external gas boilers; wall hung external boiler; external combi boiler; external boiler cabinet;external boiler housingFlexiheat UK

External Gas Boilers

External Gas Boiler – We have a full range of external gas boilers rated from 3.7 kW to 600 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 3.7 kW to 600 kW ( in Cascade), and all of the external gas boilers feature either one of the patented stainless steel heat exchangers in the domestic and commercial range of wall hung external gas boilers . We also have the option of having aluminium or stainless steel heat exchangers for 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  R2K 34 = 36.19 kW or 123,480 Btu’s

The domestic hot water performance on this external combination boiler in continuous service is – 16 litres/min (at Δt 30°C temperature rise) – This boiler will feed 2 standard showers, or a bath tap continuous – ensuring you fill a bath in a short period of time. These gas condensing external boilers are up to 106.43 % efficient @ 50C/30C Flow and Return.

This is the external combi boiler we supply as standard , as these boilers are fully modulating you do not need to worry about it being oversized. For example , this boiler will modulate down to 4.10 kW on the heating side of the boiler.

We actual do a smaller unit, as detailed below, but the larger unit has a better domestic hot water performance, and is fully modulating, and is priced at the just a little more- in our opinion, the larger boiler is the better all-round option, as the hot water performance is better and the larger boiler will modulate down to the required kW for the heating system.

Model R2K 28 = 29.40 kW or 100,312 Btu’s

The domestic hot water performance on this external combination boiler in continuous service is – 13.22 litres/min (at Δt 30°C temperature rise)

This gas condensing external boiler model is up to 105.84 % efficient @ 50C/30C Flow and Return.

These boilers are combination boilers ( 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 8 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. With the addition of ethylene glycol (anti-freeze) to the heating system, you can protect the boiler down to a temperature of minus 50 C, when using a 60% anti-freeze mix.


More detail on this range is first on this page –

 External gas system boilers

An external gas system boiler – again a wall hung condensing boiler, and available in both LPG or natural gas with an output range of 4kW up to 36 kW (dependant on the flow and return temperatures) – and is suited to domestic properties or even some light commercial applications. This boiler is designated the R1K External Boiler, and is also fully weatherproofed and rated to minus 10 degrees C (lower with an optional electrical heating kit)

Full details on the external system boilers available are shown in the following manuals

R1K 24 Rain external 24kW heating system boiler manual from Flexiheat UK

R1K 28 Rain external 28kW heating system boiler manual from Flexiheat UK

R1K 34 Rain external 34kW heating system boiler manual from Flexiheat UK


2. A Commercial Range of External Gas Wall Hung Boilers – with an output range of either 3.5 kW to 52 kW, 3.5 kW to 79 kW or  4.8 kW to 108 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 “R1K” 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.

Can a gas boiler be fitted outside?

Yes, you can fit a gas boiler outside or external to the building. There are certain things to consider, such as the wall being strong enough to take the weight of the boiler when it is full of water and also ensuring that the flue gases ( often called products of combustion ) produced are certain dimensions from doors , windows, down or waste pipes , balconies etc- all of which we show on the diagram on this page – “discharge of products of combustion”


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 ,36 kW or 29 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)


Only 370 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 boilers from Flexiheat UK;

External Gas Boilers – Heating or Combination versions are available – natural gas or LPG versions

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
  • Available in either natural gas or LPG gas
  • 1:9 modulation- provides superb heating load matching, and thus better efficiency
  • Ultra-compact dimensions
  • Easy to install
  • Designed to be installed wall-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
  • Maximum heating power regulation function
  • Sanitary hot water production up to 16 litres / min (model R2K34 )
  • High efficiency and low consumption
  • Microprocessor electronics
  • 8 litre heating expansion vessel
  • Complete with electronic modulating circulator High Efficiency (ErP ready) – high-efficiency ErP electronic circulator with PWM control with built-in air separator
  • Automatic by-pass
  • Electronic flow switch for domestic hot water priority
  • Heating and sanitary antifreeze function
  • Fittings for antifreeze fittings, standard, with thermostat for activation
  • Pump anti-blocking function and three-way valve


Finally, these external gas boilers can operate with on either for natural gas or LPG ( Propane ) Gas , depending on your site requirement !

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, these external combi boilers 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 28 mm stainless steel tube are manufactured without weld joints to have the highest resistance against corrosion, to avoid clogging and to grant:

– Large Water Flow – 28mm in diameter
– Great Heat Exchange Surface
– Low Pressure Drops
– High Working Pressure – up to 3 Bar for the heating side , and 6 Bar for the DHW side


External gas boiler heat exchanger; external combi boiler

External Gas Boiler Heat Exchanger – Made from Stainless Steel with Large Water Ways – 28mm


External Gas Boilers with Lox NoX6  Combustion System

This external gas boiler range features a control panel with a microprocessor electronic board with continuous flame modulation with P.I.D. control, which ensure low pollution levels. The achieve the stringent low NOx levels (NOx can be extremely harmful to both humans and the environment) of class 6, making these boilers environmentally friendly.

• Reduction of the gas consumption
• Lower CO/CO2 emissions
•  Not forgetting the great turn down ratio of 1:9 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 – R2K 34 K Models –  Technical Data

external gas boiler; outside gas boilers; external gas combi boiler; external combi boiler;


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 or LPG gas versions available 


External gas fired boilers lpg or natural gas from Flexiheat UK; external gas boilers; wall hung external boiler; external combi boiler gas fired;

The internals of our gas fired outdoor / outside gas boilers



If you order these gas external boilers in the external combi boiler version, these external gas boilers benefit from a patented heat exchanger system, which is a double heat exchanger,  the domestic hot water is produced by an oversized stainless steel double heat exchanger, that allows great and quick availability of Domestic Hot Water. They always work in a 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.

The other main advantage of the patented heat exchanger system, is that there is no plate heat exchanger used on the combi boiler version, with small water ways to block, that ultimately results in the poor performance or failure of the unit.

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 condensing system means a great increase in savings! with DHW production efficiencies of more than 105% and the benefit from lower fuel consumption and lower harmful gas emissions.


External gas boiler heat exchanger; external combi boiler

External Gas Boiler Heat Exchanger – Made from Stainless Steel with Large Water Ways – 28mm

These external gas combi boilers utilize stainless steel to ensure maximum reliability and robustness of its products.

External gas system boilers

We now can supply an external gas system boiler or sometimes known as a “sealed system boiler”. An external system boiler is a gas boiler that will produce hot water for the heating system, be it radiators or underfloor heating and will also supply an indirect cylinder that produces domestic hot water. System boilers have all the components such as the pump and the heating expansion vessel incorporated within the boiler casing, and unlike a regular heat only boiler, there is no need for water tanks in the loft.

System boilers are “sealed heating boilers”, and thus when used with an unvented hot water cylinder, the domestic hot water will be at mains water pressure. Heat only boilers, when used with a hot water cylinder only delivers the hot water under gravity conditions, as they are not under pressure.

We have one unit in this range of external gas system boilers, which can be specified in either natural gas or LPG gas. This boiler is a condensing external gas boiler –

An output range from 36 kW all the way down to 4 kW (output depends on your flow and return temperatures of the heating system)

This gas system boiler features the following benefits –

A large 28mm Ø stainless steel heat exchanger

1: 9 modulation for greater efficiency and reduced gas consumption. with reduced running costs

Class 6 NOx levels – reducing pollution levels

ABS plastic casing for better weather resistance


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.

Download the manual for R2K 24kW,28kW or 34kW External Combi Boilers



The “R1K Range” are a commercial wall hung heat only external gas condensing boiler, with an output range of either 4.8 kW to 53.4 kW (actual output will vary depending on the heating flow and return temperatures)with a modulation ratio 1:10 for the “50” model , and an output of 3.5 kW to 79.35 kW (again actual output will vary depending on the heating flow and return temperatures) with a modulation ratio of 1:20 for the “75” model, the “100” model , which has an output range of 4.83 kW to 108 kW, or the biggest unit available in this range the “120” model , output range 6.4 kW to 125.67 kW with a modulation ratio of 1:20 . They are ideal for small business and multi-family users. These boilers are “A ” rated ErP heating products, having a maximum working temperature of 80°C as your flow temperature, and are all compliant with class 6 NOx emission levels.

external gas boilers; external gas boiler cabinet; external gas boiler housing units; external gas boiler uk

Flexiheat R1K External Gas Boilers, 3.5 kW to 125kW – Wall mounted – with optional boiler support / frame kit if so required.


COMPACT, LIGHTWEIGHT AND ECOLOGIC – Only 41 kg for the 50 kW boiler ,89 kg for 75kW boiler, and 90 kg for the 100kW and 120kW models  – 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 6 according to EN 15502-2

WIDE TURN DOWN RATIO (of up to 1:20) – 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 Stainless Steel, with a 28mm diameter bore – which is 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

The 75 ,100 and 120 models are equipped with 2 boilers within a single boiler body weather proof casing, offering a size advantage at only 70 cm wide, as well as a benefit in terms of a high-power output.


The 75kW commercial external gas boiler has a both a 50 kW and 25 kW boiler within the casing and the 100kW boiler consists of 2 boilers (50kW + 50 kW)and the 120kW has 2 x 60 kW boilers . This innovate solution to mounting a boiler outside has brought many advantages in terms of performances and reliability, such as a 1:20 modulation (75 ,100 and 120kW),with the possibility of excluding one boiler, should one of the boilers fail, this will not shut down the heating system as the other boiler will still operate – ensuring redundancy for the heating system – another great benefit of our commercial outside gas boilers .

You can also service one of the boiler modules whilst the other is running – another great feature !

The boiler electronic consists into 2 printed circuit boards, the main PCB (master) with the parameters and settings access and the secondary one (slave), for the second boiler functioning and control.

For full technical details, dimensions, outputs etc, please download the manuals below-

Flexiheat Manual for R1K 50 External Gas Boiler -Condensing- Natural Gas or LPG

Flexiheat Manual for R1K 75 External Gas Boiler-Condensing- Natural Gas or LPG

Flexiheat Manual for R1K 100 External Gas Boiler-Condensing- Natural Gas or LPG

Flexiheat Manual for R1K 120kw External Gas Boiler Condensing- Natural Gas or LPG

Floor mounted option

We also do a floor mounted gas external boiler range from 50kW to 120kW that can cascaded up to an output of 600kW in either natural gas or LPG (propane) gas. This range is designated the “R1BK” rangge . Here is an installation example with a common flue pipe system with one hydraulic connection box, which includes the low loss header and pipework connections – the left most unit in the below pciture.

External gas boilers in cascade 500kW with common flue system from Flexiheat UK

500kW of heating power via gas powered external boilers – shown with a common flue system.

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.

External Gas Boiler Larger Sizes, External floor standing condensing gas boiler,large external gas boiler

  • 10 different capacity external boilers 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

External Gas Boilers,large external gas condensing boilers

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.


External Gas Boiler Larger Sizes, External floor standing condensing gas boiler,large external gas boiler

• 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

Option of a plate heat exchanger instead of a low loss header

External gas boiler house with plate heat exchanger option Flexiheat UK

Optional – We can supply these external gas packaged boilers with a plate heat exchanger instead of a low loss header.

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