When it comes to heating your home or business, fuel quality and compatibility are often overlooked in favour of convenience or cost. But here’s a question that deserves far more attention than it usually gets:
Can you mix different types of heating oil?
At first glance, the answer might seem obvious or even trivial. But if you peel back the layers, you’ll find that mixing heating oils is a topic fraught with misunderstandings, half-truths, and costly assumptions.
Let’s strip away the noise and get to the facts—so you know what’s safe, what’s risky, and what’s just not worth it.
The Types of Heating Oil: Not All Fuels Are Created Equal
In the UK, three major types of heating oil dominate the market:
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Kerosene (Class C2) – The go-to for domestic boilers and household heating. It’s clean-burning and efficient.
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Gas Oil (Red Diesel) – Used primarily in agriculture, construction, and older commercial heating systems.
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HVO (Hydrotreated Vegetable Oil) – A relatively new, renewable, synthetic fuel designed as a cleaner alternative to fossil-based oils.
Now, while they may all fall under the umbrella of “heating oil,” they’re chemically and structurally different—almost like trying to use cooking oil in a car engine.
Why People Try to Mix Heating Oils
There are three main reasons people are tempted to mix heating oils:
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Leftover Fuel: A supplier drops off kerosene, but there’s still some gas oil sitting in the tank.
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Emergency Top-Up: In a heating crisis, homeowners use whatever oil they can get their hands on.
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Cost-Cutting: Some think mixing a bit of cheaper fuel will stretch their budget further.
While the logic may sound practical, the reality is often expensive and irreversible.
The Scientific (and Legal) Reason You Should Think Twice
Mixing heating oils isn’t like blending fruit juices. These fuels are refined differently, have distinct burn characteristics, and are designed for very specific systems.
Key Risks:
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Combustion Imbalance
Kerosene burns at a different temperature than gas oil. Mixing them can result in poor combustion, carbon build-up, and soot accumulation—none of which your boiler will thank you for. -
Equipment Damage
Most heating systems are calibrated to handle a particular viscosity and chemical profile. Using the wrong mix could damage injectors, filters, pumps, and even the heat exchanger. -
Legal Headache
In the UK, red diesel (gas oil) is taxed at a lower rate and dyed red to discourage misuse. Mixing it into a domestic heating tank could be interpreted as tax fraud, with severe penalties—even if done unintentionally. -
Insurance Fallout
If your boiler fails and the insurer finds an unapproved fuel mix, you could be left footing the entire repair or replacement bill.
What About Mixing Kerosene with Kerosene?
If you’re talking about mixing standard kerosene with premium kerosene, you’re on safer ground.
Premium fuels contain added detergents and stabilisers to reduce smoke and improve efficiency. Mixing these two doesn’t damage your boiler, but the benefits of the additives might get diluted. Still, it won’t void warranties or cause long-term issues.
That said, always keep in mind:
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Old kerosene degrades over time. If your tank has months-old fuel, blending fresh oil can stir up sediment.
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If you’re changing suppliers, ask about compatibility. Not all additives play nice together.
HVO: The Curveball in the Room
HVO has emerged as a sustainable, cleaner option for homes looking to cut carbon emissions. But it’s not plug-and-play.
Mixing HVO with kerosene without proper boiler conversion is like trying to charge an electric car with petrol. It just doesn’t work.
Here’s Why:
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HVO is synthetic and behaves differently under combustion.
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The burner settings for kerosene won’t align with HVO flow rates or atomisation.
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Even partial mixing can throw off flame stability, reduce heat output, and trigger system lockouts.
Unless your boiler is HVO-compatible—and you’ve had a technician adjust your setup—do not mix HVO with any fossil-based oil.
Real-World Example: What Happens When You Mix the Wrong Oils?
A homeowner in rural Lincolnshire tried to stretch leftover red diesel with kerosene during a cold snap. The boiler initially worked… then failed. The technician discovered sludge in the fuel line and carbon deposits in the burner head. The fix? A full system flush and nozzle replacement—costing over £700.
That’s more than a full tank of clean kerosene would’ve cost in the first place.
Best Practices If You’re Considering a Switch or Mix
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Empty your tank before introducing a new type of heating oil.
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Get professional advice before using alternative fuels.
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Schedule regular maintenance to detect any compatibility issues early.
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Keep documentation of any fuel changes—especially if switching to HVO.
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Only buy from reputable suppliers who provide certified fuels.
Final Thoughts: Mixing Fuels Is a Gamble You Don’t Want to Take
When it comes to heating oil, compatibility isn’t optional—it’s essential. While some combinations may not result in immediate disaster, most mixtures come with consequences that creep up quietly: increased soot, inefficiency, breakdowns, and invalidated warranties.
So, can you mix different types of heating oil?
Yes—but that doesn’t mean you should.
When in doubt, treat your heating system like your car: use the fuel it was built for, and it will serve you well for years to come.
