System specification
Vaillant aroTHERM plus 8 kW air source heat pump + Vaillant ecoTEC plus 832 32 kW backup boiler in cascade configuration via eBUS protocol. multiMATIC sensorENGINE controller managing handover. 250L hybrid-mode hot water cylinder. Original cast-iron radiators preserved across 9 rooms; one bathroom radiator upgraded to towel rail at owner's request (not heat-loss-driven).
Property
4-bed Grade II listed 18th-century coaching inn, partial cavity insulation impossible
The Morpeth coaching inn is a Grade II listed 18th-century property with significant heritage constraints — listed building consent required for any external alterations, no exterior insulation permitted, original cast-iron radiators throughout. A full air-to-water heat pump retrofit was not viable; the building heat loss is too high (peak 9.8 kW at design day) for an 8 kW heat pump alone, and radiator upgrade would have required listed building consent at every emitter.
The hybrid solution was the right answer. The Vaillant aroTHERM plus 8 kW heat pump runs primary, covering ~74% of annual heat demand (all mild and cold-but-not-extreme days). The Vaillant ecoTEC plus 32 kW gas boiler kicks in as the temperature drops below approximately 0°C — covering the 12-15 days per year when whole-house demand exceeds the heat pump's 8 kW capacity.
The Vaillant eBUS cascade protocol handles the handover automatically — no manual switching, no missed comfort. The sensorCOMFORT controller monitors flow temperature and outside ambient, brings the boiler online seamlessly when needed, and shuts it back off as the day warms. The owners report no perceptible difference in comfort vs the old straight-gas system.
Listed building consent was secured in 4 months — the heat pump outdoor unit position (rear courtyard, behind existing stone wall, not visible from any public elevation) was the key consent question. Heritage officer was satisfied with the discreet positioning. No alteration to internal listed fabric required.
BUS grant of £7,500 was claimed on the heat pump portion of the install. After 4 months of winter operation, the heat pump has carried 74% of the heating load (slightly above the 70% design estimate), and the combined gas + electricity bill is tracking 32% below the prior gas-only year. The owners now have a credible decarbonisation path — if they choose to insulate further or accept slightly larger radiators in 5-10 years, the heat pump could carry 100% and the boiler could be decommissioned entirely.
