Can Existing Radiators Work with a Heat Pump? Essex aroTHERM plus Retrofit Achieving SCOP 3.7

Helping YOU find good heating engineers. We share case studies from engineers in the Guild of Master Heat Engineers to help people find top installers, help gas and oil engineers increase their knowledge around heat pumps, and provide a solution for third-sector professionals to understand the industry better.

Overview

This project demonstrates how a Vaillant aroTHERM plus 7kW air source heat pump can operate efficiently with existing radiators and underfloor heating, achieving a measured SCOP of 3.7 under real-world conditions.

The retrofit was carried out in Essex, with the heat pump installed on the flat roof for optimal airflow and a discreet footprint.

The property has a heated floor area of 181.7 m² and the system was designed to maintain 21°C indoors at an outdoor design temperature of −2°C, with a design flow temperature of 45°C at −2°C.

Monitoring is provided via a Sontex heat meter and OpenEnergyMonitor, allowing continuous measurement of electrical input, thermal output, and real-time COP.

Engineering Context

Many heat pump installations introduce buffers, low-loss headers, or hydraulic separation when existing heating systems present flow restrictions.

However, these additional components can increase complexity, reduce efficiency, and increase installation cost.

This case study demonstrates how careful hydraulic design and pipework optimisation can allow a heat pump system to operate as a single open loop circuit, without hydraulic separation, while achieving high efficiency and maintaining existing radiators.

Master Heat Engineer

Key Specifications

The Challenge:

The homeowner wanted to transition to low-carbon heating while integrating the heat pump with:

• a large solar PV array

• battery storage

• a time-of-use electricity tariff

• domestic hot water immersion connected to a MyEnergi Eddi solar diverter

Challenges included:

• underfloor heating manifold with separate circulation pump and blending valve, creating mixed temperatures and hydraulic separation

• pipework restricting flow to the index circuit, which could have required additional components if left unchanged

System Design & Engineering Approach

The installation was engineered as a complete hydraulic circuit, not just a simple heat source replacement.

Key design decisions:

1. Heat loss calculations confirmed the property could operate at 45°C flow temperature with existing emitters.

2. Pipe sizing calculations determined that sections of the distribution system needed upsizing to ensure the circulator could overcome system resistance.

3. Hydraulic simplifications included:

   • Removal of the UFH circulation pump and blending valve

   • Increasing distribution pipe diameters to 28mm in key sections

   • Reconfiguring the system as a single open loop

4. Controls and monitoring:

   • Weather compensation ensures flow temperature is automatically reduced when outdoor temperatures rise

   • Sontex heat meter allows continuous performance monitoring

5. External pipework insulated with Primary Pro insulation to maintain efficiency.

Installation

Installation was completed with minimal disruption, including:

• Removal of UFH pump and blending valve

• Upgrading distribution pipework

• Installation of a Newark Heat Geek Super Cylinder

• Installation of Sontex heat meter

• External pipework insulation

• System balancing and commissioning

The heat pump was installed on the flat roof, providing excellent airflow and a discreet footprint.

Measured System Performance

The system is continuously monitored using OpenEnergyMonitor:

• Electrical input

• Thermal output

• Real-time COP

• Seasonal performance trends

Results:

• SCOP 3.7 under real-world conditions

• No radiator replacements required

• Fully open loop heating circuit

• Integrated with solar PV and battery storage for overnight load shifting

Design Insight

Initial pipe sizing calculations revealed the index circuit resistance exceeded the pump’s residual head, which would normally require hydraulic separation.

By upsizing pipework and removing unnecessary UFH components, the system operates as a single open loop circuit:

• Radiators and underfloor heating operate at the same flow temperature

• Weather compensation maximises efficiency throughout the season

• Complexity is reduced while performance is validated with real-world measured data

Key Engineering Lessons

• Before selecting equipment, assess the heating system as a complete hydraulic circuit, rather than simply replacing the existing heat source.

• Perform accurate heat loss and pipe sizing calculations to ensure the system can operate within the available residual head of the heat pump circulation pump.

• Simplify the hydraulic layout: remove unnecessary pumps, blending valves, buffers, or low-loss headers where possible.

• Upsize distribution pipework to reduce system resistance and avoid additional hydraulic components.

• Integrate controls with weather compensation to optimize flow temperatures for efficiency across the heating season.

• Monitor, if possible, performance in real-time using meters and data logging to validate SCOP and system performance.

• Retain existing emitters where possible, as careful design can allow radiators and underfloor heating to operate efficiently without replacements.

• Design for minimal disruption to the homeowner while maintaining high system efficiency.

Estimated Calculated Heat Loss (kW) at Various Outside Temperatures

Thanks to the Patrons

The Guild of Master Heat Engineers is supported by our Patrons.

Learn more about our Patrons here.

Closing Thoughts

This Essex retrofit demonstrates that high-performing heat pump systems are achieved through smart engineering, not added complexity. By carefully addressing hydraulic layout and system resistance, Bromley Energy were able to:

• Retain existing radiators and underfloor heating

• Avoid buffers, low-loss headers, and hydraulic separation

• Achieve a measured SCOP of 3.7 under real-world conditions

• Provide transparent performance data via OpenEnergyMonitor

These design decisions, guided by a Master Heat Engineer, show that expert engineering and real-world monitoring deliver efficient, low-carbon heating without unnecessary components or disruption. This installation highlights how a Master Heat Engineer’s approach: thoughtful, measured, and data-driven consistently achieves high-performance results.

Have a great week everyone.

Nathan

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