Dual Fuel Boilers – Gas and Oil Fired

Dual Fuel Boilers – Gas and Oil Fired

duel fuel boilers natural gas and oil fired for hot water systems from Flexiheat UK

Dual Fuel Boilers – Natural gas or propane gas (LPG) and oil fired

Dual fuel boilers can keep a critical heating system operating, such as a hospital, for example, even if the primary fuel supply fails or is unavailable. They are often referred to as “dual fuel heating system”.

When it comes to hospitals and industrial operations or processes, where boilers provide heating hot water and/or domestic hot water, any loss of these vital processes can have a big effect on the health of patients and the bottom line of a company.

The other benefit is that a dual fuel boiler heating system allows you to swap your fuel depending on what’s the cheapest option. You have the choice to reduce your energy bills by using whatever is the cheapest option when using these heating systems, either as a short or long term solution.

Also, fuel security benefits: should there be an issue with supply problems of either gas or oil, you can simply swap over to what is available without having to buy a new boiler or new burner, which cuts down installation costs . You could even call duel fuel boilers a hybrid system or hybrid heating as such.

Why use a dual fuel boiler?

While most boiler installations have some form of backup system, normally in the form of additional boilers for the heating systems , in the event of a fuel loss rather than a boiler breakdown that interrupts vital heating system operations, a second boiler that utilises the same fuel will not offer any degree of redundancy to the boiler system.

Therefore, the requirement for a dual fuel system to eliminate this potential failure of a single fuel heating system or boiler system.

commercial boilers with dual fuel oil gas burners -hybrid heating system


The dual fuel or dual energy boiler is better: if one fuel supply fails, the second quickly takes over, and your facility’s heating and hot water output are unlikely to be interrupted.

Changing from one energy source to another based on which is more economical for your dual fuel boiler is a great choice for property owners.

Where essential processes exist, a dual or multi fuel boiler that can use a consistent supply of natural gas in normal operation and convert to a secondary source of fuel (such as oil or LPG) if the natural gas supply fails provides a high level of redundancy.

Dual fuel heating system

The most popular dual fuel boilers are a combination of gas and oil . The gas side of the dual fuel burner can be natural gas, which is your mains traditional gas, or propane liquid gas, also known as LPG gas.

For any old or new boiler, annual servicing by a heating engineer is recommended to ensure that the central heating systems are operating effectively, efficiently, and ready for the colder months.

Dual fuel commercial boiler

All heating boilers from 70kW to 400kW have to be condensing to comply with the UK Part L building regulations (Conservation of fuel and power-standards for the energy performance of new and existing buildings) for commercial buildings, which have strict energy efficiency requirements to help reduce carbon emissions , whether they are an oil boiler or a gas boiler or LPG fired boilers. The single fuel type doesn’t matter-they all must comply with the regulations.

A dual fuel boiler at this kW output range must also comply with the efficiency requirements just because its dual fuel makes no difference.

Even though both gas an oil are ultimately fossil fuels and produce greenhouse gas emissions, the use of a condensing boiler reduces these fossil fuels impact to the lowest possible level as the highly efficient boiler system extracts the most energy from the fuel source.

Heating boiler range

Condensing boilers are the only option for fixed or permanent duel fuel boiler installation with outputs ranging from 70kW to 400kW.

Our condensing dual fuel boiler range has some unique advantages or benefits in that you can have two different temperature heating circuits from the boiler: one can be a low temperature heating system such as a 50/30c flow and return temperatures , and the other can be a higher temperature heating system with an 80/60C flow and low and return temperatures, which is ideal for heating an indirect hot water cylinder for your domestic hot water requirements.

You need higher temperatures from your heating boiler to be able to heat the domestic hot water up to 60°C. If your boiler can only produce one temperature and is being used on a low temperature heating system of 50 °C, for example, that temperature would not be enough for the heat exchanger in the hot water cylinder. Electric heating in the form of an immersion heater would be required in combination to increase the temperatures further in the installation and achieve the 60 degrees C required for a stored hot water system.

And electricity is an expensive energy source to use for a heating system.

Condensing boiler option – from 66 kW all the way up to 1,550 kW of output dependant on temperatures run


boiler technical data for dual fuel heating systems

Condensing Boiler Option

We would always recommend our condensing boiler option for a few reasons

  1. They are more energy efficient – 102% at full load 104% at part load in condensation mode
  2. There is no lower limit on the return water temperatures
  3. The combustion chamber and heat exchanger pipers are manufactured in stainless steel AISI 316L – this ensures long product life
internal picture of a commercial boiler Flexiheat UK

Internal picture of our condensing commercial boiler with stainless steel wetted parts

But technically, after 400kW of output, you don’t have to have condensing to comply with the building regulations – they must, however, comply with the following efficiency levels – but very few boilers do if they are not condensing.

Minimum heat generator seasonal efficiency for boiler systems in existing buildings part L regulations

Part L building regulations efficiency requirements


We also supply another range, which can be used in temporary on non permanent boiler applications  – as they not require to meet the efficiency levels of Part L  as far as we understand, but please do your own due diligence.  

technical data for boilers above 400kW for dual fuel burners Flexiheat UK

These boilers must always operate in forced circulation hot water systems and with a minimum return temperature of 55 °C. So, you need to use a shunt pump, which also provides an anti-condensation function for the boiler combustion chamber, installed between the flow and return connections, upstream on the heating systems with a 3- or 4-way mixing valve.

Dual fuel burner

At the heart of every dual fuel heating system is the burner.

Dual fuel burners give you the option of using gas (natural gas or propane) or oil as your energy source, depending on your business or operational needs.

The most popular dual fuel systems are natural gas and 35-second oil, which is also known as gas oil or red diesel . As such, you get two technologies or two burners in one installed unit.

dual fuel burner natural gas propane gas and oil options for dual fuel boilers Flexiheat UK

Dual fuel burner – Gas and Oil

There are various options for your dual fuel burner-

  • single stage burner option up to 232kW output-with high efficiency


  • For large efficient heating system demands, 2 stage Hi-Low burner, 2 stage sliding or fully modulating dual fuel burner options are available.
  • Manual changeover as standard or automatic fuel changeover as an option for your dual fuel boiler

Multi-fuel burner standards

All our dual fuel burners conform to all the relevant CE legislation and quality standards where applicable –

• EMC Directive 2014/30/UE

• LVD Directive 2014/35/UE

• Directive MD 2006/42/CE-2006/42/EG-2006/42/EC

• PED Directive (Article 4, Section 3)2014/68/EU

• EN676 (gas)-EN267 (liquid fuel)-EN746-2 (industrial thermo processing equipment technologies) are the reference rules.

If you require any products support, heating solutions advice, check stock, delivery times or require a price on our dual fuel boilers or dual fuel heating system from Flexiheat UK, then please use the contact us form page on the website, or alternatively feel free to give our customer service team experts a call phone number 01202 822221.All rights reserved Flexiheat UK.