Run the sequence backwards — heat pump first, fabric later or never — and you get the disappointing installations that fuel every sceptical dinner-party story: an oversized machine working hard against walls that throw the heat straight back out.
The proof on the public register
The clearest evidence we can offer is a late-Victorian mid-terrace on Howard Street in East Oxford: solid brick walls, originally an old wall-hung gas boiler with no thermostatic controls, rated EPC E (49) in 2013. The whole-house retrofit insulated the walls internally in wood fibre and lime, rebuilt and insulated the floors and roofs, replaced the windows, and installed controlled ventilation — and only after all of that did the heat source change. The gas boiler was removed and an air source heat pump installed, feeding underfloor heating across the ground floor and radiators upstairs, with zoned room-by-room controls.
The post-works certificate, dated January 2022, records EPC B (89) — with the heat pump and its zone control rated “very good”. The same heat pump dropped into the uninsulated 2013 version of that house would have struggled. Sequencing is everything.
The worked example: oil to heat pump, in the right order
Orchard House, a detached 1970 stone house near Banbury, shows what the sequence looks like when it is run deliberately from the start.
Before: an ageing oil boiler in a kitchen cupboard running at no better than about 70% efficiency, an external oil tank, a 162-litre cylinder, and a design-stage estimate of roughly 48,300 kWh of fuel a year. Distribution pipework ran through the uninsulated solid floor — old heating pipes were, in effect, warming the ground.
The sequence was fixed before any product was chosen: services first, openings next, fabric after, heat last — so the heat pump would be sized against the building it would actually serve. Electrics were renewed first (including a dedicated supply for the future heat pump), the windows and stone-mullion reveals were upgraded, the walls were internally insulated room by room, nine sloping-ceiling zones were insulated, and whole-house ventilation went in.
Only then was the heat system designed — and three details from that design stage are the ones every older-house owner should know about:
- Heat losses were calculated room by room on post-retrofit values. Not rules of thumb, not floor-area shortcuts: the improved envelope, room by room. This is what determines the size of the machine — and a smaller, correctly sized heat pump is cheaper to run, quieter and longer-lived than an oversized one.
- Every emitter was checked, not assumed. All sixteen existing radiators were photographed, logged and checked against their output at low flow temperatures — heat pumps run water far cooler than boilers — and replaced only where they would fall short. The conservatory’s existing wet underfloor heating was connected into the new system.
- The pipework was insulated — radically. The old distribution runs buried in the uninsulated slab were abandoned. Channels were excavated across the floor, new insulated runs laid and re-buried, so the heat now travels to the rooms, not the earth. Outside, the oil boiler, cylinder and tank all left the site, replaced by an air source heat pump on a prepared base with substantially larger insulated hot-water storage (a 17 kW unit and approximately 300-litre cylinder were referenced at design stage).
The result: the house was certified EPC B in December 2023 on the public register — a striking rating for a 1970 stone house that had been heated by oil a year earlier.
Emitters: the unglamorous decision that makes or breaks comfort
A heat pump’s efficiency rises as its flow temperature falls, so the emitters — radiators or underfloor heating — decide how low it can run.
- Underfloor heating is the natural partner: a whole floor of gentle warmth needs only low water temperatures. On Howard Street the suspended timber ground floor was lifted, insulated between the joists, and underfloor heating laid over on metal trays; on a North Oxford Victorian semi the existing underfloor heating was extended across the whole lower-ground floor on diffusion plates above new floor insulation — fabric and emitters upgraded in the same operation.
- Radiators work too — checked. Some existing radiators are already generous enough at low temperatures; others need swapping for higher-output units. The Orchard House method — log every emitter, verify against calculated room loss, replace only what falls short — spends money where the calculation says, not where habit does.
- Controls and storage complete the system: zoned controls, weather compensation where specified, and a larger, well-insulated cylinder, because low-temperature systems store heat rather than blasting it.
“Designed and coordinated by us, installed with our managed certified partners” — what that means for you
We are a fabric-first retrofit contractor, and we are precise about roles. On our projects, heat pump systems are designed and coordinated by us, and installed with our managed certified partners. In practice:
- We design the whole-house programme — the insulation, airtightness and ventilation that make the heat pump viable, and the sequence that puts it last.
- The room-by-room heat-loss basis, emitter checks and system integration are coordinated within that programme, so the heating design reflects the fabric as improved, not as found.
- A certified heat pump installer within our managed team carries out the installation, registration, notification and commissioning that the system requires — the certification sits with the people who hold it.
- You deal with one accountable programme rather than refereeing between an insulation contractor and a heating firm who have never met.
That division of labour is not a caveat; it is the point. The heat pump is the last 20% of a good result. The first 80% — the fabric that lets it work — is exactly the part a heating installer alone will never touch.
This article is part of our whole-house retrofit guide. See the full projects: Howard Street, East Oxford and Orchard House, near Banbury, and our guide to ventilation after insulating.