Smouldering, sometimes called slumbering, occurs when a wood-burning stove is operated with little air, a low temperature, and a dull glow rather than a bright, lively flame. It feels like an efficient way to run a stove, being both cheap and convenient, especially overnight; however, it is one of the worst ways to operate such an appliance. Smouldering cuts heat output, wastes fuel energy, accelerates tar/creosote build‑up and dramatically raises harmful emissions such as PM2.5, black carbon, CO, VOCs and PAHs.
What smouldering really is
Wood combustion proceeds in phases:
1. Drying – free water is driven off.
2. Pyrolysis/devolatilisation – wood polymers (cellulose, hemicellulose, lignin) break down and release combustible gases.
3. Flaming combustion – those gases oxidise quickly in a hot, turbulent flame.
4. Char oxidation – the remaining carbon‑rich char burns off.
Clean burning needs all three T’s: Temperature, Turbulence (mixing with air) and Time. Smouldering starves one or more of these, most often air and temperature, so pyrolysis (decomposition by high temperature) gases and char are only partially oxidised. The result is a cooler fire that emits more carbon monoxide (CO) and organic vapours/tars that condense as fine particles (PM). Combustion completeness depends on temperature, mixing, and residence time; when combustion is incomplete, the fuel isn’t fully utilised, and emissions per unit of energy rise.
Fuel moisture is a key driver here. If the wood is above 20% moisture (considered wet or unseasoned), much of the fire’s heat goes into evaporating water, suppressing the temperature so the volatile gases can’t fully combust. Measured particle and CO emissions from moist wood are several times higher than from properly dried wood.
Scientifically, smouldering is a low‑temperature, flameless oxidation process. Compared to flaming, it tends to generate a higher fraction of incomplete-combustion products, including CO, polycyclic aromatic hydrocarbons (PAHs), and black carbon/organic aerosol, especially when oxygen is limited and surfaces are cool. Black carbon is a pollutant that results from the incomplete combustion of fossil fuels.
The wood-burning stove below is our Woodtec 8kW double-sided stove, showing a bright, lively flame, which all stoves and their owners love. Discover more about this stove here.
Why smouldering wastes usable heat:
Smouldering feels “slower”, but it is not efficient. Here is why:
Energy lost to water: wet fuel forces the fire to boil off moisture first, lowering flame temperature and slowing reactions that release heat.
Chemical energy left unburnt: with insufficient air/temperature, more of the fuel leaves as CO and unburned hydrocarbons instead of releasing their heat as CO2 and H2O in the firebox.
Weak draft: cooler flue gases reduce buoyancy (chimney “pull”), making air supply even weaker, a feedback loop that keeps the fire in the smouldering process.
Tar and creosote deposition: semi‑volatile tars condense on the cool flue surfaces, insulating the chimney and further reducing flow while storing fuel that can ignite in a chimney fire.
Smouldering wastes fuel and heat. Running the appliance hotter, within the maker’s guidance, with dry wood delivers more useful room heat.
Why wet fuel magnifies pollution
As mentioned, wet wood pushes the combustion back into a low-temperature, oxygen-starved process that emits several times more PM and CO than dry wood.
This science underpins the Air Quality (Domestic Solid Fuels Standards) (England) Regulations 2020, which prohibit the sale of small quantities of wet wood and traditional house coal, encouraging households to switch to “Ready to Burn” certified fuels.
Climate implications: smouldering and short‑lived climate forcers
Incomplete combustion increases emissions of black carbon and organic aerosol, and of gases (CO, NMVOCs, methane) that influence ozone and atmospheric chemistry. These are short‑lived climate forcers (SLCFs) that, although they don’t remain in the atmosphere like CO2, do add near‑term (over the next few decades) warming threats. SLCFs are compounds and particles that remain in the atmosphere for days to decades rather than centuries like CO2.
Defra’s own black‑carbon documentation highlights BC as a short‑lived climate forcer where emission control of such delivers health and climate gains together. Smouldering wood combustion is a known BC source; reducing smouldering is therefore climate‑positive as well as health‑protective.
Why Ecodesign stoves are a big improvement (and how they help you avoid smouldering)
What Ecodesign changes in practice:
Tighter emission limits: Ecodesign for solid‑fuel room heaters sets limits for PM, CO, organic gaseous compounds (OGC) and NOx
Higher efficiency: Ecodesign wood-burning stoves are typically 75% or more efficient, meaning more heat is retained in the room per log and less unburned material is emitted up the flue.
Combustion engineering: modern fireboxes utilise preheated secondary/tertiary air, effective air staging, baffles, and insulation to maintain higher flame temperatures and improve gas-air mixing. That makes it easier for everyday users to stay away from smouldering. Industry analyses comparing pre‑Ecodesign to Ecodesign appliances report 55% lower allowable PM and 88% lower CO in the standards framework. Those percentages describe reductions in the limits, not your personal results at home; your results depend on fuel quality and operation.
Important: An Ecodesign stove still pollutes if operated poorly. You must use dry, “Ready to Burn” wood and follow the maker’s air‑control guidance. But the design gives you a wider good‑combustion window, making clean burning more forgiving and consistent in everyday use.
How to avoid smouldering, and get the most from your stove:
Use dry fuel: Aim for 20% moisture or less; check with a moisture meter. Buy “Ready to Burn” certified logs or season your own for 1–2 years, split and stacked under cover.
Light and run hot: Use a top‑down method or approved firelighters. Let the stove reach operating temperature before trimming air.
Do not slumber: Avoid slumbering running unless your appliance is specifically designed for it.
Right appliance, right place: In SCAs, use an exempt Ecodesign stove; if buying new, Ecodesign is mandatory.
Maintain the system: Sweep regularly depending on use; service annually; keep door seals and air paths in good shape; fit and test a CO alarm.
Frequently asked technical questions
Isn’t a slow burn more economical?
No. It usually wastes energy by allowing fuel to escape as CO2 and unburned organics (chemical energy that never becomes room heat), and by lowering flue temperatures, resulting in weak draft. Running a hot, clean flame with the right amount of air is more efficient, especially in an Ecodesign stove.
What if I only have slightly damp wood?
A few percent moisture makes a big difference because it caps temperature in the firebox and shifts combustion toward smouldering.
Does cleaner burning help the climate?
Yes. Better combustion reduces black carbon and other short-lived climate forcers, which have near-term warming effects; cutting them delivers both climate and health benefits. It does not negate CO2 from biomass, but it improves the near‑term climate profile relative to high‑smoke operation.
A few per cent of moisture makes a big difference because it caps the temperature in the firebox and shifts combustion toward smouldering.
