Part L

With the transition deadline for Part L ending in June 2023, housebuilders need to ensure they’re confident about the changes and their responsibilities when it comes to constructing compliant homes.

While Part O mandates that a means of cooling a room must be included within the design, good insulation of pipes and cylinders, together with intelligent heating controls, can also work to keep properties cooler in summer, while retaining heat in cooler months.

A sum bigger than its parts

The recent changes to the building regulations provide an important stepping-stone to the Future Homes Standard, expected in 2025. These interim changes will provide the foundations on which the energy efficient homes of the future must be built. But what are the changes and what do they mean for new builds and retrofit projects?

Part O

Part O of the building regulations applies to new residential buildings only. With a primary aim to protect the health and welfare of occupants, Part O focuses on reducing the occurrence of high indoor temperatures, through the design and construction of homes that both limit unwanted solar gain and provide adequate means of removing excess heat. For housebuilders and specifiers, compliance can be demonstrated by using either the Simplified Method; reducing excess solar radiation via glass and windows by build orientation or external shading, together with adequate ventilation, or, via Dynamic Thermal Modelling; whereby the project must meet a CIBSE TM59 overheating assessment.

The uplift changes to Part L outline a 31% reduction in carbon emissions compared to previous regulations for both new builds and home renovations and extensions. With an emphasis on low carbon heating systems, importantly, Part L also focuses on a fabric first approach, with changes including an uplift in fabric insulation and air tightness standards, improved U-values, and increased thickness of insulation within extensions.

These changes come ahead of the Future Home Standard, which aims to reduce carbon emissions from all new build homes by 75-80%, when compared to 2013 regulations.

For residential buildings, Part L has been split out as follows:

New builds

Approved Document Part L1A: This document sets out the energy efficiency standard for new builds.

It outlines that all new homes will now be assessed under a new Standard Assessment Procedure (SAP) calculation called SAP10. Formerly calculated using SAP 2012, SAP10 is an update on this methodology. With the SAP calculation now made at the design stage, this sets greater insulation requirements and a new target primary energy rate, expressed as kWhPE/ (m² per year). As a result, effective heating system design will be key to achieving SAP compliance.

In addition, Part L1A states thermostatic radiator valves must be fitted on all radiators, while all new wet central heating systems should be sized for a maximum flow temperature of 55°C, (previously over 75°C), which will make it much easier for homes to be retrofitted with air or ground source heat pumps later down the line.

Existing homes

Approved Document Part L1B: This document covers the conservation of fuel and power in existing residential dwellings. Importantly, while Part L1B recognises that it is not always possible to meet new build standards, the regulations do state that if a thermal element (roof, wall or floor) is being replaced or renovated then the works must be done to Part L1A standard.

In relation to new or replacement wet central heating systems, Part L1B states these should be sized for a maximum flow temperature of 55°C, where possible

Go with the low flow

In recent years, more and more domestic heating systems – whether using a gas boiler or a heat pump – are being designed with lower temperatures in mind.

Part L puts a significant emphasis on insulation and building fabric, an important consideration for housebuilders and specifiers as the industry now readies itself for the Future Home Standard. Yet, a focus on low flow temperature systems is another key area specifiers will need to pay particular attention to.

Installing a condensing gas boiler on a system with a maximum flow temperature of 55°C has several advantages for homeowners, as the boiler would run in condensing mode when heating – which is significantly more efficient. By using a low flow temperature, the system ensures that the boiler is always capturing the latent heat from condensation. This means it will operate as close to its condensing efficiency as possible – which has the potential to lower a customer’s energy bills.

In addition, Part L states that all pipework and radiators, as part of a new or full replacement, should be sized for a maximum flow temperature of 55°C. This could mean that larger pipes are required, and radiators will need to be sized to provide sufficient heat to the room. When carried out correctly, this work can provide significant advantages in terms of preparing the property for low carbon technologies such as heat pumps, which also utilise low flow temperatures, in the future.

A design for life

The design of a low temperature system differs from that of a traditional system in several ways. It’s therefore important housebuilders and specifiers consider the wider system along with the overall design of the property, including heat load calculations, to allow any future change from a gas boiler to a heat pump to be made smoothly.

Another consideration for housebuilders and specifiers is additional accessories, such as high recovery cylinders, which can be used as part of a full Vaillant system solution to improve energy efficiency. High recovery cylinders, which can be installed alongside a traditional open vent, system boiler, and/or heat pumps, offer a high efficiency water storage solution. Vaillant’s uniSTOR range of unvented cylinders operate efficiently and offer improved insulation to minimise heat loss which can help lower running costs. For homeowners, this means they can time their hot water for whenever it is needed, without the need to run the heating system constantly.

Through the Vaillant Standard, we can offer expert consultation to housebuilders and specifiers throughout the entire specification process, from design right through to after-sales support.

Working with our team of dedicated professionals, specifiers can unlock fully indemnified system designs for all products, whether that’s a high efficiency boiler or heat pump. By understanding the bespoke requirements of your project, our team works with you to develop a heating system that’s future ready, today.

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Part L - Preparing for the future