after removal

Proper operation and maintenance of wood and pellet stoves provides efficient, renewable heat while meeting emission standards and maintaining safety.