Wood stove glossary

Short, direct answers to the terms that come up when choosing and installing a wood or pellet stove.

Ash content: Ash content is the incombustible mineral fraction left after fuel burns, quoted as a percentage by weight. For pellets it defines the fuel grade (under 1% for Premium); for cord wood it is why a stove needs its ash pan emptied every few days in season.
BTU (British thermal unit): A BTU is the energy needed to raise one pound of water 1 °F; stove heat output is rated in BTU per hour. Certified stoves publish an output range — roughly 10,000–20,000 BTU/h at low burn up to 30,000–80,000 BTU/h at peak, with about 25–35 BTU/h per square foot as a sizing rule of thumb.
Catalytic combustor: A catalytic combustor is a ceramic or metal honeycomb coated with precious metals that lets smoke ignite at around 500 °F instead of over 1,000 °F. Routing exhaust through it re-burns smoke that would otherwise leave the chimney, cutting emissions and extracting more heat — especially at low, overnight burn rates.
Clearance to combustibles: Clearance to combustibles is the minimum distance a stove and its pipe must keep from anything that can burn — walls, furniture, framing. Each listed stove specifies its own tested clearances; unlisted installations fall back to NFPA 211's default of 36 inches for a radiant stove.
Cord wood vs crib wood test fuel: Crib wood is a standardized lab fuel — dimensional lumber nailed into open cribs — used in the traditional EPA test; cord wood testing burns real split logs, closer to how people actually use stoves. The 2020 NSPS allows either: 2.0 g/hr crib-tested or 2.5 g/hr cord-wood-tested.
Emission rate (g/hr): The emission rate is the grams of particulate matter (smoke) a stove releases per hour of operation, measured over a standardized EPA lab test. The 2020 NSPS caps new room heaters at 2.0 g/hr (2.5 g/hr when tested with cord wood); the cleanest certified stoves test below 1 g/hr.
EPA certification number: The EPA certification number identifies a wood heater's certificate of compliance with the NSPS emission limits. It appears on the stove's permanent label and in the EPA Certified Wood Heater Database, and it covers a specific list of model numbers tested as one design.
Firebox volume: Firebox volume, in cubic feet, is the size of the chamber that holds the wood — the best single proxy for a stove's size class. Under 1.5 cu ft is a small/cabin stove, around 2 cu ft heats a mid-size home, and 3+ cu ft is the large-home, overnight-burn class.
Forced-air furnace (wood): A wood forced-air furnace is a central heating appliance that burns cord wood to heat air and pushes it through the home's ductwork with a blower — the wood-fired equivalent of a conventional furnace, usually installed in a basement alongside or instead of a gas or oil unit.
Hearth pad: A hearth pad is the noncombustible floor protection under and around a wood stove — stone, tile, or a listed board — that guards the floor against embers and radiant heat. The stove's listing specifies the required size and thermal rating (ember protection only, or a specific R-value).
HHV efficiency: HHV (higher heating value) efficiency is the share of the total energy in the wood — including the energy lost evaporating its moisture — that a stove delivers as useful heat. It is the conservative efficiency measure and the one the EPA database certifies; certified stoves typically run 60–85% HHV.
Hydronic heater (wood boiler): A hydronic heater is a wood- or pellet-fired appliance that heats water rather than air, distributing heat through radiators, radiant floor loops, or a heat exchanger in a furnace plenum. They include outdoor wood boilers, indoor cord-wood gasification boilers, and automatic pellet boilers.
NSPS 2020 rule (Step 2): The NSPS (New Source Performance Standards) for residential wood heaters is the EPA regulation capping particulate emissions from new stoves. Its "Step 2" limits, in force since May 2020, cap room heaters at 2.0 g/hr (2.5 g/hr cord-wood tested) — only compliant models may be manufactured or sold new in the US.
Pellet fuel grades: Wood pellets are graded chiefly by ash content and consistency: PFI Premium pellets run under 1% ash, Standard up to 2%, and Utility up to 6%. Pellet stove warranties and clean operation generally assume Premium-grade fuel.
Seasoned wood (moisture content): Seasoned wood is firewood dried to below roughly 20% moisture content — the threshold at which a modern stove can burn it cleanly and efficiently. Fresh-cut ("green") wood is 35–60% water, and burning it wastes heat boiling water while producing smoke and creosote.
Secondary burn (secondary air): Secondary burn is the re-ignition of smoke inside the firebox using preheated air injected through tubes or a baffle above the fire. It is how non-catalytic EPA-certified stoves achieve low emissions — the visible "light show" of flames rolling along the top of the firebox.
Stove pipe vs Class A chimney: Stove pipe (chimney connector) is single- or double-wall pipe that connects the stove to the chimney and may only run through open room space; Class A chimney is insulated, listed high-temperature pipe that carries the flue through ceilings, attics, and roofs. They are different products with different clearances and are not interchangeable.
Turndown ratio: Turndown ratio is the span between a heater's maximum and minimum steady output — a stove rated 12,000–48,000 BTU/h has a 4:1 turndown. A wide turndown lets one stove burn cleanly in both mild October weather and a January cold snap.