Low temperature heating: what installers need to know

Changes to the Building Regulations mean domestic heating systems now need to be designed to run at a maximum flow temperature of 55°C in new build properties. Here’s the low temp lowdown

Decarbonising domestic heating is a vital step on the UK’s path to net zero – and the latest changes to Part L of the Building Regulations aim to cut emissions by 31% for both new build properties, home renovations and extensions.

These changes provide an important stepping stone to the Future Homes Standard, which aims to slash emissions from all new builds by 75-80% by 2025.

The interim measures came into effect in June 2022 – and with the year-long transition period ending on 15 June 2023, it’s vital that heating installers understand the new requirements.

While Part L1(a) covers the energy efficiency of new build homes (read more about what Part L means for new builds here), the key section of the regulations for most installers will be Part L1(b), which focuses on the conservation of fuel and power in existing residential dwellings.

Crucially, it states that new or replacement wet central heating systems should be designed to run at a maximum flow temperature of 55°C, instead of the 75°C typical of traditional heating systems.

Nowadays, more and more domestic heating systems – whether they use a gas boiler or a heat pump – are being designed with lower flow temperatures in mind. In fact, installing a condensing gas boiler on a low temperature system could even save homeowners money.

At a flow temperature of 55°C, the boiler will always run in condensing mode when the heating is on. This ensures it’s always capturing the latent heat from condensation and is much more efficient. So, the boiler will operate as close to its condensing efficiency as possible – which has the potential to lower a homeowner’s energy bills.

Part L also states that all pipework and radiators installed as part of a new or replacement system 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 appropriately sized to provide sufficient heat to the room.

When carried out correctly, this work can go a long way towards future-proofing a property so low carbon technologies such as heat pumps – which also use low flow temperatures – can be fitted at a later date.

"At a flow temperature of 55°C, a gas boiler will operate as close to its condensing efficiency as possible – which has the potential to lower a homeowner’s energy bills"

As the design of a low temperature system differs from that of a traditional system, it’s important that installers consider the wider system along with the overall design of the property – including heat loss calculations – to ensure any future change from a gas boiler to a heat pump can be made smoothly.

Installers should also think about using accessories as part of a Vaillant full-system solution to improve energy efficiency. For example, Vaillant’s high-recovery cylinders can be installed alongside an open-vent boiler, system boiler or a heat pump to offer a highly efficient 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. Available in sizes from 150 to 800 litres, they also have a Global Warming Potential of less than 5 – so, when combined with a heat pump, they’ll provide a high-performing, sustainable heating system.

For homeowners, they can time their hot water for whenever it’s needed – and our high-recovery cylinders heat up much quicker than traditional boiler cylinders, too.

1. Minimum insulation thicknesses apply to all pipes connected to hot-water storage. A pipe with an internal diameter of 10mm must have insulation at least 5mm thick, a 22mm pipe needs 10mm insulation, and a 28mm pipe requires 15mm insulation

2. TRVs are required on every install (excluding heat pump installations where volume for defrost is required)

3. The maximum flow temperature of 55°C applies to new and fully replaced systems

4. All primary hot water pipes for domestic hot water must be insulated

5. Secondary central heating pipes must be insulated when passing through voids

6. If the total floor area is 150m² or more, multi-zoning is required

7. Primary central heating pipes must be insulated when passing through voids

8. All central heating and primary hot-water circuits should be thoroughly cleaned, flushed out and dosed with inhibitor on every install according to BS 7593