--- title: Kerosene Grades Explained: 1-K vs 2-K, Dyed Fuel and What Burns Clean Indoors url: https://upfuel.online/blog/kerosene-grades-1k-vs-2k topic: Kerosene author: Marcus Hale (Contributing gas engineer) published: 2026-05-06 updated: 2026-08-02 source: UpFuel — https://upfuel.online --- # Kerosene Grades Explained: 1-K vs 2-K, Dyed Fuel and What Burns Clean Indoors The sulphur difference between 1-K and 2-K kerosene, why dyed fuel is a tax marker, and how to store fuel so a wick heater burns clean. ## Summary 1-K caps sulphur at 0.04% and is the only grade for unvented indoor appliances. 2-K allows 0.30% — seven times more — and burning it indoors puts sulphur dioxide in the room with you. ## Key takeaways - 1-K: max 0.04% sulphur, water-white, correct for indoor wick and radiant heaters. - 2-K: max 0.30% sulphur, for vented and outdoor equipment only. - Dye is a tax marker; clear pump kerosene is the safe default for indoor use. - Never substitute diesel, petrol, camping fuel or heating oil in a wick heater. - Kerosene absorbs atmospheric water — buy what you will use in one season and store sealed, opaque and cool. Two identical heaters, two owners. One burns with a clean blue crown for six seasons. The other smells, sputters, and needs its wick scraped every few weeks, and the owner concludes they bought a bad heater. Almost every time, it is the fuel. Usually it is the grade, sometimes it is water that has crept in over a summer in a half-full container — and both are easy to avoid once you know what you are looking at. ## The grades Kerosene sold for heating is graded under ASTM D3699, which sets limits on sulphur, flash point and other properties. | Property | 1-K | 2-K | | --- | --- | --- | | Max sulphur (wt%) | 0.04 | 0.30 | | Typical appearance | Clear, water-white | Clear or dyed | | Intended appliances | Unvented indoor heaters, wick lamps | Vented equipment, outdoor use | | Suitable indoors | Yes | **No** | That sulphur ratio — seven and a half to one — is the entire story. Sulphur in the fuel becomes sulphur dioxide in the exhaust. In a flued appliance it leaves through the flue. In an unvented heater it stays in the room with you. Sulphur dioxide is an upper-airway irritant at low concentrations and a genuine trigger for asthmatic bronchoconstriction. It is also what makes a badly fuelled kerosene heater smell "sharp" rather than faintly oily. ## Clear vs dyed: a tax question that became a safety heuristic In the US, red dye marks fuel sold without road tax, for off-road and heating use. In the UK, "red diesel" plays the same role. The dye is chemically irrelevant to combustion. But the practical correlation is strong: dyed kerosene at a bulk pump is usually 2-K, sold for outdoor equipment and vented heating systems, while the clear kerosene sold in sealed containers at hardware stores is 1-K. Using clear as your rule is not chemistry, but it is a reliable proxy — and it has a second benefit: with clear fuel you can *see* contamination. Cloudiness means water. Straw colour means age. A rust tinge means the container or tank is failing. Dye hides all three. ## What never goes in a wick heater - **Petrol/gasoline.** Flash point around −43°C against kerosene's +38°C. A wick heater is not a container for a fuel that produces an ignitable vapour at room temperature. This is the single most common cause of fatal kerosene heater fires. - **Diesel or heating oil.** Too viscous for the wick to draw properly. Result is a starved, sooty flame, heavy carbon on the wick and an appliance you will be cleaning constantly. - **Camping fuel / white gas / naphtha.** Formulated for pressurised stoves, far too volatile for a wick. - **Used motor oil, lamp oil blends, citronella oil.** Additives and metals that combust into things you should not inhale. - **Jet fuel (Jet A / A-1).** Chemically close to 1-K but carries anti-icing and static-dissipating additives not intended for indoor combustion. If the container does not say kerosene, it does not go in the heater. ## Buying and storing **Buy for the season, not for the decade.** Kerosene is hygroscopic. Water enters via the vapour space every time the container breathes with temperature swings, then settles under the fuel where microbes live at the interface. A season-old jug is usually fine; three-year-old fuel in a partly filled container usually is not. **Container rules:** - Use an approved blue container. Blue means kerosene in the US colour convention (red is petrol, yellow is diesel). Colour discipline in a shed prevents the mistake that starts fires. - Keep it sealed, opaque and cool, out of direct sun, away from ignition sources and out of living space. - Fill containers on the ground, never in a truck bed or on a plastic liner — static discharge is a real ignition path. - Store no more fuel than local fire code permits inside a domestic building; many jurisdictions cap this at a few gallons. **Judging fuel by eye:** good 1-K is water-clear with no cloud, no sediment when swirled, and smells faintly of paraffin rather than sour or sharp. If you can see a distinct layer at the bottom of a clear container, that is water — do not use it. ## What bad fuel does to a heater The failure chain is consistent: 1. Water or heavy fractions in the fuel disrupt the wick's capillary draw. 2. The flame runs uneven and yellow rather than a stable blue crown. 3. Incomplete combustion deposits carbon on the wick's upper edge. 4. The carbonised wick draws even less fuel; the flame degrades further. 5. Combustion product output — odour, CO, particulates — rises steadily in the room. Owners often read step 5 as "the heater is getting old". The heater is usually fine. Perform the dry-burn cycle the manual describes (outdoors), trim or replace the wick, drain and dispose of old fuel properly, and refill with fresh clear 1-K. Most "worn out" wick heaters come back with that treatment. ## Disposal Old kerosene is hazardous waste in most jurisdictions. Do not pour it down a drain, onto ground, or into household waste. Municipal household hazardous waste sites accept it, and many fuel merchants will take back small quantities. Never use degraded kerosene as a fire accelerant — a slightly cheaper disposal route is not worth the fire it starts. ## The one-line rule For anything burning inside your home without a flue: **clear 1-K, bought this season, stored sealed**. Every other kerosene grade and substitute belongs to outdoor or vented equipment. Grade matters most in an unvented appliance, which is why it sits alongside [ventilation and CO alarms](/blog/are-kerosene-heaters-safe-indoors) in the safety picture. For the wider question of what kerosene actually is, see [is kerosene a fossil fuel](/blog/is-kerosene-a-fossil-fuel). ## FAQ ### Can I burn 2-K kerosene in an indoor heater? No. 2-K permits up to 0.30% sulphur against 1-K's 0.04%. Burned in an unvented heater, that sulphur becomes sulphur dioxide in the room you are sitting in — an airway irritant, particularly for anyone with asthma — and it accelerates corrosion and wick carbonisation inside the heater. ### Is red-dyed kerosene bad for my heater? The dye itself is a tax marker for off-road fuel and does not harm combustion. The problem is what it usually accompanies: dyed kerosene is typically 2-K grade, and the dye can stain wicks and make it harder to judge fuel cleanliness by eye. For an indoor heater, buy clear 1-K. ### Can I use diesel in a kerosene heater? No. Diesel is heavier, has a higher flash point and a different viscosity; a wick designed for kerosene will not draw it correctly, producing a smoky, sooty, under-powered flame and heavy carbon build-up. Petrol is far worse — it will not merely burn badly, it will burn explosively. ### How long does kerosene last in storage? Treat it as one heating season. Kerosene is hygroscopic and slowly picks up water from the air, and stored fuel supports microbial growth at the fuel-water interface. Symptoms of old fuel are a cloudy appearance, a sour smell, sputtering flames and repeated wick fouling. --- Safety note: indoor fuel burning carries carbon monoxide, fire and air-quality risks. Follow your appliance manual and local regulations, fit a certified CO alarm, and use a qualified engineer for installations.