Oil Condensing Boiler ,commercial and industrial condensing oil boiler, commercial oil boilers , best quality and price for the UK and IrelandFlexiheat UK

Commercial Oil Boilers – 69 kW to 1,550 kW


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Condensing oil boiler – With conventional oil boilers, waste energy, as a large portion of the thermal energy produced by the combustion, is thrown away, with the flue gas, which exits the central heating boilers to the environment at high temperature. Technology answers this problem, through the condensation of the flue gases, by reducing the temperature of the flue gas until the condensation point. With this process, it is possible to exploit the flue gas latent heat, which is transferred to the water of the heating system. This way, we exploit the full potential of the fuel resulting in a significant reduction of oil heating fuel consumption and thus fuel bills as well as emissions to the environment, with this efficient oil boiler range.

Conventional oil fired boilers, may have an efficiency between 65% and 93%. With modern condensing technology, we can achieve up to 102% efficiency for the combustion of oil heating. This improvement of efficiency and the consequent reduction of fuel consumption leads to savings to your heating bill and protection of the environment in which we live in.

This range of condensing oil boilers / Commercial oil boilers are produced according to the new directives of the European Union for energy products, in which very strict limits have been set, regarding efficiency and fuel consumption. Since April 2007 , the regulations state that all new central heating boilers that are oil fired must be high-efficiency condensing boilers unless there are exceptional circumstances.

Oil Fuel

These condensing oil boilers must be fuelled by either gas oil, which is known by a variety of names such as 35 second burning oil, Class A2 or Class D, or to standard EN590. or the new “drop-in replacement oils,” which are hydrotreated vegetable oil and thus designated  “HVO”

HVO fuel oil , a 100 percent renewable diesel or biofuel, is essential for the UK to reach its net zero carbon goal.

HVO fuel is ecological and biodegradable because it is created entirely from renewable raw resources. It drastically lowers NOx, PM, and CO emissions while removing up to 90% of net CO2 emissions.

HVO, a fuel that belongs to the paraffinic family, is almost chemically identical to conventional diesel, making it a drop-in replacement for red and white diesel (i.e., gas oil) that can cut net CO2 emissions by up to 90%. (Source Wikipedia)

They cannot be powered by kerosene, also known as 28 second heating oil or just kero. This oil will not achieve the emissions required by the regulations in force.

High efficiency Condensing Oil Boiler

Modern condensing oil boilers are designed to use virtually all of the energy content of the oil – often referred to as the higher heating value. Compared to standard boiler technology, oil condensing boilers also recover the heat of condensation of the water vapour contained in the exhaust flue gas. Oil-fired condensing boiler technology therefore achieves the highest efficiency and very minimum emissions along with the lowest fuel bills. It is important to know that to get your oil commercial boiler to work at its maximum efficiency, you need your return water temperature to be under 47C , which is the “dew point temperature” for heating oil .An efficient oil boiler will guarantee that your heating costs are minimal while reducing your carbon footprint.

Commercial Oil Boiler

Oil boilers ( which all now have to be condensing ) with an input of over 70 kW net for the installation are considered to be a commercial oil boiler. An example of this is if you had a 60 kW net oil fired boiler this would be deemed to be a domestic oil fired boiler, but if you had 2 x 45 kW oil fired boilers connected to the same heating system, then this would be classified as a commercial boiler installation because the total net input is greater than 70 kW ( i.e. 2 x 45 kW boilers = 90 kW in total )

This range of Condensing Oil Boilers has –

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Technical specifications of oil fired combustion

What is NOx ?

NOx is the term used to describe Nitrous oxide which is released during normal combustion. When combined with water and released into the atmosphere it produces acid rain, and results in significant long term damage to any structure that is repeatedly exposed to it. Besides being a single toxin, NOx combines with other substances and creates smog when it is exposed to sunlight. This creates a major health hazard for people that live in affected areas. NOx penetrates into the lung tissue and can cause permanent damage to lung tissue. Children, the elderly, people with asthma and other breathing difficulties, and people that work outside are especially susceptible to the negative health effects of NOx. The way to reduce that health risk is for companies to employ NOx reduction measures.

The new Low NOx Oil Burner differs to the older type oil fired burner, with regards to the  following points:

  • New combustion head design
  • New air disperser design
  • New way of mixing air and fuel (turbulation)
  • Oil pre-heating for better fuel spraying
  • Ultra-violet photo-resistance (New!)
  • New fan for combustion air supply

Condensing oil Boiler Heat Exchanger 

Flexiheat UK is committed to offering the most technically advanced and extensive range of commercial oil fired boilers available. The heat exchanger on our condensing oil fired boilers is made from 316 grade stainless steel. This high specification has been achieved through extensive research and testing to ensure our commercial boiler products achieve the highest quality and they are manufactured to satisfy the demanding standards of commercial premises or organizations.


Power Range – 69 kW to 1,550 kW

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Commercial Oil Boilers / Industrial Oil Boilers

This Commercial oil condensing boiler series, called the Flexiheat XC-K has 13 models covers a output range from 69 kW to 1,500 KW, in order to choose the most suitable boiler for the different requirements of the heating system. These commercial oil boilers can be be fitted with a relevant burner that either uses HVO or Gas Oil as the fuel to be burnt.

The” XC-K” range of commercial and industrial oil fired condensing boiler from Flexiheat is suitable for all those applications in which, for installation reasons, it is necessary to use a large water content condensing boiler.

The construction fully complies with the requirements stated in EN 303: Part 1.The components of the pressure vessel parts, such as steel plates and pipes, in contact with the combustion gases, are in stainless steel AISI 316 L and all the other pressure vessel parts are manufactured in certified carbon steel, according to the tables of  Euro norm 25 and Euro norm 28.

Cascading up to 9,200Kw of output , as a amximum of 8 boilers can be cascaded.

These commercial oil boilers can be used in a cascade configuration of up to 8 boilers, meaning the highest output available is 9,200 kW or 9.2 Megawatt in cascade.

Thermal efficiencies of our Commercial Condensing Oil Boilers –

102% at full load
104% at part load in condensation mode

Commercial Condensing Oil Boiler  – Advantages

Stainless Steel Combustion Chamber-  in stainless steel AISI 316L, entirely water cooled, above the tube bundle, so that the assembly could form a structure apt to favour the thermal exchange and the evacuation of the condensate from this efficient oil boiler.

Two Return Connections – allowing both high and low temperature heating circuits

Special “progressive” and Patented  Smoke Pipes in stainless steel AISI 316L with, inside, 6 special sectorial pipes and turbulators in AISI 304, that guarantee a very high heat exchange and resistance to the acidic condensate. The hot water tube bundle of the heat exchanger is slightly inclined towards the smoke chamber in order:
– to let the condensate to be collected from the heat exchanger
– to avoid the acidic wet deposits remaining in the pipes of the heat exchanger
– to clean, thanks to gravity, the heat exchanger surfaces

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Commercial Oil Fired Boilers – Internal picture showing the stainless steel smoke pipes, perfectly suited to handling the acidic condensate

Central Heating Flow connection placed on the rear upper part and two return connections, for low and high temperature, in the lower part. For models 69 and 100 the Flow and Return connections are towards the back side; for models 250 to 1550 output range the flow and return connections are towards the upper side (as shown in the picture)

The high temperature return connection is placed so that it doesn’t interfere with the low temperature return connection

The low temperature return connection is placed on the extension of the lower outer shell to that it can exploit at the maximum the heat recovery

These commercial oil boilers are equipped with a blind cylindrical furnace, in which the central flame of the burner is reversed peripherally towards the front. When the combustion gases have reached the front part, they are sent through the door into the tubes of the third pass to reach the rear flue gas chamber and then the chimney.

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The combustion chamber is always pressurised while the burner is operating, within the power output range of the oil fired boilers. The chimney must be calculated so that no positive pressure is detected at its base.

– Robust & Quality construction.
– Reliability.
– Energy savings.
– Friendly to the environment

Commercial Oil Condensing Boiler Applications –

As to Standard EN 303-1 the Flexiheat XC-K Commercial Oil Condensing Boilers are intended for water heating (up to 100°C), for central heating installations, as well as for other similar use. The maximum operating pressure allowed is 6 Bar. There is also an option to have a higher working pressure of up to 8 bar if so required.

As well as benefiting from –

– Perfect design and compact design
– Easy maintenance with easy access to the heat exchanger and the control unit
– Large heating surface but a relatively compact design
– Three passes of the burned gases.
– Uniform thermal charge.
– Long lifespan without problems.

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Commercial Oil Condensing Boilers – Technical Data according to the ErP Directive 2009/125/CE – The models with an Output higher than 400 kW are not covered by the directive

Commercial condensing oil boiler technical data according to the ErP directive 2009/125/CE

Download Flexiheat Brochure for XC-K Oil Boilers

Download the XC-K Oil condensing boilers installation manual – Flexiheat UK

Easy Maintenance

The door of XC-K Commercial Oil  boiler opens to the left or to the right ( as standard we supply the boilers with the hinges on the LH side ), so that there is easy access to the combustion chamber for cleaning and is also equipped with a Pyrex covered hole for easy visual inspection of the flame during operation. The smoke chamber cover is fastened with screws and has a special cleaning opening, which also acts as expansion opening.

Control Panel

Our Commercial oil condensing boilers are accompanied by a control panel or control unit, which is equipped with all the necessary instruments and functions. The control unit includes: general switch, safety thermostat, burner thermostat, water pump thermostat, and thermometer and operation lamps. There is also provision for additional installation of all automatic components of a modern boiler facility for automatic function and energy saving.

Insulation Level

These commercial / industrial condensing oil boilers benefit from a heavy insulation of 100mm of fabric finished mineral wool around the boiler body. There is also insulation on the boiler’s door and smoke chamber in order to minimize the thermal radiation losses. The commercial oil boilers  door is constructed in steel with insulation in special super light refractory cement which is able to reduce by more than 30% the heat losses by radiation from this efficient oil boiler range.


Our commercial and industrial oil boilers operate with oil, in accordance to DIN 51603,or can also be fitted with burners such as natural gas, or liquid gas according to specifications to DIN 4788.

Specifications / Standards of our energy-efficient products – Commercial Oil Boilers

Our boilers are manufactured following the German specification DIN 4702. The manufacturing process is based on the European Norm DIN EN ISO 9001, as to the equipment, the manufacture and the control of sub assemblies, as well as to specialized personnel and the design. These commercial and industrial condensing oil boilers have the CE approvals and quality mark.

Should you require anymore assistance or information on our comprehensive range of  Condensing Oil BoilersCommercial Oil Boilers, please don’t hesitate to contact our sales team on 01202 822221 or click here for our email contact form