Engineering efficiency into Exchange Square
A performance-led approach to electrifying commercial hot water
In the heart of one of London’s most ambitious commercial developments, a forward-thinking collaboration between Twenty One Engineering, and Mixergy has delivered a pioneering hot water generation system, focusing on a heat recovery solution at Exchange Square.
The result? A fully electrified hot water system powered by recovered energy from building cooling operations, proving it’s not only possible to eliminate gas, but to do so efficiently and cost-effectively.
The system cut total energy use by approximately 600 kWh per week, delivered a Coefficient of Performance (COP) of >5.0, and delivered a payback in under a year, evidence that performance-led electrification is commercially viable today.
This is a blueprint for how modern commercial estates can decouple commercial hot water from traditional building heating systems, delivering measurable carbon and energy savings.
Context. A strategic shift away from gas
One of the UK’s largest real estate investment trusts, is targeting a 75% carbon reduction and elimination of fossil fuels by 2030. In Exchange Square, legacy gas-fired cylinders from 2003 were due for replacement.
The challenge was clear, remove reliance on gas for hot water and avoid over-sizing replacement systems.
Twenty One Engineering’s forward thinking built in:
- Heat recovery from other building services by Integrating with the building’s VRF (VariableRefrigerant Flow) cooling systems
- Prove performance against real-world operational data
Traditional electric or gas replacements would have meant missed opportunities for energy reuse and system efficiency. A smarter solution was needed.
The Solution: Smart Heat Recovery with Mixergy Cylinders
Twenty One Engineering designed a system that recovered waste heat from the building’s lift motor room via a VRF unit. The recovered heat feeds a sanitary water heater unit, which delivers 70°C water to two 300L Mixergy smart hot water cylinders. The Mixergy cylinders operate using a volume control methodology, enabling them to intelligently store and discharge heat when available, rather than on demand.
Why Mixergy?
Conventional cylinders couldn’t cope with the variable heat input from the VRF. Mixergy’s ability to:
- Store heat flexibly using intelligent volume stratification
- Modulate how and when heating occurs based on system input*
- Operate efficiently with irregular heat sources
…made it uniquely suited to this integration. In essence, the cylinders act as thermal batteries, charging with excess cooling energy and discharging when hot water is needed.
*In this context the system does not require instant heat input whenever hot water is drawn. Instead, it stores heat in advance when recovered energy is available. Hot water is still available when needed, but it is produced more efficiently by pre-heating and storing at times of energy availability.
Highlights
| Element | Detail |
| Hot water Storage | 2 x 300L Mixergy cylinder |
| Heat Source | Recovered VRF heat via a sanitary water heater unit (Mitsubishi) |
| Peak Output Temp | 70°C water |
| Energy Source | Fully electric, REGO-backed (renewable electricity) |
| Control Integration | Modbus interface to BMS |
| Installation Location | Lift motor room – heat capture zone |
| Former System | Gas-fired cylinders (c.2003) |
Challenges & Learnings
Controls Integration
Due to BMS constraints, Mixergy’s machine learning was able to be disabled in favour of a Modbus control strategy.
Future updates will allow dual-mode operation: retaining machine learning while supporting external BMS control.
Right-Sizing: Rethinking Demand
Our client analysed two years of metered water use across their portfolio to move away from oversizing based on CIBSE defaults. They leveraged real-world data, especially from COVID-impacted usage patterns, to accurately size for actual demand.
Traditional designs were too big. Operational data gave us the right size for the system, and Mixergy’s smart storage enabled that flexibility.
Phil Draper, 21 Engineering
Performance. Evidence of commercial viability
Performance Gains
| Metric | Result |
| Gas Usage (Pre-2020) | ~850 kWh/week |
| Post-Electrification | 0 kWh/week from gas |
| Coefficient of Performance | 5.0 |
| Net Electrical Increase | +200 kWh/week (VRF) |
| Total Energy Reduction | Approx. 600 kWh/week |
| System Payback | Estimated <1 year on incremental cost |
Operational insights
Controls integration
The cylinders were operated via Modbus, enabling the BMS to coordinate heat recovery events. Mixergy’s machine learning was paused to align with the building’s wider control strategy. Future updates will allow dual-mode operation, combining AI optimisation with BMS integration.
Right-sizing strategy
The site used two years of metered consumption data to avoid oversizing. Mixergy’s volumetric control supports smaller storage volumes without compromising performance. This reduces capital cost, heat loss, and footprint.
Strategic Significance
This project redefines the role of hot water in commercial building design. Rather than a passive load, hot water becomes:
- A flexible energy sink for recovered heat
- A storage asset for grid-aligned operations
- A critical lever in reducing total gas dependency
Scalable Lessons for the Industry
Heat recovery combined with smart storage provides a commercially viable alternative to gas, enabling estates to decarbonise without compromising performance. Intelligent cylinders support accurate right-sizing, improving efficiency and reducing whole-life energy use. Success depends on close collaboration between controls engineers, product specialists and data teams to ensure systems operate as intended. The experience at Exchange Square shows that when these elements come together, electrification can deliver positive financial returns within the first year.
