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What is a Condensing Oil Boiler?

Adam Cherry
By: Adam Cherry
Updated: 7th October 2021

Oil boilers are the heating system of choice for off-grid homes.

And modern condensing oil boilers turn more fuel into heat than non-condensing models. So if you have an old non-condensing oil boiler, it’s time to get a replacement.

As of April 2007 all new oil boiler installations must be condensing.



What is a condensing oil boiler?

Condensing oil boilers work like any oil boiler. The oil is stored in a tank at the property and is as efficient as natural gas. When you need heating or hot water, oil travels from the tank to the boiler where it’s burned.

The key difference between condensing and non-condensing oil boilers is that they’re more efficient at turning oil into heat for your home.

They achieve efficiencies of over 90%. So they use almost all of the heat made when the oil is burned. Non-condensing boilers let much of the heat they make go straight out the flue pipe. Which isn’t good for the planet or your heating bills.

By holding onto more of the heat, you won’t need the boiler on for so long. And it will burn through less fuel. That means you’ll be paying less to heat your home and won’t have to fill up the oil storage tank so often.

Does my next oil boiler have to be condensing?

All new oil boiler installations must be condensing. And this has been the case since April 2007.
Blink condensing oil boiler
However, it’s still possible to install a non-condensing boiler if certain conditions are met.

For this to be possible, installing a condensing boiler in your home must be too difficult or expensive. To decide this, an inspection will need to be carried out at your property.

A number of factors will be considered, such as building type, whether you’re moving the boiler and if the flue can be fitted inline with regulations. A score will be given for each of these and if this score is over 1,000 then a non-condensing oil boiler can be installed.

If a condensing boiler can be installed then it is by far your best option in terms of efficiency and reliability.

Types of condensing oil boiler

There are three types of oil boiler: combi, system and regular. The same goes for gas boilers too.

Let’s see how they work:

Combi boilers

The combi boiler is the most recent type of oil boiler. They’re compact, cost-effective and heat your home and hot water on demand. That means there’s no need for any tanks or cylinders.

The mains water supply feeds straight into the boiler and is heated as and when you need it. So there’s no waiting around for the cylinder to fill up with warm water again (a problem with system and regular boilers). The water is then pumped from the boiler to the radiator or taps.

This can help to cut your heating costs by quite a chunk.

System boilers

System boilers take their water supply directly from the mains (like a combi) but store hot water in a cylinder. This is great for homes with 2+ bathrooms and a high demand for hot water.

Regular boilers

Regular boilers go by many names. Conventional, open-vented and heat-only for starters.

They’re the oldest type of boiler and most often found in homes built in the 1950s and 60s. But while they have been around longer than combi and system boilers, modern models are reliable and efficient.

However, you should only think of installing a regular oil boiler as a direct replacement for an old one. So if you have a combi or system boiler, rule out a regular boiler. This is because you’ll need a hot water cylinder and tanks in the loft as well as the boiler iself. So a large pipe network is needed to connect them all up.

When you turn the heating on, cold water is fed down to the boiler from a tank in the loft. The boiler then heats up the water before sending it round to the radiators.

Which type of oil boiler is right for your home?

Large homes with 2 or more bathrooms will need a system or regular boiler. They far better at meeting high demand for hot water than a combi as they store hot water in a cylinder. If you have a regular boiler then you may want to move to a system boiler as you won’t need the tanks in the loft.

If you live in a smaller property with 1 bathroom then a combi boiler is perfect. They heat water on demand so you’ll have hot water available at all times and won’t need a cylinder.



Condensing oil boiler costs

A new condensing oil boiler can cost anywhere between £1,000 and £4,500. Before you add on the installation costs – around £500-£2,000.

The total you end up paying will depend on the manufacturer, model and output rating. As well as whether it’s designed for internal or external installation.

You may also need a new oil storage tank. They need replacing every 10-20 years and cost between £1,00 and £2,000.

Best condensing oil boilers

The best condensing oil boilers are made by:

  • Firebird
  • Grant
  • Mistral
  • Navien
  • Viessmann
  • Warmflow
  • Worcester Bosch

Between them, they make over 500 oil boilers. So you have plenty of choice.

Let’s take a look at the best models:

Combi boilers

Internal combi boilers

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen HE 93% 2 years
Grant Vortex Blue 90.81-94.56% 10 years
Mistral Combi Standard 92% 2 years
Navien LCB700 93% 10 years
Worcester Greenstar Heatslave II 90-91% 5 years

External combi boilers

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen Combipac HE 93% 2 years
Grant Vortex Blue 90.81-94.5%% 10 years
Mistral Outdoor Combi Standard 92% 2 years
Navien LCB700 93% 10 years
Worcester Greenstar Heatslave II 90% 5 years

Internal system oil boiler

Internal system boilers

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen System Internal 92.5-93.8% 2 years
Grant VortexBlue Kitchen/Utility 90.8-91.7%% 2 years
Mistral Sealed System 92% 2 years
Navien LCB700 Internal 93% 10 years
Viessmann Vitorondens 200-T 97% 7 years
Worcester Greenstar Danesmoor 90-91% 5 years

External system oil boilers

While the system boiler itself can be fitted outside, the cylinder will need to be inside your property.

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen Systempac 92.5-93.8% 2 years
Grant Vortex Eco 90.3%% 5 years
Mistral Outdoor Sealed System 92% 2 years
Navien LCB700 External 93% 10 years
Worcester Greenstar Danesmoor External 92% 5 years

Regular boilers

Internal regular oil boilers

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen Kitchen 91.6-92.6% 2 years
Grant VortexBlue 94.5%% 10 years
Mistral Kitchen/Utility Classic 92% 2 years
Navien LCB700 Internal 93% 10 years
Worcester Greenstar Danesmoor 90% 2 years

External regular oil boilers

These regular boilers can all be installed outside in a garden area. However, the hot water cylinder and cold water tank must still be within the property.

Oil boiler Efficiency Max Warranty/Guarantee
Firebird Envirogreen Heatpac External 92.6% 2 years
Grant VortexBlue External 94.5%% 10 years
Navien LCB700 External 93% 10 years
Worcester Greenstar Danesmoor External 90% 2 years

Compare condensing oil boilers with our Boiler Comparison Tool.

Why you need a condensing oil boiler

If you have an old non-condensing boiler then we recommend upgrading to a condensing boiler.

Not only do boilers lose their efficiency over time but non-condensing boilers are outdated. You’ll have a more reliable heating system, start saving money on your bills and reduce your carbon footprint. What’s not to like?

More plus sides to condensing oil boilers include:


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Disadvantages of oil boilers

Before you turn to an oil boiler, there are a few things to think about:

  • Oil prices are slowly rising
  • Leaks from tanks and piping are an environmental concern
  • There’s a good chance that you’ll have to invest in another fuel for cooking
  • Oil installers don’t have to be OFTEC registered, so there is no governing body protecting property owners

Alternatives to heating oil

Oil is the most popular way to heat homes that aren’t connected to the gas network. But they’re not you’re only option. So you may want to consider the alternatives to an oil heating system.

You could turn to LPG. A type of gas that’s stored in a tank – much like oil. That way, you’d still have a boiler so wouldn’t need to get used to a new type of heating system. However, LPG is a little more expensive than oil.

If you’re keen to do your bit for the environment then a renewable heating system is worth thinking about.

Heat pumps are a renewable heating system that take heat from the air or ground. Depending on whether you turn to an air source or ground source heat pump. And if you’re interested in a heat pump, you could pair it with your oil boiler rather than going all in. This will take some of the workload off your boiler and could cut your heating bills.

You could also pair your oil boiler with solar thermal panels. These panels absorb heat from the sun and use it to warm the water in a cylinder. Saving your boiler a job.

Adam Cherry

About the author

Adam Cherry

Adam is our resident home heating expert. His experience and advice has helped millions of customers improve the efficiency of their homes and save money.

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