Heating oil vs natural gas: which boiler saves more?

Heating oil or natural gas: the battle for energy savings in heating

Anyone thinking that switching from natural gas to heating oil will reduce their energy bill is in for a disappointment. Available data debunks this intuition: the cost per kWh of heating oil is around €0.068 compared to €0.047 for natural gas, and a good heating oil boiler barely reaches 94% efficiency, while a condensing gas boiler exceeds 108%. The difference is not negligible, and the initial investment does not favor heating oil either.

Energy costs: the numbers speak for themselves

The price per kWh of natural gas stands at €0.047, while heating oil reaches €0.068, according to verified sector estimates. This does not even account for the fact that heating oil has peaked at €0.09/kWh when crude oil prices surge. Additionally, a new heating oil boiler costs at least €3,000 in areas like Galicia, while condensing gas boilers range from €1,500-2,000 (only €300 more than a low NOx unit in 180 m² homes). The equation is clear: natural gas wins on both operating and acquisition costs.

Efficiency: the trick behind the 108%

How can a boiler achieve over 100% efficiency? The key lies in the lower heating value (LHV). Condensing boilers recover latent heat from combustion gases by condensing water vapor, achieving efficiencies of 108% based on LHV. This is not magic, but real thermal efficiency. For it to work, return water must be cold, ideally with underfloor heating at low temperatures (below 45 °C). With conventional radiators, savings are reduced, but still higher than any heating oil boiler, whose efficiency stays at 94% at best.

Alternatives: pellets, biomass, and heat pumps

Pellets emerge as a recurring option: €280 per pallet of 1,050 kg, equivalent in energy to about 525 liters of heating oil (2 kg of pellets = 1 liter of heating oil). The cost of pellets is around €0.22/kg, making it the cheapest fuel per kWh, although the boiler costs double or triple that of an oil one. Furthermore, it requires a silo and certified quality supply (DIN PLUS). In Andalusia, olive pits are subsidized up to 40% of the investment. Air/water heat pumps or geothermal systems offer efficiencies (COP) of 3 to 4, but with high investments (€7,000 or more). Geothermal energy, with very high installation costs, allows reducing bills from €3,000/month to €500 in high-consumption homes, but it is not for everyone.

Maintenance and fees: the hidden cost of gas

Natural gas has unavoidable fixed costs: subscription fee, annual checks (around €30), and mandatory inspections every 4-5 years costing €240-250. If contracted power exceeds a certain threshold, a maintenance contract is required. In total, you might pay €140-150 per year with little consumption. Heating oil, on the other hand, has no fixed fee, but requires its own maintenance (filter and burner cleaning), and the tank takes up space and generates pump noise. In the long run, the electronics of oil boilers tend to fail before the hydraulic circuit.

Prediction with reservations: today, natural gas with a condensing boiler remains the most balanced option in terms of cost, comfort, and efficiency for most households. However, regulatory pressure towards decarbonization and rising gas prices could tip the balance towards biomass or aerothermal systems in the coming years. Those who can afford a higher initial investment should study each case with their own data. General calculations do not always fit every home.

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