Preparing Your London Home for a Heat Pump: What Needs to Change

Why Heat Pumps Need Different Distribution

A gas boiler in a London Victorian terrace property typically operates at a flow temperature of 70 to 80 degrees Celsius — this is the temperature of water leaving the boiler and entering the radiators. Radiators are sized on the assumption of this high flow temperature. An air source heat pump operates efficiently at flow temperatures of 35 to 55 degrees Celsius. At lower flow temperatures, the same radiator emits significantly less heat — the heat output from a panel radiator is approximately proportional to the temperature difference between the radiator surface and the room, and a radiator running at 45 degrees emits considerably less heat into a 20-degree room than the same radiator running at 75 degrees. If a heat pump is connected to an existing London property without assessing and upgrading the radiators, the rooms will be underheated and the system will either run continuously at elevated flow temperatures — reducing efficiency — or fail to maintain the design room temperatures in cold weather.

Heat Loss Calculation

A heat pump installation carried out to a professional standard begins with a room-by-room heat loss calculation. The calculation determines how much heat each room loses to the outside in the design cold weather condition — typically minus 3 degrees Celsius for London, based on the CIBSE design winter temperature for the South East. The heat loss depends on the room dimensions, the construction of the external walls (solid brick, cavity brick, insulated cavity), the floor construction (suspended timber, concrete slab), the window type and area (single-glazed, double-glazed, secondary-glazed), and the ceiling construction. For a typical London Victorian terrace with solid brick external walls, the heat loss per square metre of external wall is considerably higher than a post-1990 cavity wall construction. The heat loss calculation produces a required heat output for each room in watts, which determines the minimum radiator size needed at the design heat pump flow temperature.

Radiator Sizing and Replacement

Existing radiators in London Victorian terrace properties are almost always undersized for heat pump operation at 45-degree flow temperatures. A radiator that was correctly sized for a gas boiler at 75-degree flow will need to be significantly larger to deliver the same heat output at 45 degrees. The replacement approach is typically to fit double-panel double-convector radiators in place of existing single-panel or double-panel single-convector radiators — the additional convector fin surface on a double-panel double-convector model approximately doubles the heat output for a given surface area and flow temperature. In some rooms with restricted wall space, fan-assisted radiators or low-surface-temperature fan convector units can deliver the required heat output in a more compact form factor.

In London properties where the bathroom, kitchen, or hallway layout makes standard panel radiator replacement impractical, fan-assisted skirting board heaters or active low-temperature ceiling panels are alternative distribution solutions. The heat loss calculation tells the installer what heat output is needed in each room; the distribution solution is then chosen to deliver that output at the design flow temperature within the physical constraints of the space.

Underfloor Heating Compatibility

Underfloor heating systems operate at flow temperatures of 35 to 40 degrees Celsius, which aligns closely with efficient heat pump operation. For London properties with ground-floor extensions — the rear kitchen extension that is standard in Victorian terrace conversions — underfloor heating in the extension is an excellent heat pump-compatible distribution choice. The screed or slab floor acts as a thermal mass that absorbs heat slowly and re-emits it gradually, which suits the continuous low-temperature operation of a heat pump better than the intermittent high-temperature operation of a gas boiler. Mixing underfloor heating on the ground floor with upgraded radiators on the upper floors is a common and effective distribution strategy for London terrace heat pump installations.

Hot Water Cylinder Requirement

A combi boiler — the dominant boiler type in London flats and smaller terraces — provides domestic hot water on demand by heating mains water through an internal heat exchanger as it is drawn off. This instantaneous hot water production requires high-temperature heat input that a heat pump cannot efficiently provide. A heat pump cannot replace a combi boiler directly: a hot water storage cylinder is required. The heat pump heats the cylinder to 55 degrees Celsius during off-peak hours, and domestic hot water is drawn from the stored volume. An unvented cylinder — which connects directly to the mains cold supply and delivers pressurised hot water — is the preferred option for London properties. A 200 to 250-litre cylinder is typically required for a two to four-person household.

BUS Grant and Eligibility

The Boiler Upgrade Scheme (BUS) provides a grant of 7,500 pounds toward the cost of an air source heat pump installation in England and Wales as of 2026. The grant is paid directly to the MCS-certified installer and reduces the cost to the homeowner. To be eligible, the property must have a current Energy Performance Certificate (EPC) with no outstanding recommendations for loft insulation or cavity wall insulation — if the EPC lists these measures as required, they must be completed before the BUS grant application can be submitted. London Victorian terrace properties with solid brick external walls cannot have cavity wall insulation by definition, so this requirement typically does not apply to them. Loft insulation, however, is required to be present or confirmed as already installed. Contact Prestige Engineers for heat pump readiness assessments and radiator upgrades across all London boroughs.