Vaillant aroTHERM Retrofit: 4.7 SCOP Achieved in an Oxford 1930s Detached Home

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.

System Snapshot

Technology: 10kW Vaillant aroTHERM plus (R290)

Property Type: 1930s Detached with loft and rear extensions

Design Flow Temperature: 40°C at -2.9°C DOT

Peak Heat Loss: 8.83kW

Measured Efficiency: 4.7 SCOP

Water Quality: VDI 2035 (Demineralised)

Overview

The transition from an ageing gas combination boiler and restrictive microbore pipework to a low temperature air source heat pump (ASHP) highlights the value of first principles engineering. Smiths Heating Services Ltd, a multi generational firm of Vaillant Master Technicians, designed this system to deliver elite efficiency in a traditional Oxford property without further fabric improvements.

Master Heat Engineer

Technical Specifications

• Primaries: 35mm copper

• Hydraulics: Open loop with weather compensation

• Controls: Vaillant sensoCOMFORT with external sensor and internet gateway

• Buffer/Volumiser: 100L unit installed in loft eaves

• Hot Water: Unvented cylinder installed in a modified bedroom cupboard

• Water Treatment: Full system flush and fill to VDI 2035 standards

• External Pipe Insulation: Primary Pro

The Engineering Challenge:

The project encountered significant administrative and spatial hurdles. Because the installation was planned for late 2024, prior to the May 2025 changes in permitted development rules, planning permission was a mandatory requirement. Despite the system easily meeting MCS noise criteria, Oxford City Council insisted upon a full noise impact assessment to BS4142. This added £750 to the project costs and caused significant delays before the eventual April 2025 start date.

A primary technical challenge was the decommissioning of the existing microbore heating system, which was in poor condition and restrictive to flow. Transitioning from a combination boiler also required a new domestic hot water (DHW) plant space. The team extended an existing small cupboard in a bedroom study to accommodate the cylinder, diverter valve, and controls. Furthermore, the required 100L volumiser proved too large for the bedroom cupboard and was strategically relocated to the loft room eaves space.

Property Background & Fabric

The detached 1930s residence featured a complex thermal envelope including a single storey rear extension and a loft conversion. While the property had retrofitted cavity wall insulation, double glazing, and loft insulation, the suspended wooden floors remained uninsulated. The homeowner sought a high comfort, low carbon solution and specifically selected the installer via the Vaillant 'find an installer' as they had been very happy with their Vaillant boiler.

System Design & Innovation

Using Spruce software, the team established a room by room heat loss of 8.83kW. To achieve a 40°C design flow temperature without further radiator upgrades, the team identified one specific room that fell short of the required output using standard bottom opposite end (BOE) connections.

By converting this specific radiator to a Top Bottom Opposite End (TBOE) configuration, the team gained a 9% increase in output over standard UK BOE connections. This innovative use of hydraulic connection geometry allowed the entire system to maintain the target 40°C flow temperature while still achieving a 21°C room temperature on the design day.

Performance and Results

During the first annual service, Tommy Smith, a former Worcester Bosch WAI Apprentice of the Year, confirmed an SCOP of 4.7. Water quality testing verified a conductivity of 88µS and a pH of 8.4. The homeowner reports a transformation in thermal comfort and significant economic gains.

Thanks to the Patrons

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Closing Thoughts

I first met Richard and his family at the Installer Show. It’s nice when I meet people who have been listening to the podcast. One of the things we immediately discussed was his son was having to teach his college lecturers what best practices in the industry were. A reminder that this industry is very disjointed.

You will notice this install was filled with demineralised water to reduce corrosion rates in the system. This method, which was introduced by boiler manufacturers out in Germany, is being viewed by engineers as a better way to control corrosion than chemical inhibitors.

Unfortunately, we have a sad, sad situation where the chemical inhibitor manufacturers such as ADEY don’t like engineers using other methods and are wrongly telling engineers that inhibitors are mandatory under building regulations… It seems ADEY wouldn’t know what a building reg was if it fell on their head.

Have a great weekend everyone.

Nathan

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