Boiler flow temperature for central heating regulations Part L June 2023

Boiler flow temperature

Boiler flow temperature ? hot water heating systems in newly constructed homes or commercial buildings must now be designed to operate at a maximum flow temperature of 55°C as a result of changes to the Building Regulations.

The low flow temperature information

The latest revisions to Part L of the Building Regulations seek to reduce emissions by 31% for both new build houses and home renovations and additions, which is a crucial milestone in the UK’s journey towards net zero.

Since the year-long transition period finished on June 15, 2023 (it did not apply to work subject to a building notice, full plans application, or initial notice submitted before that date, provided the work for each building is started before June 15, 2023), the original measures that went into effect on the 15th June 2022 apply. Heating installers must be aware of the new regulations that now apply without fail.

These modifications served as a significant first step towards the Future Homes Standard, which targets reducing emissions from all new construction by 75–80% by 2025.

Part L of the building regulations

For both new builds and home extensions or renovations, the stipulated modifications to Part L define a 31% decrease in carbon emissions compared to prior requirements. Part L, which places a strong emphasis on low-carbon heating systems, also emphasises a fabric-first strategy. Changes include an improvement in fabric airtightness and insulation standards, better U-values, and thicker insulation in homes.

For buildings, Part L of the building regulations is divided as follows:

New-build homes and buildings: Part L1A Regulations

This document outlines the energy efficiency requirements for new construction. Approved Document, Part L1A

It describes how all new houses will now be valued using the SAP10 Standard Assessment Procedure (SAP) formula. This approach, which was previously derived using SAP 2012, has been updated to SAP 10. Greater insulation needs and a new target primary energy rate, expressed in kWhPE/m2 per year, are defined as a result of the SAP calculation being performed at the design stage. As a result, achieving SAP compliance will rely heavily on good water heating system design.

Additionally, 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 were allowed), as required by Part L1A. This will make it much simpler for houses to be retrofitted with air or ground source heat pumps in the future, when they replace a condensing boiler, be they condensing gas boilers or an oil condensing boiler as the radiators and pipework will be sized correctly for the central heating to work at these temperatures and produce maximum efficiency.

Existing properties and buildings, Part L1B

 Part L1B, an approved document, discusses energy and fuel conservation in existing homes.

Even though Part L1B acknowledges that it is not always practicable to achieve new build standards.

Quote: “Where it is not feasible to install a space heating system that can operate at this flow temperature of 55 °C or lower (e.g., where there is insufficient space for larger radiators, or the existing distribution system is provided with higher temperature heat from a low carbon district heat network), the space heating system should be designed to the lowest design flow temperature possible that will still meet the heating needs of the dwelling.

Source: HM Government – Part L 2021 edition incorporating 2023 amendments, for use in England, Page 36

Part L1B specifies that wet central heating systems that are new or being replaced shall, whenever practicable, be sized at a maximum boiler water flow temperature of 55°C.

Importantly, the regulations stipulate that if a thermal element ( wall, roof, or floor) of the home is being renovated or replaced, the work must be done to Part L1A requirements.

Commercial/non-domestic buildings are covered in Part L2a

Again, same regulations for the maximum flow temperature of 55°C or lower, but this document refers to commercial and industrial heating applications that are not domestic dwellings.

Referring document / legislation is here –

Part L2a requirements for energy efficiency compliance for commercial/non-domestic buildings

Low temperature heating advantages

Lower heating flow temperatures are now mandatory in the design of an increasing number of household heating systems, whether they utilise a gas boiler or a heat pump. Installing a condensing gas boiler on a low temperature system will result in cost savings for homeowners and reduce energy bills.

The boiler will always operate in condensing mode while the boiler temperature is set to a flow temperature of 55°C or lower. This makes the boiler and heating system far more effective and guarantees that they are always capturing the latent heat from condensation. As a result, the boiler will run as close to its condensing efficiency as possible, which will result in cheaper energy costs for homeowners.

All radiators and piping installed as part of a new or replacement system must be sized for a maximum flow temperature of 55°C, according to Part L. As a result, it could be necessary to use larger pipework and install radiators of the correct size to adequately heat the space.

When done appropriately, this work can significantly contribute to future-proofing a building so that low carbon technologies like heat pumps, which also employ low flow temperatures, can be installed in the future.

“A gas boiler will function as closely to its condensing efficiency at the boiler’s flow temperature of 55°C (55°C is the dew point temperature at which the condensing mode starts to work in gas boilers, as the flue gases condense and they can recover the latent heat), which will reduce a homeowner’s energy heating bills.”

In short, a lower boiler water flow temperature means reduced heating bills for your central heating system.

How to design and install a low temperature heating system and what to think about

Installers must take into account the wider system as well as the overall design of the building, including heat loss calculations, because the design of a low heating flow temperature system differs from that of a traditional higher boiler’s flow temperature system. By doing this, they can ensure that any future switch from a gas boiler to a heat pump can be made smoothly.

Heating flow temperature

The most important component of the regulations for the majority of installers will be Part L1(b), which focuses on the conservation of fuel and electricity in existing residential dwellings. Part L1(a) addresses the energy efficiency of newly constructed homes (learn more about what Part L means for newly constructed homes here).

It is crucial to note that it specifies that new or replacement wet central heating systems should be built to operate at a maximum flow temperature of 55°C rather than the 75°C typical of conventional heating systems to ensure boiler efficiency in modern boilers that are all now condensing boilers.

Heating system

1. A minimum of insulation All pipes linked to the hot-water storage are subject to thicknesses. Insulation must be at least 5mm thick for pipes having an internal diameter of 10mm, 10mm for pipes with an internal diameter of 22mm, and 15mm for pipes with an internal diameter of 28mm.

2. Thermostatic radiator valves are needed for every installation, with the exception of heat pump installations, which need volume for defrost.

3. For brand-new and completely replaced systems, the maximum flow temperature is 55°C.

4. Insulation is required for all principal hot water pipes used for home hot water.

5. When passing through voids, secondary central heating pipes must be insulated.

6. Multi-zoning is necessary if the total floor space is 150 m2 or greater.

7. When passing through voids, primary central heating pipes must be insulated.

8. In accordance with BS 7593, all primary hot-water and central heating circuits should be properly cleaned, flushed out, and dosed with inhibitor on each installation.

What does this mean for condensing combi boilers?

A combi boiler produces hot water on demand and has no hot water cylinder attached to the boiler, unlike a system boiler. The boiler flow temperature setting on a combi boiler for the heating side of the condensing combi boiler can be set to a thermostat temperature of 55C, and you can achieve the standard hot water temperature required of 40C for the domestic hot water side of the combi boiler with ease with our combi boiler range.

These are listed here

Our fully modulating 24kW combi boiler in natural gas or LPG  

our 35kW fully modulating combi boiler in natural gas or LPG 

or our largest combi boiler 50kw combination boiler in natural or LPG gas 

Hot water cylinders

Lower flow temperatures can cause issues for a hot water cylinder, as domestic hot water needs to be stored at 60 °C to avoid legionella. This means that the boiler flow temperature to the heat exchanger coil in an indirect hot water cylinder must be higher than the hot water temperatures within the cylinder for the heat to transfer to the domestic hot water. Newer boiler models these days have a system that will ramp up the boiler’s flow hot water temperature temporarily to achieve the correct temperature and heat the hot water cylinder.

Alternatively, immersion heaters can be used to top up the heat required for the hot water cylinder


Changing your boiler temperature, or now, as required, designing a lower flow temperature system, is now mandatory. Whether you’re using a combi boiler or system boilers, the boiler temperature settings must be a maximum of 55 °C and the system designed accordingly, as they have to be future-proof for low carbon heating solutions such as heat pumps.

With a boiler flow temperature of 55 °C and a return temperature that is even lower, this allows the boiler to recover the latent heat for the condensing process, lowering the heating bill / gas bills, as recommended by the energy saving trust.