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External Gas Boilers

External Gas Boiler – We have a full range of external gas boilers rated from 2.4 kW to 2,240 kW that can be fuelled by either natural gas or LPG (Propane) Gas. The smaller boilers are wall hung, and the large boilers are floor standing. The range consist of three external gas boiler ranges which cover the range from 2.4 kW to 2,240 kW ( in Cascade), and all of the external gas boilers feature either one of the patented stainless steel heat exchangers in the domestic models with the commercial range of wall hung external gas boilers having aluminium heat exchangers as well as  the floor mounted range of external gas boilers

External Gas Boiler – The Range

We supply 3 different types of External Gas Boilers all listed on this page  –

1.  A Domestic / Smaller Output External Gas Combi Boiler range – These are wall hung gas boilers, available in both natural gas of LPG ( Propane ) Gas  and at a 50C Flow / 30C Return central heating temperature these boilers will give an heating Output of –

Model 25K = 21 kW or 71,652 Btu’s

Model 35K = 29.5 kW or 100,654 Btu’s

They are a combination boiler only ( heating and domestic hot water configuration) , and both boilers are “sealed system boilers” incorporating an expansion vessel and ErP rated heating pump and are A rated efficiency – SAP 2005 seasonal efficiency  . They are IP5XD weatherproof rated and are rated down to minus 10 degrees C , although they have a anti freeze system that kicks in automatically at 5 Degrees C , which means as long as you have gas available the boiler will never get down to minus 10 degrees C , as it fire’s up to keep the water inside the boiler and also the central heating system from freezing.More detail on this range is first on this page –

2. A Commercial Range of External Gas Wall Hung Boilers – with an output range of either 19.2 kW to 105 kW or  10.5 kW to 136 kW Output (actual output will vary depending on the heating flow and return temperatures), these are IP5XD rated weatherproof gas condensing boilers ,that can be used for heat only or can be coupled to a domestic hot water cylinder or calorifier for your DHW requirements. More detail is in the middle of this page for this range – The Flexiheat “FHKON-EXT” range of external gas boilers

3. A Commercial and Industrial Range of External Gas Boilers – Which are floor mounted – with an output range from  65 kW to 300 kW (actual output will vary depending on the heating flow and return temperatures), these are IP5XD Boilers , supplied with a purpose built boiler housing , encompassing all the pipework, low loss header system as well as a gas leak detection system, as one of the many safety features on this range . Basically the unit is a pre built packaged plant room as such, there are more details at the end of the page on this range.



These are the new concept boiler born to satisfy current housing / residential field needs, characterized by increasingly tight spaces,energy saving, comfort, and reliability. The output options on this range are ,21 kW or 29.5  kW and are Class 6 NOx and ready for the new requirements in force from the 26 September 2018 , with regards to emission limits for nitrogen oxides (NOx) in terms of nitrogen dioxide (Eco-design Regulation. 2013/813 CE)

COMPACT AND ELEGANT – Only 240 mm depth contains all the power of these External Gas Boilers , whether it be the heating only or combination boiler versions , in an elegant and refined design.Its small size and light weight simplify installation. It has been given great attention to the user usability with simple and intuitive controls, and ease of maintenance with front access to the boiler. A range of condensing gas boilers able to satisfy the new household needs,that feature smaller spaces, energy saving requirements, comfort, ease of use, reliability and a light impact on the environment.

External Gas Boiler Internal Picture

External Gas Boiler Internal Picture

This External Gas Boiler range are highly efficient boilers benefiting from:
• A Modulation ratio of 1:10
• Always optimum ignition (the system is self-tuning to operating conditions)
• Perfect combustion (getting a longer boiler life for an increasingly reliable product)
• Reduction of on/off cycles (increasing efficiency and reducing boiler noise)

Key Features

  • Primary condensing heat exchanger in stainless steel with increased water passages
  • RANGE RATED certified
  • Class 6 NOx according to UNI EN 15502-1
  • ICS (Intelligent Combustion System) system that guarantees perfect combustion in all conditions
  • Possibility to work with all types of gas (adaptive gas function) without replacing the nozzles
  • 1:10 modulation- provides superb heating load matching, and thus better efficiency
  • Hydraulic unit in brass with DIN sequence
  • Ultra-compact dimensions
  • Easy to install
  • Designed to be installed flush-mounted, outdoors, in a partially protected place (temperature from -10 to + 60 ° C)
  • Designed to facilitate maintenance
  • High and low temperature operation in heating
  • Prepared for operation with zone systems (with the possibility of excluding the internal circulator)
  • Prepared for operation with external probe
  • Remote control as standard with weekly climate programmer and self-diagnostics
  • Simple and intuitive user interface
  • Solar compatible, in combination with the Solar Kit with fittings (optional accessory)
  • IP X5D electrical protection
  • Chimney sweep function and boiler regulation menu (TSP) by remote control
  • Maximum heating power regulation function
  • Hydraulic unit made entirely of brass
  • Sanitary production up to 18.7 l / min (model 35 K)
  • High efficiency and low consumption
  • Microprocessor electronics
  • Drip-trap trap with dry closure
  • Stainless steel plate heat exchanger
  • Electric three-way valve
  • Complete electronic modulating circulator High Efficiency (ErP ready)
  • Automatic by-pass
  • Timed post-circulation
  • Heating and sanitary antifreeze function
  • Fittings for antifreeze fittings, standard, with thermostat for activation
  • Pump anti-blocking function and three-way valve
  • Water filter entering the boiler
  • Automatic system pressure reset with safety limitations on the number of loading cycles and their duration
  • Solenoid valve for loading the system with the possibility of manual opening and additional filter with narrow meshes


Finally, thanks to the adaptive gas function, these external gas boilers can operate with all types of gas without requiring the replacement of the nozzles (the boilers are supplied with a single product code), so it’s the exact same boiler for natural gas or LPG ( Propane ) Gas , ensuring that you can not go wrong on site !

From initial conception, these external gas boilers have been designed and developed to benefit from a new stainless steel heat exchanger with extra wide water passages: the inner section of the coils has been increased by 4 times in comparison to the market standards.Thanks to the new heat exchanger, the City Open and the City Box have better thermal efficiency, better resistance to limescale and clogging up, making these external gas boilers ideal even when replacing boilers on pre-existing heating systems; In addition, the single coil design of the new exchanger guarantees high flow rates, low hydraulic load losses, easier cleaning and maintenance as well as exceptional resistance against corrosion.

External Gas Boiler – Heat Exchanger Features

The external gas boiler heat exchangers are made of a stainless steel flattened tube are manufactured without weld joints to have the highest resistance against corrosion, to avoid clogging and to grant:

– Large Water Flow
– Great Heat Exchange Surface
– Low Pressure Drops
– High Working Pressure  (up to 3 Bar)


External gas boiler heat exchanger

External Gas Boiler Heat Exchanger – Made from Stainless Steel with Large Water Ways


External Gas Boilers with Intelligent Combustion System

This external gas boiler range has been designed and developed the ICS – Intelligent Combustion System – a system making the City Open and the City Box smart boilers, the only external gas boilers controlling and adjusting themselves independently.The ICS – Intelligent Combustion System – controls the combustion values and adjusts the gas flow rate in order to always get the correct air / gas ratio, thus obtaining:

• Reduction of the gas consumption
• Lower CO/CO2 emissions
• Less installation time (there is no manual commissioning to be done to the gas burner -it’s fully automatic ! controlling and adjusting itself  as required )

Not forgetting the great turn down ratio of 1:10 with a constant air/gas ratio at each point of the turn down range of the burner, decreasing polluting emissions and streamlining efficiency

External Gas Boilers – 25 K  and 35 K Models –  Technical Data

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Ecologic premix boilers have a constant air/gas ratio in each point of the turn-down range of the burner, decreasing polluting emissions and stream lining efficiency. The gas premix burner is made of “Fecralloy” a special metal fibre and has a round shape. The premix burner spreads short and perfectly nourished flames.

High-efficiency combustion
Low polluting emissions (Class 6 NoX Compliant )
Natural gas and LP gas operation without the need of a nozzle change


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External Gas Boilers – available in Combi Versions from 21 kW to 29.5 kW



If you order these gas external boilers in the external combi boiler version, these external gas boiler benefit from an over sized plate heat exchanger, the domestic hot water is produced by an over sized stainless steel plate heat exchanger, that allows great and quick availability of Domestic Hot Water. They always works in condensing mode, even during domestic hot water supply, thereby reducing the consumption and costs involved in the production of hot water of an additional 10% – compared to boilers that do not condense during Domestic Hot Water production.

Double Condensing = Double Savings

This external gas combi boiler range always works in a condensing mode, both in heating and hot water production, thereby reducing the consumption and costs involved in the production of hot water of an additional 10%.Considering that hot water is used 365 days a year, the choice of these External Gas Combi Boiler’s double condensing means a great increase in savings!

External Gas Combi Boiler Brass Hydraulic Group

External Gas Combi Boiler Brass Hydraulic Group with Domestic Hot Water Plate Heat Exchanger


These external gas combi boilers utilize brass on its hydraulic units to ensure maximum reliability and robustness of its products.

The new brass hydraulic group is equipped with:
• Automatic filling valve
• Three-way valve
• External bypass
• Insulated stainless steel plate heat exchanger
• One-way valve
• Flow regulator
• NTC DHW probe

External Gas Combi Boiler Range and Heat Only Boilers 

We often get asked if we do an external heat only gas boiler, at present there is not enough volume to produce this type of boiler, however there is a work around to get these external gas combi boilers to work in this manner. You simply have to connect a cold-water inlet (mains cold water), to the domestic hot water side of the boiler. You can either cap off the hot water outlet, or use this hot water outlet to feed either a tap inside the property – e.g. the hot water tap in the downstairs toiler, or the kitchen tap etc, or even connect it to an external tap, the choice is yours , but it must be connected to the cold water mains for the boiler work at all.

There is a solution to have a the City External Gas Combi Boilers coupled to a DHW Cylinder and more details on this can be found here –

Solution to have a the City External Gas Combi Boilers coupled to a DHW Cylinder by Flexiheat UK

External Gas Boiler / External Gas Combination Boiler Install Locations 

Even though our range of external gas combination boiler are located externally to the property or building, their installed location, with regards to the flue pipe exit point still has to comply with the latest building regulation 2010 – Part J2 – Discharge of products of combustion.Below is a schematic of where you can locate the boiler in relation to doors, car ports, windows, boundaries etc.

External gas boilers, external combi boiler


This range of gas condensing boilers is available for use outdoors with the additional the CE certified, and weather resistant cover made of epoxy coated steel, which is resistant to sun, rain, frost and salt gives an IPX5D electric protection degree and protects the boiler from frost down to Minus 10 Degrees Centigrade .The boiler remote control allows remote control over the external gas boiler internally from the house or building.

Flexiheat Brochure for External Gas Boilers Domestic Range – Combination Boiler


Download User Manual for City Open 25K & 35K External Combi Boilers


The “FHKON- EXT” are a commercial wall hung heat only external gas condensing boiler, with an output range of either  19.2 kW to 105 kW (actual output will vary depending on the heating flow and return temperatures)with a modulation ratio 1:5.75 (5.0) for the 100 model , and  an output of 10.5 kW to 136 kW  (again actual output will vary depending on the heating flow and return temperatures) with a modulation ratio of 1:13 for the 140 model  . They are ideal for small business and multi-family users, and can be connected in cascade sequence up to 12 units for the 140 model and 8 units for the 100 model to achieve a maximum power of 1,120 kW of boiler heating power.These boilers are  “A ” rated ErP heating products, and have a maximum working temperature of 85°C as your flow temperature.

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Flexiheat KON-EXT External Gas Boilers, 100kw or 140kW shown with option boiler support / frame kit or can be wall mounted


COMPACT, LIGHTWEIGHT AND ECOLOGIC – Only 96 kg for 100kW boiler, and 172 kg for the 140kW model- Please ensure that any external wall that these boilers are to be located can take the weight of the boilers. This range of Commercial Gas Wall Hung Boilers is extremely ecologic thanks to the premix burner which develops short and perfectly nourished flames.

Very low polluting emissions , Low NOx, class 5 according to EN 15502-2

WIDE TURN DOWN RATIO (of up to 1:13) – thanks to patented air/gas premix system ,the boiler output adapts according to real need for room heating, guaranteeing great gas savings and low CO2 emissions.

HEAT EXCHANGER – made from Al/Si/Mg alloy, the best for:

– 100% wet surfaces of the boiler body
– for long time guaranteed efficiency, thanks to the absence of scaling
– reliability, thanks to the optimized circulation that avoids thermal overloads
– long lasting and proven – benefiting for the many years of our metallurgical experience with boiler heat exchangers

AVAILABLE MODELS – With an inbuilt High efficiency ErP  boiler shunt pump supplied and a Pump anti-jamming time controlled function as standard , without expansion vessel

Usually a standard condensing boiler works in condensing mode only in combination with low temperature heating systems. This happens because the under floor heating system uses water at low temperatures and the condensing boiler reaches its maximal efficiency in these conditions. But traditional radiators need to be heated with high temperature water and traditional condensing boilers do not condense when they work at high operating temperatures. Thanks to the innovative, wide surface,heat exchanger of these commercial external gas boilers, the flue gases condense at 55°C return temperatures,working always in the condensing mode when installed with a radiator heating system which have been sized correctly.For this reason the “FHKON” range are the ideal commercial condensing external boilers not only for new installations,but also in the replacement market for commercial heating systems

Download the Flexiheat UK KON-100 External Gas Condensing Boiler Brochure 105kW @ 50/30C

Download the Flexiheat UK Alkon-140- Larger External Gas Boiler Brochure 136kW @50_30C


Larger External Gas Boiler Range , Floor  Standing , IP5XD Rated , For External Locations 


These Larger external commercial gas boilers are produced from 65 kW to 300 kW is and are modulating commercial gas condensing boilers, completely fully assembled  , in a floor standing external gas boiler configuration for commercial and industrial applications.These external gas boilers can be cascaded up to 2,240 kW of Heat Output , if so required.

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  • 10 different capacity option between 65 kW-300 kW in our “ECODENSE”-  RT One or Twin series (i.e one boiler is the RT one or two boilers, designated the RT Twin)
  • High efficiency with premix condensing technology and ecologic micro-flame metal fibre coated steel burner
  • Provides energy saving through 5:1 modulating operation,that optimizes the supplied output according to real thermal needs of the heating system – ensuring maximum efficiency, and allow high seasonal efficiency in systems characterized by frequent and high output variations.
  • Can be fuelled by natural gas or LPG
  • Easy hydraulic and gas circuit connections,
  • Allows simple control with illuminated LCD panel which provides ease of use; and error codes and boiler information can be displayed on this panel,also cascade sequence control of the boilers
  • Daily and weekly operation schedule can be programmed,
  • Seasonal heat program can be prepared during summer and winter times,
  • Operation in lower noise values,
  • Environmentally-friendly with lower NOx and CO emission rates,
  • Easy installation, operation and maintenance.Thanks to their relative lightness they are easy to manoeuvre  and to place to the location desired
  • Aluminium spiral heat exchanger – Each heat exchanger is made without weld joints and it is able to bear a working pressure up to 6 bar.If you require a higher working pressure, then you can use a plate heat exchanger to separate the boiler circuit from the heating circuit

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Why choose an External Gas Boiler:

BOILER SAFETY – All of our RT Floor Mounted External Gas Commercial Boilers come with a Gas Leak Detection System as standard

  • When outdoor temperature goes below +4 °C, the boiler automatically protects itself and prevents it and the heating system from freezing.
  • The boiler protects itself, when the temperature of flue gas exceeds the set value during operation,which is  < 70 Degrees C
  • The boiler protects itself should it encounter any unexpected high temperatures within the combustion chamber via a  safety thermostat.
  • Gas Pressure Sensors ensure that these external gas boilers can sustain the gas pressure between the minimum and maximum limits.


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• Primary hydraulic circuit pump / pumps
• Inlet and return collectors in size according to installed total power
• Check valve in inlet line and safety valve in return line of the boiler
• Ball valves in both inlet and return lines of the boilers
• Balance container with residue, dirt and air separator return collectors in size according to total installed power
• Gas detector for the gas leakage control
• Pressure gauge and ball valve in gas line
• Thermometer in hydraulic circuits
• Drain trap for each boiler
• Ventilation grills for the air inlet and outlet
• Eye bolt for easy transport

Savings and efficiency

  • Condensing technology
  • Total flame modulation (heating and sanitary)
  • Certified efficiency up to 107%
  • Suitable water flow to the external gas boiler heat exchangers with suitably sized flow and return pipework, and hydraulic separation of the boiler circuit
  • Cascade sequence control and heat exchangers in“rotation” so all boilers have equal ageing , and thus expanding the life-cycle of the boiler plant


  • Very quiet in operation
  • Outdoor sensor and climate control options available
  • Easy installation and maintenance
  • Reduced size and weight

Construction quality

  • Modern, innovative and attractive design
  • Aluminium heat exchangers – Which offers excellent thermal capacity, meaning water can move more slowly around the system and the return water temperatures are cooler, increasing efficiency.
  • Turn down ratio up to 1:10 for the 300 kW model

Ecology & Environmentally Friendly External Gas Boiler

  • Fecralloy fibre premix burner
  • Reduced gas emissions in the atmosphere – With  low CO and NOx

Dimensions of the RT Floor Standing External Gas Boilers

Large External Floor Standing Gas Condensing Boilers Dimensions RT Series

Flexiheat Manual for Commerical and Industrial External Gas Boilers – Floor Mounted- RT Series

These three Boilers make up a full range of external gas boilers, that can cover any application. Please remember that by mounting these boilers externally , you will dramatically reduce or eliminate your flueing costs , by locating your external gas boiler in a position that suits you and your site conditions. All of the covers that we do are CE certified, insulated and frost protected, and made from anti Ultra Violet ABS material for the smaller ranges or Anodized Aluminium for the floor standing range. So whatever your application we have the External Gas Boiler to provide the solution to your needs.

For more information on our External Gas Boilers , please don’t hesitate to contact our sales team on 01202 822221 or email us here