Overview

Bromley Energy delivered a full heat pump retrofit in SE13, South London, to an early 1900s end-of-terrace property extended in the early 2000s.

The existing system was a 36kW Intergas combi boiler that was leaking, significantly oversized and approaching failure.

Rather than replace like-for-like, we redesigned the system from first principles using full heat loss calculation and the Heat Geek ZeroDisrupt™ framework.

The property’s true design heat loss was measured at 7.72kW at -2°C outside and 21°C inside.

We installed a correctly sized Vaillant aroTHERM Plus 7kW system, backed by a Heat Geek efficiency guarantee of 379 percent.

The Problem

Like many period homes across South London, this property had been running an oversized combi boiler for years.

The 36kW unit was almost five times larger than the property’s actual heat demand.

Issues included:

Boiler leakage and declining reliability
Severe oversizing leading to inefficient cycling
Poor long-term system design
Rising repair risk

The client did not want disruption, trenching or internal floor lifting.

They wanted:

Correct sizing
Long-term efficiency
Minimal disruption
Clean aesthetics
A future-proof heating solution

The Solution

We designed the system under the Heat Geek ZeroDisrupt™ methodology.

This meant:

No internal floor lifting
No 700mm ground trench excavation
Minimal aesthetic compromise
Full low-temperature system design
379 percent efficiency guarantee

> Heat Pump: Vaillant aroTHERM Plus 7kW
> Cylinder: Vaillant uniSTOR PURE 210L Slim
> Controls: Vaillant sensoCOMFORT
> Radiator upgrades: 2 replacements
> Weather compensation active
> Starting heat curve: 0.9

The unit output closely matches the calculated 7.72kW design heat loss, avoiding oversizing and ensuring stable continuous operation.

This is engineered matching, not guesswork.

Technical Design Strategy

Primary Pipework

Rather than excavating the garden, we designed an above-ground primary pipe route:

Pipework mounted on rubber feet
Fully insulated in Primary Pro lagging
Deliberate, clean routing
Fully serviceable installation

This preserved budget and garden integrity without compromising performance.

Outdoor Unit Positioning

The homeowner considered placing the unit on a flat roof.

After assessment, we agreed on a ground-level position that:

Protected established bamboo planting
Maintained airflow compliance
Preserved aesthetics
Ensured future service access

It was not the closest position possible, but it was the correct solution for the client.

Plant Room Constraints

The cylinder cupboard space was extremely tight.

We:

> Maintained symmetrical layout
> Preserved full service access
> Optimised pipe geometry

Due to space constraints, the hot water expansion vessel was installed horizontally within manufacturer tolerances. Not our preferred orientation, but safely executed and fully compliant.

Controls & Cabling

No control space was available within the cylinder cupboard.

We relocated system controls beneath the stairs and ensured:

Clear separation of power and communication cabling
Logical routing
Future accessibility
Clean finish

These details protect long-term reliability.

Performance & Efficiency

Design heat loss: 7.72kW
Installed unit: 7kW aroTHERM Plus
Low temperature curve: 0.9 starting point
Radiator optimisation: 2 upgrades
Weather compensation active

The system replaces a 36kW gas combi with a correctly sized, low-temperature heat pump operating at nearly four times the efficiency of a traditional boiler.

This is not a boiler swap.

It is a full system redesign.

The Result

The homeowner now has:

> A properly matched heat pump
> Stable, continuous low-temperature heating
> ZeroDisrupt pipe strategy
> 379 percent efficiency guarantee
> Garden-sensitive outdoor placement
Engineered plant room layout under tight constraints

This project demonstrates what modern retrofit looks like when executed with calculation, restraint and engineering discipline.