Condensing water heaters

Condensing water heaters

Condensing water heaters with high efficiency domestic hot water heating with storage or condensing tankless water heater options from Flexiheat UK


Condensing water heaters have become the norm since Part L of the UK building regulations came into force in June 2022.

The more stringent water heater requirements of the Part L building regulations apply to both newly built and existing non-domestic building stock.

This basically eliminates the option to swap out existing non-condensing water heaters with the same type of water heater. Any replacement hot water heaters must utilise condensing technology within the domestic hot water heater to produce savings on both a monetary and emission level i.e., carbon emissions, as a condensing water heater will deliver cutting edge efficiencies.

A low emission level hot water system helps the UK achieve our drive to a lower carbon economy and ErP compliant condensing water heaters help this.

However, in the rare event that the Building Control authority determines that replacing a non condensing gas fired water heater water is not suitable or feasible in non domestic buildings, a minimum seasonal efficiency of 80% for natural gas or 79% for LPG may be utilised.

But there are very few circumstances in which you can’t use a condensing water heater, to be fair, so these exceptions would literally be “once in a blue moon,” as they say, if at all.

The energy efficiency benefits of condensing water heaters now mean that the residential hot water systems market has also rapidly moved to this type of water heater’s installation, as they deliver cutting edge efficiencies and produce savings on your water heating bills.

As of yet, there are no regulatory requirements to use a gas fired condensing water heater in a domestic property, but the economic benefits of using less gas to produce domestic hot water for a home or flat through higher efficiency alone have driven the domestic market this way.

However, the commercial hot water regulations or the commercial water heaters market often lead with regards to regulations, and the domestic market then follows, so we can envisage that condensing water heaters will soon be mandatory in this sector before long.

Energy efficient water heating

Condensing gas fired water heaters have a hot water efficiency level of up to 105%. Which is essentially an extra 12 to 15% more performance than an older gas fired non condensing water heater.

A second heat exchanger in a condensing water heater preheats the incoming cold water by using waste heat from the exhaust gases. This process lowers the exhaust gas temperature, boosts energy efficiency, and increases the hot water flow rates of the water heaters.

A cooler exhaust allows for additional possibilities for flueing materials, such as polypropylene plastic flues. Condensing gas fired water heaters need to dispose of the acidic condensation water through a drain and possibly a pH neutralizer because water vapour condenses inside the appliance when the exhaust gas cools, and this is acidic in nature.

Gas fired water heaters

We have a large, high efficiency gas fired condensing water heater range that are direct fired water heaters for small or high hot water demand that can be required in commercial or industrial hot water heating applications such as hotels, leisure centres, and hospitals.

Wall hung water heaters

SFC or SFK gas fired condensing water heater range from Flexiheat UK

SFC or SFK Condensing water heaters range – in natural gas or LPG


Our wall hung condensing water heaters range consists of three models a 28kW, 34 kW or 50 kW output water heaters.

These water heaters benefit from various flueing options: horizontal or vertical flue systems, room sealed or conventional flued with various flue terminal options for easy installation.

The two smallest water heaters come with the option of a pump for a domestic hot water recirculation system (designated the SFK model) or without a pump (our SFC version) for straight through water heating, which are the most common appliances we sell. The larger 50kW model comes with a pump as standard, which is capable of being turned off for straight through direct water heating.

With a large 28mm single-pipe coil heat exchanger made of stainless steel, the SFC or SFK condensing water heater range benefits from very low pressure drops, less chance of blockage, increased thermal efficiency and easier cleaning and delivers high volumes of temperature accurate hot water. Other safety features such as protection to IPX5D rating are standard.

There is no water storage within these units, so there is no heat loss compared to a storage water heating system.

This is the basic rule: the higher the kW output, the higher the DHW flow rate performance

When heating cold water to a certain temperature, this is known as the temperature rise or Delta T (shown as Δt in the charts below). The normal bathing or showering water temperature is 38°C to 40°C, depending on personal preference.

For winter performance

We recommend sizing the water heater on a Delta T of 30 degrees C, shown as “Δt 30” for the majority of the UK, in Scotland, we recommend using “Δt 35”.

During the summer period

You can use “Δt 20” as the temperature rise.

SFK or SFC 28kW Gas fired condensing tankless water heater


Water Temperature RiseΔt 20Δt 25Δt 30Δt 35Δt 40Δt 50Δt 60Δt 75Flow rates
SFK / SFC 2820.6616.5313.7811.8110.338.276.895.51Litres/minute


Download the SFC 28kW Gas condensing water heater with no pump manual

Or with pump for hot water recirculation systems

Download the SFK 28kW Condensing water heater with pump manual 


SFK or SFC 34kW Gas fired condensing tankless water heater


Water Temperature RiseΔt 20Δt 25Δt 30Δt 35Δt 40Δt 50Δt 60Δt 75Flow rates
SFK / SFC 3425.0920.0716.7314.3412.5510.048.366.69Litres/minute


Download the SFC 34kW Gas condensing water heater manual with no pump manual

Or with pump for hot water recirculation systems

Download the SFK  34kW Condensing water heater with pump manual 

SFK 50kW Gas fired condensing tankless water heater


Water Temperature RiseΔt 20Δt 25Δt 30Δt 35Δt 40Δt 50Δt 60Δt 75Flow rate

Download the SFK  50kW Condensing water heater with pump manual 

Please be aware that the highest temperature that the water temperature can be set to is 60 degrees Celsius on the standard water heaters ; on request, we can, however, build a higher temperature unit that can reach up to 85 degrees Celsius. These water heaters are frequently used for the pasteurisation process in the dairy and farming industries, as well as in other applications that call for high-temperature washdowns and water for process applications.

For high demand hot water heating that commercial applications can require, these high efficiency commercial water heaters can be used in cascade for example- we go into more depth on the of this type installation here – Commercial gas water heaters

These on demand high efficiency gas water heaters have various essential safety features, and can be used to top up heat-pump water heaters, which can struggle to achieve the desired water temperatures.

Control options

You can set the water temperature on the control panel of the water heater; however, should you wish the facility to change the water temperature, then we have a remote controller that you can site in your kitchen to boost the water temperature to 50 °C for dishwashing, which is the recommended temperature for removing grease from cooking utensils and dishes.

Easy remote controller from Flexiheat UK; Class 5 modulating boiler or water heater controller that is wired.

Easy remote controller – Wired – for our condensing water heaters from Flexiheat UK


Storage options

We also supply three condensing storage tank water heaters options – which are available as either LPG or natural gas fired water heater and are all ErP compliant water heaters.

We do a 50kW water heater option with a 330-liter storage cylinder

A picture of a gas hot water tank or gas fired hot water cylinder from Flexiheat UK, which are used for water heating in large domestic, commercial, and industrial applications. These units are condensing gas water heaters.

Gas water heaters with storage systems for high volumes at intermittent times demand. These ErP compliant condensing water heaters are energy efficient and cost effective to run.


A 34kW water heater option with a 150 Litre stainless steel storage cylinder

gas hot water tank or gas fired hot water cylinder from Flexiheat UK

or a 28kW water heater option with a 150 Litre stainless steel storage cylinder

internal picture of our hot water heater with storage tank which is gas fired from Flexiheat UK

SFKA 28 or 34 KW Output models – Natural gas or LPG powered

We go into more depth on these storage condensing water heaters that are often used in commercial buildings here – Condensing gas hot water tanks or cylinders

Where are condensing water heaters used?

Everywhere, from residential and smaller business settings such as homes, cafes, hair salons, bed and breakfasts establishments to industrial and large commercial applications such as sports complexes, hospitals, colleges, or schools, we have a complete range of high-efficiency water heaters for every warm water heating requirement.

Contact us

Should you require anymore assistance, information, check stock or get a price / cost on our effective (both in a monetary and emission level) condensing water heaters range please contact our customer services sales team on 01202 822221 or use our email contact above on the web page.