Hertfordshire Retrofit: 400%+ Efficiency Achieved With No Radiator Changes

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 involved the retrofit of a 5-bedroom, 250m² home located in Broxbourne, Hertfordshire. The property featured a mixed emitter system of existing Underfloor Heating (UFH) and radiators. Crucially, due to the home's good levels of insulation, the installation team was able to find a comfortable combined maximum flow temperature of 40 degrees Celsius without needing to alter any existing radiators.

The fundamental design utilised a fully open-loop system driven by weather compensation, with no influence from any room thermostats. The entire system was meticulously balanced via the manifold flow meters and the radiator lockshields to ensure optimal performance across all emitters.

Master Heat Engineer

Key Specifications

• Property Type: 5 bedroom, 250m² home (Retrofit)

  Emitter System: Mixed (Underfloor Heating and Radiators)

  Maximum Flow Temperature: 40°C (Achieved without radiator changes)

  Estimated Design Heat Loss: 8.68kW @ -2.5°C Design Outside Temperature (DOT)

  Validated Heat Loss (OEM Data): 7.63kW (1kW less than expected)

  Heat Pump Installed: Viessmann Vitocal 150-A 10kW

  System Configuration: Fully Open Loop, Weather Compensation driven, No room influence

  Cylinder Installed: 300L Heat Geek Cylinder (Newark) with 6m² coil

  Performance Validation: Achieved just over 400% efficiency (SCOP 4.0+)

System Design & Selection

The total heat loss for the property was initially estimated at 8.68kW. Based on this calculation, the team selected the Viessmann Vitocal 150-A 10kW unit, which comfortably covers the heat requirement at the design outside temperature (DOT) of -2.5°C.

The customer was initially nervous that the heat pump would be undersized for the house. However, the installation team reassured them the 10kW model was the correct choice. Post-installation, the real-world data validated this decision: after a year of monitoring, the heat demand tool on HeatpumpMonitor.org actually puts the heat requirement at 7.63kW—a full kilowatt less than initially estimated.

To support the customers' high hot water usage (heating the 300L cylinder several times daily), the team installed a high-specification Heat Geek cylinder from Newark, featuring a dedicated 6m² coil for enhanced heat transfer efficiency.

Performance Validation and Results

After a year of rigorous use, the system has achieved an exceptional efficiency of just over 400% (SCOP 4.0+). This is a great result, particularly considering the high usage pattern for domestic hot water.

Overcoming the Planning Challenge

A key challenge for this install was the need to secure planning permission for a double fan unit of this size. The process was complicated when the neighbour initially rejected the application, leading the council to attempt to mandate a costly acoustic assessment (around £1500).

The lead engineer successfully intervened, urging the council to accept the official MCS noise calculation that had been performed. After extensive communication, the application was approved, but under the proviso that the heat pump was positioned away from the house and boundary, facing down the garden. This specific positioning was executed precisely to meet these noise stipulations. The accompanying image visually confirms how the unit’s required placement was achieved.

Installation and Component Choice

For the primary pipework, 40mm MLCP was used to ensure plenty of unrestricted flow, critical for high efficiency. The hydronic side of the system was completed using quality components to ensure system integrity.

The entire installation was completed by a two-man team in approximately five days.

Estimated Heat Loss (kW) at Various Outside Temperatures

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Podcast

INTRO

I have just seen some news about Air to Air systems being allowed under the BUS scheme so thought it a good idea to revisit a recent episode I did about them.

Closing Thoughts

Hi everyone, first I would like to thanks everyone who has subscribed to the newsletter. Please tell your friends and work colleagues about it. I am trying to create a useful resource that will not only help customers find good engineers but also help heating engineers learn what current good practice looks like. It is also a useful resource for those working in the third sector around the decarbonisation of heat.

It was great to have another case study featuring my friend Alex. The interesting thing with this particular case study was planning and the positioning of the heat pump. Lot’s of good work has been happening around noise assessments and planning from the team at Nesta. If you don’t know who they are check them out. They have been brilient with engaging with the good on the ground engineers and helping bridge the gap between the engineering community and policy.

Have a great week everyone.

Nathan

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