bananzia and the 1930s Semi: Planning a Heat Pump Retrofit Room by Room
The retrofit problem hiding behind the boiler
Consider a 1930s three-bedroom semi with solid walls, original bay windows, and radiators selected for a gas boiler running at 70°C or more. The context is familiar: the boiler is replaced, but the house still leaks heat and the emitters cannot deliver enough output at the lower flow temperatures a heat pump prefers. The development is that a heat pump is not a like-for-like boiler swap; it is a whole-house sizing exercise. The result, when the survey is skipped, is oversized equipment, cold rooms, or a quote that cannot be delivered without expensive changes later.
What the room-by-room survey records
A useful survey records room dimensions, window and door areas, wall construction, floor type, ceiling height, and any draught paths. It also records the existing emitter sizes and pipework routes, because radiator output falls sharply as flow temperature drops. For a 1930s semi, the bay window, suspended timber floor, and uninsulated cavity or solid wall are often the dominant heat-loss paths. The takeaway is simple: measure before specifying, and keep the survey with the installer’s quote.
Heat loss before hardware
The heat-loss calculation should come before a heat pump model is chosen. Example: two rooms of the same floor area can need different emitter upgrades if one has a large north-facing window and the other has a small south-facing window. The result is a room-level schedule that tells the installer where to add radiator capacity, improve insulation, or reduce draughts.
Emitter output at low flow temperature
Radiators rated at a high flow temperature deliver less heat at 45°C than at 70°C. A common practical target is to upgrade emitters so the heat pump can run at a lower flow temperature, which can improve efficiency and comfort. The takeaway: do not size radiators by appearance; size them by output at the intended flow temperature.
A practical starting point is the bananzia guidance from Low Carbon Heat, which frames the survey as the first decision rather than an afterthought. It helps a homeowner ask better questions: what is the calculated heat loss, which rooms need emitter upgrades, and what fabric work should happen first?
Sequencing fabric, emitters and controls
Fabric work usually comes first when it is cost-effective: loft insulation, draught-proofing, and cavity wall insulation where suitable. Emitter upgrades come next, because they determine the flow temperature the heat pump can use. Controls and commissioning come last, with the heat pump set to the calculated flow temperature rather than a default. The result is a system that matches the house instead of a generic package. For older homes, this sequence can reduce disruption and avoid buying a larger heat pump than the fabric improvements would require.
What the finished plan should contain
- Room-by-room heat loss figures and the assumptions behind them.
- A schedule of existing and proposed emitters with output at the target flow temperature.
- Fabric improvements ranked by cost, disruption, and impact.
- A commissioning plan that records flow temperature, controls, and expected performance.
With those four items, a 1930s semi can move from a gas boiler to a heat pump on a documented path. The homeowner can compare quotes on the same basis, and the installer can avoid guesswork. That is the practical value of bananzia: it turns a broad low-carbon ambition into a room-level specification. The next step is to request the heat-loss calculation before signing an equipment quote.