Heat Pump Retrofit Alongside an Existing Oil Boiler
A customer with an ageing oil boiler and radiators wants to reduce running costs and is considering an air source heat pump. The house is a 1970s semi with cavity walls (unfilled), 100 mm loft insulation, and double glazing. The existing radiators were sized for a 70 °C flow. The customer wants to keep the oil boiler as a backup and asks you to 'just add the heat pump in parallel' so they can switch between the two. The incoming electrical supply is single-phase with limited spare capacity.
1. Explain why a direct parallel connection of the heat pump and oil boiler is unlikely to be satisfactory, and what you would assess first.
Read the worked answer
A heat pump and an oil boiler have very different flow temperatures, modulation behaviour and control requirements. Simply plumbing them in parallel risks:
- Low flow temperature mismatch - the heat pump will run inefficiently if it must match a 70 °C design; weather compensation and lower flow temperatures are needed.
- Control conflict - two heat sources fighting each other, short cycling and poor efficiency.
- Hydraulic issues - correct separation, bypass, buffer or low-loss header may be needed.
- Fabric and emitter suitability - unfilled cavities and modest loft insulation mean high heat loss; radiators sized for 70 °C will be undersized at heat-pump flow temperatures.
Assess first: a full room-by-room heat loss calculation, radiator output at the proposed flow temperature, domestic hot water demand and cylinder suitability, electrical supply capacity, and the intended control strategy (e.g. heat pump as primary with oil as backup via a proper changeover arrangement, not a simple parallel tee).
2. Outline the checks and design decisions needed before recommending a heat pump here.
Read the worked answer
- Heat loss calculation room by room to establish design load at the intended internal/external design temperatures.
- Emitter assessment - existing radiator outputs at the lower flow temperature; identify which need upsizing.
- Fabric improvements - recommend cavity wall insulation and topping up loft insulation, as these reduce load and improve heat pump performance.
- DHW - cylinder size and reheat capability at heat pump temperatures.
- Electrical - confirm single-phase capacity for the heat pump and any immersion/backup; check supply and earthing arrangements.
- Hydraulics and controls - buffer/low-loss header if needed, weather compensation, and a proper changeover or bivalent control strategy with the oil boiler.
- Siting and noise - outdoor unit location and permitted development considerations.
Only recommend the system once these checks support it; explain to the customer that 'adding it in parallel' is not a valid design.