Are we doing enough to design for low temperature heat in net zero buildings?

In its report to parliament in June 2025, The UK’s Climate Change Committee underscores how buildings and heating decarbonisation are crucial to meeting carbon budgets and net zero commitments. It highlights the scale of transition required in the heat sector and the urgency for effective delivery, beyond just technology adoption.

That transition, however, needs to lean on new ways of delivery, not just technology, says Harry Davis, founder and managing director at Eden Energy Partners. Here, he argues that as the industry accelerates towards electric, heat pump-led buildings, many designs still rely on legacy assumptions about heat distribution.

On paper, the journey towards net zero buildings looks clear. Across many sectors, gas boilers are being replaced with air and ground source heat pumps, electric networks are expanding, and policy is steadily tightening around operational carbon.

But there still exists a growing disconnect between the heat sources we are specifying and the buildings we are designing to receive that heat. If buildings are not designed from the outset to operate efficiently at lower flow temperatures, the performance gap is effectively designed in – regardless of how low-carbon the heat source may be.

Heat pumps are fundamentally different from the fossil fuel systems they replace. They work best when delivering heat at lower flow temperatures, typically in the 35 – 50°C range rather than the 70 – 80°C that traditional boiler-led systems operate at. Yet many building designs still assume high-temperature heat distribution as the default.

Low temperature heat is now a design parameter

For decades, the make up of heat distribution meant that systems would still deliver comfort because high temperatures masked inefficiencies.

That landscape has shifted somewhat, however, because when a building is served by a heat pump, system efficiency, and therefore running cost and carbon performance, is highly sensitive to return water temperatures. The lower the temperature the system can operate at while maintaining comfort, the higher the seasonal coefficient of performance (SCOP) and the lower the electricity demand. The Renewable Energy Consumer Code (RECC) backs this up, when in tests, found “significant performance gap between the design SCOPs and the in-situ efficiencies remains for both ASHPs and GSHPs (air and ground source heat pumps).”

In essence, this means heat distribution directly affects operational carbon, and flow temperature has become a core design parameter. If the building cannot comfortably be heated at low temperatures, the design team might be forced into a compromise by raising system temperatures, for example, which reduce efficiency, and increase plant size (and cost). But the outcomes don’t align with net zero guidance.

The ability of a building to operate at low flow temperatures is largely determined long before the detailed services design is finalised. By RIBA Stage 2, many of the most important decisions affecting future system temperatures have already been made.

For example, emitter type and area become critical because low temperature systems need larger heat transfer surfaces to deliver the same level of comfort. Underfloor heating, oversized radiators, trench heaters or active chilled beams can all be effective, but they require physical space, careful integration and early coordination with the architectural design. When emitter strategy is deferred with the assumption that it can be resolved later by the mechanical & engineering team (M&E), the result is often compact radiators sized around higher flow temperatures, which undermines heat pump efficiency from the outset.

Glazing ratios and overall facade performance also play a major role. High perimeter heat losses drive up peak heating loads, which in turn pushes system designers towards higher flow temperatures simply to maintain comfort near the facade during cold weather.

Ceiling heights and service zone allowances are another enabling factor. These dimensions influence whether larger emitters, deeper floor build-ups for underfloor heating, or alternative low-temperature distribution strategies can be accommodated without compromising the architectural intent. Tight zones and minimal build-ups tend to limit options and make higher temperature systems more likely by default.

Finally, zoning and spatial layout matter more than is often recognised. Deep-plan areas with high internal gains behave very differently from cellular spaces dominated by perimeter losses. System design temperatures should respond to that diversity, allowing different parts of the building to operate in different ways. That is only realistic, however, if these variations are understood and planned for early in the design process.

The danger of “net zero ready” in theory only

We are starting to see a pattern in performance evaluations, with buildings designed with heat pumps that technically meet compliance targets, but only achieve their modelled performance under narrow operating conditions.

In use, facilities teams raise flow temperatures to deal with cold complaints at the perimeter, slow heat-up times, or control instability. The heat pump still runs, but at a lower COP. Electricity consumption rises, carbon intensity increases during peak grid periods, and the operational performance gap widens.

On paper, the building is all-electric and therefore “low carbon”. In reality, it is working much harder than necessary to deliver the same comfort.

True net zero readiness is not just about swapping the heat source. It is about ensuring the building can maintain comfort at the lowest practical system temperatures from day one.

Heat transfer efficiency matters as much as heat generation

There is an understandable focus on the carbon intensity of heat generation, but we talk far less about the efficiency of heat transfer within the building. Yet improving heat transfer can deliver carbon savings just as significant as selecting a marginally more efficient heat pump.

Architects and specifiers influence this more than they might think. The choice to prioritise floor build-ups that can accommodate underfloor heating, to allow wall space for larger radiators, or to design facades that reduce perimeter loads are all decisions that support low-temperature operation.

If low temperature heat is treated as a core performance objective from concept stage, the design conversation changes. Some practical questions worth raising at RIBA Stage 2 include:

• What is our target system flow temperature at peak design conditions?

• How does the facade strategy affect perimeter heat loads?

• What emitter strategies are we assuming?

• Are plant space and riser routes sized for higher flow rates?

• How will different space types be zoned and controlled?

In doing so, the team can test whether the architectural concept is aligned with low-temperature operation, rather than assuming it can be engineered in later.

By technical design, RIBA Stage 4, the focus shifts from principle to proof. Useful questions at this stage include:

• What are the calculated return water temperatures under typical and peak loads?

• Are emitters sized based on realistic room temperatures and occupancy patterns?

• How robust is the system to part-load operation?

• Is there a clear strategy for commissioning and seasonal optimisation?

• What happens if occupants override setpoints or change usage patterns?

Designing out the performance gap

The industry has made huge progress in reducing fabric heat demand and adopting low-carbon technologies. But if we continue to design buildings around legacy assumptions about heat distribution, we will undermine those gains.

The performance gap is often discussed in terms of construction quality or controls. This is important, but so too is the design decisions that quietly determine whether a building can ever operate efficiently at low temperatures.

The conversations around net zero design seems to be changing, which is a good thing, and we are now having more conversations with architects and specifiers around not just the generation of heat, but also around is not only about how effectively we can deliver it.

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