Kerosene vs gas oil: which does my boiler use?

What do 28-second and 35-second mean?

The numbers describe how thick the oil is. They come from an old lab test, the Redwood viscometer, which times how long a set amount of oil takes to flow through a small hole. A longer time means a thicker oil. Wärtsilä's glossary of Redwood seconds explains the measure.

So 28-second oil (kerosene) is thinner and lighter. 35-second oil (gas oil) is heavier. Oil suppliers still use the old names on their order forms and dockets, so it helps to know both.

Revenue says the markers added to marked gas oil give it a green colour and those in marked kerosene give a red colour (Revenue: fuel marker). In a clear sight gauge kerosene can look much paler than that, so do not judge your fuel by colour alone. Certa, one of the larger Irish oil suppliers, says most new and modern home heating systems in Ireland use kerosene.

Which oil does my boiler use?

Check in this order. Stop as soon as you have a clear answer.

1. Look at the boiler data plate. Most oil boilers have a label inside the front cover or on the casing with the model number and fuel. Turn the boiler off at the switch before you take a cover off, and do not open the burner itself.
2. Read the manual. If you have the model number, the maker's manual will say which fuel the burner is set up for.
3. Check the tank. Some tanks have a fuel sticker near the fill point, and Irish oil suppliers such as Certa ask you to look for one before ordering.
4. Find an old delivery docket or invoice. It names the fuel delivered. If you have just moved in, ask the previous owner or the estate agent which oil supplier they used, then ask that oil supplier what they delivered.
5. Ask the technician who services the boiler. They set the burner up for a fuel and can tell you which one.

If the answers disagree, or there is nothing to go on, ask an OFTEC-registered technician to check the burner before you order. It costs far less than fixing a boiler after the wrong delivery.

Why you must not mix them up

An oil burner is set up for one fuel. The pump pressure, nozzle and air setting all suit a certain thickness of oil. Put gas oil into a burner set for kerosene and it can burn poorly, soot up the boiler or lock out. Kerosene in a gas-oil burner is also wrong for the settings. A small top-up of the wrong fuel still mixes into the whole tank.

If you think the wrong oil has been delivered:

• Leave the boiler or range switched off.
• Phone the oil supplier straight away and keep your docket.
• Get an OFTEC-registered technician to look at the burner and tank before you use it again.

If you smell fumes or have any sign of carbon monoxide, such as headaches or dizziness that ease when you go outside, get everyone out into fresh air and call 112 or 999. Our guide to a kerosene smell in the house covers what to do next.

Can a boiler be converted from gas oil to kerosene?

Only a qualified technician can tell you, because it depends on the boiler and burner. A change of fuel means a change of burner set-up, so ask your OFTEC-registered technician whether your model can be changed and what it would involve. Do not switch fuel at your next delivery and hope the boiler copes.

Aga and range cookers

Oil-fired range cookers, such as Aga and Stanley models, have their own burners, set up for one fuel just like a boiler. Do not assume yours takes the same oil as a neighbour's. Check the cooker's data plate, which is often behind the lower door or on the back panel, or look up the model number with the maker. If your range and boiler share one tank, they must both use the same fuel. If you are not sure, ask the technician who services the range.

Is kerosene cheaper than gas oil?

Tankful tracks kerosene prices from Irish oil suppliers. The typical delivered price for 500 litres of kerosene is €828.50 (prices checked 12 October 2026). Prices vary by county and oil supplier, so compare oil suppliers near you before you order.

We do not track gas oil prices, so we will not quote one here. Gas oil and kerosene are taxed and priced differently, and the gap moves. If your boiler uses gas oil, ask two or three local oil suppliers for a delivered price for the same amount. Price is not a reason to switch fuel. Your boiler decides which oil you buy.

Tax and the legal use of green diesel

Revenue describes marked gas oil as diesel on which Mineral Oil Tax at a reduced rate has been paid, on condition it is used only for purposes that qualify for that rate. It is chemically marked and dyed green to deter misuse (Revenue: what is marked gas oil?). Revenue's manual on marking gas oil and kerosene lists home heating among the qualifying uses.

Using marked gas oil or marked kerosene in a road vehicle is illegal, Revenue says. Do not keep it in cans for a car or van, and do not sell or give heating oil to anyone for road use. If you have farm or business questions about reduced-rate fuel, check with Revenue directly, because the rules depend on the use.

Before you place the order

• Confirm the fuel from the boiler plate, docket or your technician.
• Write it down near the tank so the next person knows.
• Say “kerosene” or “gas oil” clearly when you order, and check the docket when the driver leaves.
• Check how much room is in the tank. Our guide to checking your oil tank level shows how.

Common questions

Next step

Once you know your fuel, compare heating oil prices from local oil suppliers, or read how to order heating oil. New to oil heating? Start with our guide to moving into a house with oil heating.