OFTEC Heat Pump cheat sheet

Design heat loss is calculated room by room using the fabric U-values, ventilation/infiltration allowance and the design external temperature (commonly -3 °C to -5 °C in the UK); the heat pump is sized to the whole-dwelling total, not to boiler output.

Heat pump output falls with source temperature: an air-source unit rated 10 kW at 7 °C ambient may deliver only 6-7 kW at -5 °C, so sizing must use the output at design ambient, not the nominal A7/W35 rating.

Heat emitter output is governed by the mean water temperature above room temperature; a radiator rated at ΔT50 (e.g. 75/65/20 °C) delivers roughly half its rated output at ΔT30 (55/45/20 °C), so oversizing emitters is normal in retrofit.

Flow temperature drives efficiency: design to the lowest achievable flow temperature (35-45 °C for underfloor, 45-55 °C for well-sized radiators); every 1 °C reduction in flow temperature improves COP by roughly 2-3 %.

SCOP/SPF is seasonal performance including defrost, cycling and standby; a good air-source SCOP is around 3.0-3.5 for radiators at 45-50 °C and 4.0+ for underfloor at 35 °C, while ground-source typically achieves 4.0-4.5.

Domestic hot water cylinder sizing: allow roughly 40-50 litres per occupant for a cylinder heated to 48-55 °C, with a coil/heat-exchanger surface sized for the heat pump's lower flow temperature and a legionella cycle to 60 °C.

Buffer volume rule of thumb: provide 10-20 litres of buffer per kW of heat pump output where system water volume is low, to prevent short cycling and to give defrost energy on air-source units.

Pipe sizing and pump head: keep design flow velocity below about 1.0-1.5 m/s in primary pipework and check available external static pressure; heat pumps need higher flow rates than boilers because of the smaller ΔT (typically 5-7 K, not 11-20 K).

Outdoor unit siting: maintain manufacturer clearances (commonly 300-1000 mm) for airflow and service access, avoid recirculation of discharge air, and mount on anti-vibration feet or brackets rated for the unit weight plus snow load.

Refrigerant pipework: pressure-test with dry nitrogen (typically to 1.5x design pressure or per manufacturer, e.g. 40+ bar), then evacuate to below 500 microns (about 66 Pa) and hold a vacuum decay test before releasing charge.

Flushing and water treatment: flush the system to remove debris, fit a magnetic filter and dose with corrosion inhibitor and biocide; glycol antifreeze is required where the outdoor unit or exposed pipework can freeze.

Fill and vent thoroughly: air in the system causes high flow temperatures, noise and reduced COP; commission with all vents open, then record flow rate and differential pressure across the heat exchanger.

Commissioning records must include: design and actual flow rate, flow and return temperatures, compressor/ambient readings, measured electrical input, calculated COP, refrigerant type and charge weight, and the completed benchmark/commissioning checklist.

Controls set-up: configure weather compensation curve, DHW schedule and legionella cycle, set the heat pump to the correct capacity/defrost parameters, and confirm the back-up heater is locked out or staged correctly.

Electrical supply: heat pumps are continuous loads; size the dedicated circuit with diversity NOT applied (no cooker/immersion diversity), check supply capacity and earthing, and install Type A or Type B RCD as required by the manufacturer.

Handover: demonstrate operation to the occupant, provide the MCS/installation certificate, warranty registration, user manual and a maintenance schedule; record the measured SPF or metered performance where monitoring is fitted.

Heat pump circuits are treated as continuous maximum demand: protective device and cable are sized to the full rated input (including back-up heater and crankcase heater) with no diversity applied.

Cable sizing uses the standard formula: Ib ≤ In ≤ Iz, with correction factors for ambient temperature, grouping, thermal insulation and installation method applied to Iz.

Voltage drop should not exceed 3 % for lighting and 5 % for other circuits (BS 7671 guidance); long runs to outdoor units frequently fail this and require a larger conductor.

RCD selection: manufacturers commonly require Type A (or Type B where DC components are present) at 30 mA for socket outlets, or 300 mA for fixed equipment per manufacturer instructions; check the inverter's DC leakage guidance.

Earthing and bonding: confirm the means of earthing and main protective bonding of incoming metallic services; heat pump outdoor units need equipotential bonding where extraneous conductive parts are present.

Outdoor unit isolator: fit a lockable, weatherproof local isolator within sight of the unit, plus a means of isolation at the consumer unit, and label the circuit clearly.

Controls wiring: use manufacturer-specified cable for sensor and communication buses (screened where required), keep sensor leads separate from power cables, and observe polarity on modulating/0-10 V or bus connections.

Notification and certification: heat pump installations are notifiable in England and Wales (Part P applies to the electrical work); issue an Electrical Installation Certificate or Minor Works Certificate and a Building Regulations compliance certificate.

Annual service scope: check refrigerant pressures/charge and look for leaks, clean evaporator and condenser coils, check fan blades and motor, inspect electrical connections and contactors, verify safety devices and record flow temperatures and COP.

Refrigerant handling: F-gas regulations require certificated personnel, leak checks at intervals based on CO2-equivalent charge (e.g. 5 t CO2e or more triggers periodic checks), and records of charge added or recovered.

Legionella control: stored DHW should be raised to at least 60 °C periodically (typically weekly) with distribution reaching 55 °C; record the cycle and any temperature log as part of the risk assessment.

Water quality: check inhibitor concentration, pH (typically 6.5-8.5), and glycol strength at service; top up and record; replace magnetic filter elements and clean strainers.

Compliance: MCS certification (or equivalent) is required for government grant eligibility and building control sign-off; installations must comply with Building Regulations Part L (conservation of fuel and power) and Part P for electrical work.

F-gas and ODS records: maintain a log of equipment, refrigerant type, charge, leak test dates and engineer certification; keep records available for inspection.

Performance review: compare measured SPF/COP and electricity consumption against design; falling COP with rising flow temperature indicates fouling, low charge, air or emitter problems.

Breakdown decision rules: high-pressure trips point to poor airflow or overcharge; low-pressure trips to leak or restriction; frequent defrost cycles to coil icing or sensor faults; check fault codes before replacing parts.