120kW Boiler Gas Fired Condensing

120kW Boiler Gas Fired Condensing


120kw Boiler gas fired condensing boiler for central heating be it radiators or underfloor heating and domestic hot water Flexiheat UK


Our gas fired 120kW boiler for heating commercial systems is available in natural gas or LPG gas (LPG biofuel is compatible as well).

This regular boiler can be used for space heating, central heating (radiators or underfloor heating), and domestic hot water production via accessories such as a plate heat exchanger or an indirect domestic hot water calorifier or cylinder.

They are a high-performance “A “rated efficiency , ErP-compliant condensing boiler and offer enhanced performance that exceeds the Part L building regulations for seasonal energy efficiency requirements. With low NOx Class 6, environmentally friendly emissions

Efficiency features: with a broad range of output modulation ratios of 20:1, automatic part load adjustment offers superb outputs down to 5.75 kw of heat output at 80/60 °C or 6.44 kW heat output at 50/30 °C, which leads to better heat load matching of the system, higher energy efficiencies, lower fuel consumption, and energy saving results.

This energy-efficient commercial boiler also has two stainless steel heat exchangers (2 x 60 kW) in one boiler, which ensures redundancy for your heating system; if one heat exchanger module fails, you still have  50% of your heat duty available. This enhanced performance feature can be vital for commercial customers on a site that can’t be without some heat.

120 kW wall hung gas condensing boilers

With the option of installing the boiler as a wall-to-wall solution or arranged on a suitable free-standing frame kit for floor-mounted installation, depending on site requirements.

120kw output gas fired commercial condensing boiler in natural gas or propane gas fuel with high thermal efficiencies for energy savings.

Our 120kW gas fired condensing boiler on a free standing frame kit, with low loss header and plate heat exchanger for Domestic Hot Water production.

We can provide a solution for every commercial heating specification or commercial system, as we can cascade these high performance gas boilers up to 960 kW with automatic part load adjustment with an intelligent fault system that will monitor and report any faults on these commercial specification boilers.

Commercial gas boilers high efficiency condensing boilers for heating either floor standing boilers or wall hung boilers for central heating in commercial applications - Flexiheat UK

Floor standing or wall hung gas commercial boilers from Flexiheat UK


120kW commercial wall hung gas boilers

As each boiler (heat exchanger) has its own flue non-return valve, you can service either boiler heat exchanger module without shutting the space heating system down.

120kW gas boiler with two heat exchangers for energy efficient low carbon central heating and domestic hot water via an indirect hot water cylinder or heat exchanger- R1K120 from Flexiheat UK

Two heat exchangers – redundancy for the heating system – a wall to wall solution


Our energy-efficient condensing gas 120 kw boilers are fitted with fully modulating low NOx burners (Class 6 combustion levels) for lower environmental impact.

R1K120 gas condensing boiler output and energy efficiency figures—Part L building regulations

The latest changes to Part L of the Building Regulations, which went into effect on June 15, 2023, say that all new wet central heating systems should be sized for a maximum heating flow temperature of 55°C (previously, 75°C plus boiler flow temperature was allowed) and preferably lower if possible to get the most condensing heat transfer from condensing boilers, which results in high energy efficiencies.

Thus, our gas boiler will produce up to 125.67 kW heat output when the heating system flow and return temperatures are 50/30 °C.

The unit will also provide 114.64 kW at 80/60 °C heat output if you have a process heating application that is not covered in the Part “L” building regulations.

Full data is shown in the following:


This is detailed in our full manual, which can be downloaded here.

Manual for R1K120 Gas Condensing Boilers, Internal Version, Flexiheat UK

Should you require a larger output gas boiler (we do up to 240 kW in a single boiler and up to 980 kW in a cascade system), then we do a full range of gas condensing boilers for commercial use, which are listed here.

Gas commercial condensing boiler range

Condensing boiler specifications

  • “A” ErP efficiency rating wall-hung boiler designed for commercial use or a very large home domestic central heating system and DHW for multiple bathrooms via a hot water cylinder or plate heat exchanger

  • Low Class 6 NOx flue gas emissions make these more environmentally friendly as low carbon solutions today.

  • Straightforward and easy install: all plumbing connections are at the bottom of the unit, saving installation costs.

  • Energy efficient condensing boilers up to 105.8 %  at 50/30°C flow and return hot water temperature.

  • Stainless steel heat exchanger with 28mm diameter water ways that ensure the boiler benefits from very low pressure drops, guarantees lower risks of clogging (more resistance to fouling up), greater thermal efficiency, and greater ease of cleaning.

  • Can be arranged inline for larger output requirements to cover any commercial specification, but not back to back at present.

  • These 120-kW gas-fired boilers have the identical outstanding efficiency and performance characteristics as their smaller domestic counterparts.

  • Intelligent fault system: it will monitor system pressure, i.e., if the central heating system installation pressure is too low, etc.

System pressure

  • Maximum heating operating system pressure = 3 bar
  • Maximum heating operating system pressure = 0.3 bar


  • An “Opentherm” thermostat controller, such as a “Hive” or “Nest” type, can control the boiler’s operation and temperature either locally or on the go using your phone, tablet, or computer. You can also use a basic thermostat; the choice is yours.
  • BMS compatible with our 0–10 volt free contact kit

Boiler Fuel type

This 120kw high performance boiler can be powered by natural gas or LPG applications

Dimensions of this regular boiler: 120 kW Output for central heating and domestic hot water heating

This boiler has the following compact dimensions: 670 mm wide, 485 mm deep, and 642 mm high.

Dimensions of our 120kw condensing boiler gas fired in natural gas or LPG- ErP "A" rated for central heating (radiators or underfloor heating) and domestic hot water via a calorifier from Flexiheat UK


Connection details at the bottom of the unit: flow and return connections, gas connection, and condensate drain.

Connection details of our 120kW condensing gas fired boiler Flexiheat UK

Free standing frame installation detail

Floor standing mounting kit

Gas fired 120kw condensing boiler on a free standing frame with low loss header Flexiheat UK

Boiler on our optional free-standing floor frame kit with low loss heating system header

120kW gas system boiler

Are these an 120kW gas system boiler? Yes, in that they have a pump (two pumps actually, i.e., one for each heat exchanger module), but at this size of system boiler, we can’t fit an heating expansion vessel within the boiler casing, so one needs to be fitted to the plumbing pipework external to the boiler to make the unit a full system boiler.

Flue options

Each heat exchanger module features its own flue exhaust connection. You can either individually flue each unit with an 80mm or 100mm flue up to 25 or 30 metres, respectively, either vertically or horizontally in range. However, if you use a split flue system, i.e., an 80mm flue diameter for the combustion air and 80mm for products of combustion, you can go up to 40 metres vertically or horizontally in range.

The last accessories option is a 100mm common flue header for the boiler.

R1K120 gas fired boiler with common flue header

Common flue header kit

Gas-fired 120 kW boilers

Should you require more product information, check stock, technical support, budget options, prices, or to order on our 120kw boiler in natural gas or LPG, then please don’t hesitate to contact our sales team on 01202 822221 or use our email contact form via the contact tab above to message us.