35kW Combi boiler – Combination boilers

35kW Combi boiler

35kw combi boiler - a high efficiency combination boiler for central heating and hot water being a condensing boiler with a long warranty. No hot water cylinder is required as your producing domestic hot water on demand.

35kW combination boiler – “R2C34” from Flexiheat UK – high performance domestic hot water output flow rate of 16 litres/min with a 30C temperature rise for commercial or domestic installations

Our modern 35kw combi boiler is a cost effective heating solution that employs condensing technology to effectively heat the system water by removing latent heat from the flue gases which enables these boilers to have a “A” ErP rating for space heating efficiency and “A” ErP water heating efficiency rating, which means higher efficiency and thus lower running costs and for this gas combi boiler.

A combination boiler is the most popular boiler type in the UK, as the size of a combi boiler is one of its main benefits. As Combi boilers do not require external hot water cylinders / water storage cylinder, this type of boiler provides both hot water and central heating in a single compact unit for more than one bathroom properties.

Having “heating and hot water” in one boiler has many benefits and will save installation space on site and the users or customers will have a lower installation cost as a combi boiler installation is quicker that a system boiler and indirect cylinder installation.

Our gas boiler has a great modulation or turn down ration of almost 10:1 in range, which means the boiler will match or address the central heating load requirement very closely with high efficiency, ensuring you’ll save money on your energy bills for heating and hot water.

With low noise level, this reliable gas boiler, which is available to natural gas or propane gas (LPG) will produce a domestic hot water output of 16 litres/min with a temperature rise of 30C and is up to 108.6 % efficient and provides a cost effective heating solution for domestic installations with tested technology.

35 kW Combination Boilers

A gas burner in the combi boiler is lit when you need hot water or heating. The primary circuit receives the heat from the burner through the main heat exchanger, where it is then either moved straight to the radiators or through a plate heat exchanger to your hot water taps in larger homes.

One of the key features of our 35 kw ErP compliant modern gas combi boiler is a “pre-heat” system, which enables the boiler to contain a small volume of water that can be used instantly for fast hot water response. The water is kept in the boiler all day and kept at the desired temperature so that it can be quickly delivered to the hot water taps.

Fuel options for our 35kw output gas combi boiler

This 35kw combi is available in either natural gas or LPG (propane) gas options depending on local availability of the fuel to the property or house. Should you ever need to change your gas type, just speak to our customer services as we can supply a gas adaptor kit to change the gas boiler to your required fuel type. This is an easy installation and takes less than quarter of an hour to do at most if ever needed as the unit has easy access to all internal components. 

Stainless steel heat exchangers in our combi boilers

Our full range of condensing boiler models use stainless steel heat exchangers because of their high corrosion resistance across a broad pH range and are better than a aluminium heat exchanger. Your combi boiler will last longer as a result and has a long warranty period, making them a good choice gas boiler for consumers who require a quality solution and  high domestic hot water output.

When discussing our combi boilers dependability, the development of our modern heat exchanger from one single continuous coil permits the water to move through more quickly, efficiently, and at a higher flow rate performance.

This reduces the possibility of debris blockages, increasing reliability and lengthening the boiler’s lifespan. A Stainless steel heat exchanger construction will withstand high flow rates without corroding or wearing out more than a aluminium heat exchanger.

Patented A rated ErP water heating efficiency

Inside the high performance patented stainless steel main heat exchanger is a 12mm diameter internal stainless steel pipe that produces domestic hot water (installed within the 28 mm diameter heating exchanger). This ensures you have a quantity (approximately 7 litres) of “warm water ready to go,” as they say. It’s total length is 8 metres. Because these boilers produce domestic hot water by continuously condensing, which makes them very efficient, gas energy costs and energy bills are reduced.

Pipe in pipe stainless steel heat exchanger for heating and hot water production for our 35kW combi boiler suitable for glow worm controls.


Additionally, these design features reduce the possibility of scale accumulation, which you can expect in older combi boilers with a plate heat exchanger that many companies still use.

Blocked combi heat exchanger with scale and debris

In areas with hard water, this is crucial because our heat exchanger’s high velocity and flexing while in use reduce the potential for scale to build up and prevent it from happening, helping to ensure a long life, trouble-free operation, and a good hot water flow rate for domestic installations.

Good mains water pressure is needed for combi boilers

One potential drawback of a combination boiler is that if multiple showers and taps are initiated simultaneously due to combi boilers pulling water directly from the local availability in the mains cold water supply, the water pressure can drop quickly for example.

Our boiler needs a minimum pressure of 0.5 Bar and will operate up to a maximum pressure of 6 Bar. For good hot water performance we recommend around 3 Bar pressure for your hot water system.

Boiler specifications, features and details

Our 35kw heat output combi boiler has both “A” rated ErP water heating efficiency performance for both heating and domestic hot water, with low noise level outputs and is suitable for weather compensation, when used in conjunction with the external temperature sensor that we can supply for our full range of heating and hot water boilers. Weather compensation reduces the heat output to suit the weather conditions which can vary so much on a day to day basis in the UK.

This 35 kw boiler range is suited for space heating or central heating with radiators or underfloor heating systems with a high efficiency of 106.43% when used on a 50/30C range flow and return system which will save you money and cost on your gas energy bills.

With a useful central heating output of 36.19 kW when used on a 50/30C system , or when used on a 80/60C system the central heating output is 33.35 kW.

We use a Wilo pump rather than a Grundfos pump in our 35kw gas combi, as we consider them a more reliable pump than a Grundfos pump.

R2C34 combi boiler 34kW technical data, showing the central heating or space heating and domestic hot water performance of this condensing boiler by Flexiheat UK

Download the Flexiheat R2C 34kW Combi Boiler Manual

For larger homes, or homes that have larger hot water performance demands then please look at our next size up combi boiler our – 50kW gas combi boiler


Our 35 kw combi boiler dimension are – Height = 642 mm , Width = 410 mm  and a Depth of 350 mm – with a weight of 44 kg making these robust units easy to fit in any installation.

combi boiler 35kw dimensions Flexiheat UK

Flow and return and gas connections – 20 mm

Hot and Cold water connections – 15 mm

Flue lengths and options

With extensive flue options of up to 60 metres in length – when using a sperate 80mm diameter twin flue system, or up to 10 metres in length when using our 80/125mm co-axial horizontal flue system option – both options make this combi boiler a room sealed gas appliance.

Horizontal flue kit co-axial room sealed for our boilers Flexiheat UK

Horizontal flue kit co-axial – the 80/125mm Ø will go 10 metres


Or our vertical flue option

Vertical flue kit co-axial for our boilers Flexiheat UK

Vertical flue kit co-axial – the 80/125mm Ø will go 10 metres vertical

Control options

This boiler can be controlled via the inbuilt digital display, which displays digital operating status and diagnostic display or you can opt for our smart thermostat range option, which is an OpenTherm control system, that allows you to control these combi boilers from an app on your phone, tablet, or computer.

cloud warm app to control the boiler from your mobile phone or tablet

Cloud Warm App – control on the go !

Nest and Hive controllers are Opentherm compatible, so you can use them with what we consider the best 35 kW combi on the market and a cost effective heating solution for homes heating

Common question

35kw combination boiler how many radiators can it run ?

The average size radiator in a new build property is now 1.5 kW (kilowatts) output – so your looking at 23 radiators, but for older properties your looking at around 11-15 radiators on average for this size gas boiler type.

Please ensure that the local area boiler installer or a company engineers are gas safe registered and can provide a labour warranty and service agreement on the install.

Contact us for free boiler quotes for your combi boiler requirements

Should you require anymore assistance, information or best deals/prices on the best 35kw combi boiler with great hot water performance in natural gas or LPG gas on the UK market, please contact our customer services sales team on 01202 822221 or use our email contact above on the web page for our email contact form. All of sales are supplied with a vat invoice.