The boiler in this SE13 end-of-terrace was dying. An Intergas combi, 36kW, leaking and unreliable. The homeowner knew it was on borrowed time and started thinking about what came next. They wanted something that would last, something efficient and something that would not require the garden to be dug up or the floors to be lifted. They had heard about heat pumps. They were open to them. But they also had a very clear message: do not make a mess of my house.
What they did not know (what most homeowners do not know until someone actually runs the numbers) was that their 36kW boiler was almost five times larger than their home actually needed. The property’s design heat loss, calculated properly at -2C outside and 21C inside, came out at 7.72kW. A heat pump, sized correctly to that figure, would cover everything the house required with room to spare.
This is the starting point that separates a well-designed heat pump installation from a disaster. Not the brand. Not the price. The heat loss calculation.
We installed a Vaillant aroTHERM Plus 7kW, paired with a Vaillant uniSTOR PURE 210-litre slim cylinder and sensoCOMFORT controls. Two radiators were upgraded to handle low-temperature flow. Weather compensation was set active from day one, with a starting heat curve of 0.9. The system carries a Heat Geek efficiency guarantee of 379 percent, because that figure was not a marketing claim, it was the product of the design calculation.
The pipework question is often where retrofits get complicated, and where costs can spiral. The instinct is to bury the primary pipes. The client did not want that, and in this case it was not necessary. We ran the pipework above ground on rubber feet, fully insulated in Primary Pro lagging, routed cleanly and deliberately. It looks considered rather than improvised, it is fully serviceable and it preserved both the garden and the budget.
Where to position the outdoor unit was its own conversation. The homeowner initially raised the idea of the flat roof. After a proper assessment (airflow, access for future servicing, aesthetics) ground level was the right answer. It was not the shortest pipe run we could have taken, but it protected a bed of established bamboo planting and kept the unit accessible without a ladder. These are the kinds of decisions that only get made correctly when there is a genuine conversation between installer and homeowner rather than a site visit followed by a quote.
The indoor space presented its own constraints. The cylinder cupboard was tight, tight enough that the hot water expansion vessel had to be installed horizontally. That is not the preferred orientation, but it sits within manufacturer tolerances and is fully compliant. The controls could not fit in the cupboard at all, so they were relocated beneath the stairs, with power and communication cabling separated cleanly and routed for future accessibility. None of this is heroic engineering. It is just the kind of careful thinking that tight spaces demand and that too many installations skip.
The result is a home that has moved from a leaking, wildly oversized gas boiler to a correctly sized heat pump system running at close to four times the efficiency. The homeowner got what they asked for: no trenching, no floor lifting, a clean finish and a system designed to last. What they perhaps did not fully anticipate was how different a heat pump designed from first principles feels to live with compared to the boiler it replaced.
That is the point. This is not a boiler swap dressed up in green language. It is a system redesign, carried out with calculation and constraint, in a period property that most people would assume was unsuitable for heat pumps. It was not unsuitable. It just needed someone to do the maths.
Michael Waring is the founder of Bromley Energy, a Heat Geek accredited installer based in South London and a member of the Renewable Heating Hub Installer Network.
