How Does Oil Central Heating Work?

April 26, 2017 | Heating Oil Advice

Why do people use central heating oil?

There is one reason above any other that central heating is fuelled by oil. The property has no connection to mains gas.

Most homes in the UK use gas fired boilers. However, there are around 4 million homes in the UK where a property isn’t connected to the mains gas, of which 1.5 million heat their homes using oil as the preferred option.

There is another (but, less common reason) that a property is fuelled by oil.

In some cases, areas have had mains gas installed in recent years, but the property has not been converted and continues to be fuelled by oil.

The cost of a new gas boiler installation outweighs any savings that are going to be made.

How does oil central heating work?

The operation of an oil-fired boiler is much the same as a gas-fired boiler. That means that the same heating and boiler problems will occur regardless of whether a gas or oil boiler is being used.

a boiler
The operation of an oil-fired boiler is much the same as a gas-fired boiler.

The key difference between oil and gas boilers is that homes using central heating oil will need to plan further in advance. The availability of gas is almost instant, so if a home needs to be heated quickly then it is easily achieved.

Heating oil, however, needs an arranged delivery so it can be stored in an oil tank. Only then is it available for home use. So if an order is not placed in advance of running out, then there could be a few days of colder temperatures inside whilst waiting for the delivery.

 Here’s more detail on how each of the oil boiler elements work:

Hot water storage

The main difference is that gas fired combination boilers have no storage facility (they heat water on demand). Many newer combination oil boilers will have a small built in storage tank to ensure hot water and heating needs are met.

Firing your oil boiler

When you switch on your heating, ignition leads fire up the boiler. At the same time, a valve will open to allow oil into the boiler. The oil then burns to heat up the water that is flowing around the system.

Flow and return

BoilerMax heating additives
BoilerMax heating additives

The water flows on a circuit, passing through radiators and towel rails. The water coming out of the boiler is called the “flow”. Once it passes through the circuit, it re-enters the boiler (the “return”). After just a few minutes, the water on the return pipe should be hot. If the return is not close in temperature to the flow after 10-20 minutes, there’s likely to be something hindering the flow of water in your heating system which is usually linked to radiator sludge.

If the sludge is sat within the oil tank itself, heating oil additives can help produce a cleaner burn.

How a boiler heating pump works

The key element to the flow of water around this “circuit” is the heating pump. This can be found behind the boiler casing. This will pump the hot water around the heating circuit in your property.

Many older boilers used to use Wilo pumps. Nowadays, most manufacturers will use Grundfos pumps.

Remember, less energy is required to keep this circuit warm, than is needed to initially get it to temperature. Therefore, turning oil boilers on for short periods (multiple times a day) is likely to be more expensive than just leaving it on a low temperature constantly.

Combination boilers and diverter valves

An important part of the puzzle for all combination boilers (gas, oil and LPG) is the diverter valve. The diverter valve diverts water to wherever you ask it to.

When you fire up your central heating, the diverter valve will push all hot water around pipe work, radiators and towel rails.

Remember, a combination boiler will always give preference to hot water over your heating.

So, when you turn on a tap or shower, the diverter valve will flip to send the heated water there rather than the heating. Once you turn it off, it will flip back into the heating position, sending hot water back around the pipe work, radiators and towel rails in your property.

Therefore, on heating systems using older boilers (or boilers that are not producing enough KW of heating output), you might physically feel a dip in your property’s temperature if you have a long shower, or run a bath.

Storing heating oil in a tank

Heating Oil tank
Heating Oil tank

The supply of oil is not the same as gas.

If you’re looking to buy a property that has an oil-fired boiler, you are likely to notice something slightly unusual.

A huge heating oil tank. It’s going to look something like this.

Unlike mains gas that is provided “on tap”, oil needs to be stored. Storage tanks vary in size, but they’ll usually hold enough oil to fuel a property for 6-9 months.

There are several benefits to this. The UK average heating oil price fluctuates throughout the year, so the wonderful thing about heating oil is that you can buy it in bulk when the price suits you.

Consumers could save, but on the flip side you could find yourself buying when oil prices are high, so it’s worth keeping up to date.


Before purchasing a property that has oil central heating, consider potential access routes for large vehicles (or ask the current owner how oil is delivered).

Heating oil is delivered in tankers. So, there must be reasonable access for tankers to top up your storage tank.

When ordering your heating oil, carefully select the right tanker size for you. The available options include:

  • Standard tanker
  • 4 wheeler
  • Baby tanker

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