Summary

Kerosene and gas oil are both distillate petroleum fuels used for heating and machinery, but they have different physical properties that make them suitable for different burners and applications. The key difference is viscosity: kerosene (Class C2) is lighter and thinner; gas oil (Class D) is heavier and more viscous, burns with a yellower flame, and produces more soot if the burner is not correctly set up for it.

In domestic UK heating, virtually all oil-fired boilers and Aga/Rayburn type appliances are designed for kerosene. Gas oil is predominantly used in agriculture (for tractors, machinery), construction plant, and some industrial and commercial boilers. Using gas oil in a kerosene-rated burner will result in poor combustion, sooting, increased emissions, and potentially burner lockout.

The "Class C2" and "Class D" designations come from BS 2869:2017 (Fuel oils for agricultural, domestic, and industrial engines and boilers). These classifications define viscosity limits, sulphur content, pour point, flash point, and other parameters that determine burner suitability. OFTEC publishes guidance on fuel specifications alongside boiler commissioning requirements.

Key Facts

  • Kerosene — BS 2869 Class C2; 28-second viscosity; flash point ≥38°C; clear or light amber in colour; the standard domestic heating oil in the UK
  • Gas oil (red diesel) — BS 2869 Class D; 35-second viscosity; flash point ≥56°C; dyed red by HMRC regulations (duty-rebated fuel); heavier and more viscous
  • Class C2 kerosene — also marketed as "heating oil", "kero", or "boiler fuel"; sometimes slightly different formulations between brands but all within BS 2869 Class C2
  • Premium kerosene — additives (biocide, anti-wax, stabiliser); not a different class, just additive-enhanced; reduces diesel bug, prevents winter waxing and storage degradation
  • Diesel bug — microbial growth (Hormoconis resinae) in oil tanks; more common in kerosene than gas oil due to water retention; causes filter blockages and burner lockout
  • Flash point — kerosene (≥38°C) vs gas oil (≥56°C); affects storage and fire safety regulations (both require same separation distances under AD Part J in practice)
  • Pour point — kerosene: approximately −45°C; gas oil: approximately −15°C; means gas oil can wax (gel) and block filters in cold weather far more readily than kerosene
  • Viscosity impact — gas oil in a kerosene burner results in poor atomisation (oil spray droplets too large), yellow sooty flame, carbon deposits on the heat exchanger, and eventual burner lockout
  • Boiler approvals — manufacturers approve boilers for specific fuel classes; using unapproved fuel voids warranty and may invalidate insurance
  • OFTEC requirement — OFTEC engineers must record the fuel type on the commissioning certificate (OFTEC Form OFT105); if fuel type changes, re-commissioning is required
  • HVO (Hydrotreated Vegetable Oil) — third category emerging; compatible with some kerosene boilers per manufacturer guidance; see biofuel hvo oil boilers
  • Kerosene plus additive — some installers add Exocet or similar additive to standard kerosene; this does not change the fuel class, it enhances stability and reduces microbial risk
  • Delivery identification — kerosene arrives in an unmarked or silver tanker; gas oil (red diesel) arrives with red-dyed fuel — visually obvious if the wrong fuel is delivered

Quick Reference Table

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Property Kerosene (Class C2) Gas Oil (Class D)
BS 2869 Class C2 D
Common name Heating oil, kero Red diesel, gas oil
Viscosity (at 40°C) 28 seconds (≤2.0 cSt) 35 seconds (2.0–5.0 cSt)
Flash point ≥38°C ≥56°C
Pour point −45°C (approx) −15°C (approx)
Sulphur content ≤0.1% ≤0.1% (ULSD)
Colour Clear to light straw Red (HMRC duty marker)
Typical domestic use? Yes — standard domestic boiler fuel No — agricultural, plant, commercial
Typical burner type Pressure jet, vaporising Pressure jet (larger nozzle)
HMRC duty rate Full duty paid Duty-rebated (lower rate)
Storage degradation risk Higher (diesel bug) Lower
Cold weather performance Excellent Can wax below −15°C

Detailed Guidance

Why Viscosity Matters for Burners

Pressure jet burners (the most common type in domestic oil boilers) work by forcing oil through a small nozzle under pressure (typically 100–120 psi) to produce a fine mist (atomised spray) that mixes with air and ignites. The atomisation quality depends critically on fuel viscosity.

Kerosene at 28-second viscosity atomises easily, producing a fine white-yellow flame. Gas oil at 35-second viscosity does not atomise as well through a nozzle sized for kerosene — the droplets are larger, combustion is incomplete, and the flame burns yellow and sooty. Over time, soot deposits build up on the heat exchanger, combustion chamber, and flue, reducing efficiency and eventually causing lockout.

A burner approved for gas oil uses a larger nozzle size, higher air settings, and sometimes a different pump pressure to compensate for the higher viscosity. These settings are calibrated during commissioning with a flue gas analyser.

Bottom line: never put gas oil in a kerosene boiler or vice versa without first confirming the boiler manufacturer approves the fuel and, if so, re-commissioning with the correct burner settings.

Vaporising Burners (Aga, Rayburn, Esse)

Range cookers and traditional solid-fuel-effect appliances often use vaporising burners rather than pressure jet burners. Vaporising burners work by heating the oil until it vaporises and burns as a gas. These burners are particularly sensitive to fuel viscosity and quality.

All vaporising burners in domestic use in the UK are designed for Class C2 kerosene. Gas oil will not function correctly in these burners — it is too viscous to vaporise at the correct temperature, leading to incomplete combustion, sooting, and carbon deposit build-up on the vaporising pot.

If a customer has accidentally filled with gas oil and has a vaporising burner appliance, the system must be drained completely and flushed before refilling with kerosene. An OFTEC engineer should re-commission the appliance after any fuel change.

The Fuel Mix Scenario

A common on-site problem: a customer has mistakenly ordered gas oil, or a delivery driver has filled the wrong tank (rare but does happen). What to do:

  1. Do not light the boiler — if the fuel has not yet been burned, keep the system off
  2. Call an OFTEC engineer — the tank must be drained by a registered engineer using a licensed waste oil carrier
  3. Confirm via appearance — gas oil has a distinctive red colour due to HMRC duty marker; if the tank contents appear red, it is gas oil
  4. If already burning — if gas oil has been running in a kerosene boiler, a full service is required: nozzle replacement, heat exchanger inspection, flue inspection, and re-commissioning with flue gas analysis

In cases where the wrong fuel is discovered, document everything with photographs — this supports any insurance or delivery company complaint.

HMRC Regulations on Gas Oil (Red Diesel)

Gas oil is duty-rebated fuel — it attracts a lower rate of excise duty than road diesel or fully-duty-paid heating oil. The red dye is added by HMRC to identify its use. It is illegal to use gas oil as a road fuel (the red dye is detectable in DVSA roadside tests).

Domestic heating oil (kerosene) is fully duty-paid. There is no rebate, so there is no incentive to use gas oil as a cheaper domestic fuel — and any attempt to do so would involve using fuel in an unapproved appliance, which has technical consequences as described above.

Some agricultural properties have gas oil tanks (for farm machinery) alongside separate domestic heating oil tanks. Both tanks should be clearly labelled to prevent cross-contamination during deliveries.

Seasonal and Storage Considerations

Kerosene is a very stable fuel in cold UK winters. Its pour point of approximately −45°C means it will not wax or gel even in the coldest conditions experienced in mainland UK. However, kerosene is more prone to water absorption and diesel bug growth than gas oil, particularly when tanks are part-filled over summer. Premium kerosene with biocide additive is recommended for any tank that regularly sits partly empty between seasons.

Gas oil has a pour point of approximately −15°C, which means in harsh winters (Scottish Highlands, high ground in Wales, exposed parts of Northern Ireland), gas oil in tanks and pipes can partially gel. Agricultural users in cold areas often add a 'cold flow improver' additive to their gas oil supply in winter months. This is not a concern for domestic kerosene users.

Identifying Fuel Type on Site

When attending a service on an unfamiliar system:

  1. Check the tank label — should state "Kerosene" or "Heating Oil" or "Gas Oil"
  2. Check fuel colour via the filter bowl (if fitted with a transparent bowl) — kerosene is clear or very slightly amber; gas oil is red
  3. Check the OFTEC commissioning form (OFT105) — should state fuel type
  4. Check the boiler manual or data plate — will state approved fuel grades

If there is any doubt, take a fuel sample in a clear container and compare to known references, or use a fuel contamination test strip (available from OFTEC-approved suppliers).

Frequently Asked Questions

Can I use gas oil in my domestic heating boiler to save money?

No. Gas oil (red diesel) has a lower excise duty rate, but using it in a domestic heating boiler that is approved only for kerosene will cause combustion problems, sooting, and burner damage. The boiler warranty will be voided. Your OFTEC engineer will identify the wrong fuel at the next service. More fundamentally, gas oil in a domestic context is not a legal grey area — the duty rebate is for agricultural and industrial use only.

My old boiler seemed to run on gas oil for years without problem — was that okay?

Older pressure jet boilers with simple burners were sometimes operated on gas oil with the nozzle and air settings adjusted to compensate. This was not ideal (increased servicing intervals, more soot) but some installers in agricultural areas did this pragmatically. Modern high-efficiency condensing boilers are far less tolerant of incorrect fuel — the heat exchanger design, burner electronics, and flue requirements are all optimised for kerosene.

What is premium kerosene and should I use it?

Premium kerosene (brands like Emo Premium, Certas Firebird, Crown Premium) is standard Class C2 kerosene with added biocide (to suppress diesel bug growth), a wax stabiliser (to prevent cold-weather waxing near the pour point), and sometimes a combustion improver. It costs a few pence per litre more than standard kerosene. For tanks that regularly sit partly full over summer (the common condition for most domestic tanks), premium kerosene is worth the extra cost to reduce filter blockages and servicing issues.

Does the fuel type affect what I charge for a service?

Not directly, but fuel type affects what you find on site. A system running on standard kerosene that hasn't had a biocide additive may show diesel bug contamination in the filter and pump strainer — this adds time to the service. Premium kerosene tanks tend to be cleaner. If you find the wrong fuel type in a boiler on arrival, this is a chargeable intervention separate from the annual service.

Regulations & Standards

  • BS 2869:2017 — Fuel oils for agricultural, domestic and industrial engines and boilers. Specification; defines Class C2 (kerosene) and Class D (gas oil) properties

  • Building Regulations Approved Document J — combustion appliances requirements; fuel type is part of commissioning record

  • OFTEC Technical Book 3 — OFTEC guidance on oil fuel types, storage, and commissioning requirements

  • HMRC Excise Notice 75 — hydrocarbon oil reliefs; governs rebated fuel (gas oil) use restrictions

  • Energy Act 2004 — framework legislation covering domestic heating fuel regulation

  • OFTEC OFT105 — commissioning certificate; requires fuel type to be recorded by registered engineer

  • OFTEC: Fuel oil types and specifications — primary guidance on fuel classes

  • BSI: BS 2869 Fuel oils specification — standard for fuel oil classification

  • HMRC: Excise Notice 75 — hydrocarbon oil reliefs — rebated fuel rules

  • GOV.UK: Approved Document J — building regulations combustion requirements

  • oil boiler fault finding — diagnosing fuel-related faults including nozzle blockage and lockout

  • oil boiler service procedure — annual service steps including filter, nozzle, and combustion analysis

  • biofuel hvo oil boilers — HVO as a third fuel category compatible with some oil boilers

  • oil tank inspection maintenance — tank inspection covering fuel quality checks