OFTEC Heat Pump flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Design heat loss

The calculated whole-dwelling heat requirement at design external temperature, summing fabric and ventilation losses room by room. It determines heat pump capacity and must be used instead of boiler output.

Output at design ambient

Air-source heat pump capacity falls as outdoor temperature drops. Sizing must use the manufacturer's output at the design ambient (e.g. -3 °C), not the nominal A7/W35 rating.

Delta-T emitter rating

Radiator output depends on mean water temperature minus room temperature. A radiator rated at ΔT50 delivers roughly half its rated output at ΔT30, so emitters are usually oversized in retrofit.

Flow temperature and COP

Lower flow temperatures improve efficiency: each 1 °C reduction typically improves COP by about 2-3 %. Design to the lowest flow temperature the emitters can support.

SCOP versus COP

COP is an instantaneous ratio at one operating point; SCOP is the seasonal average including defrost, cycling and standby. SCOP is the realistic figure for design and comparison.

DHW cylinder sizing

Allow roughly 40-50 litres per occupant, stored at 48-55 °C with a legionella cycle to 60 °C. The coil must be sized for the heat pump's lower flow temperature.

Buffer vessel rule

Provide about 10-20 litres of buffer per kW of heat pump output where system water volume is low, to prevent short cycling and supply defrost energy.

Heat pump flow rate

Heat pumps use a smaller temperature difference (typically 5-7 K) than boilers, so design flow rates are higher. Keep primary velocities below about 1.0-1.5 m/s.

Outdoor unit clearances

Maintain manufacturer-specified clearances (commonly 300-1000 mm) for airflow and service access, avoid discharge air recirculation, and mount on vibration-isolating brackets.

Pressure test and evacuation

Pressure-test refrigerant pipework with dry nitrogen, then evacuate to below about 500 microns (66 Pa) and confirm a vacuum decay test before releasing refrigerant charge.

System flushing and dosing

Flush the water circuit to remove debris, fit a magnetic filter, and dose with corrosion inhibitor and biocide. Add glycol where pipework or the outdoor unit can freeze.

Commissioning record contents

Record design and actual flow rate, flow and return temperatures, ambient and compressor readings, electrical input, calculated COP, refrigerant charge and the completed commissioning checklist.

Weather compensation set-up

Configure the heating curve so flow temperature falls as outdoor temperature rises. Confirm DHW schedule, legionella cycle and correct staging or lockout of the back-up heater.

No diversity on heat pumps

Heat pump circuits are continuous loads. Size the protective device and cable to the full rated input, including back-up heater and crankcase heater, without applying diversity.

Handover documentation

Provide installation certificate, warranty registration, user manual, maintenance schedule and a demonstration of operation, plus metered performance records where monitoring is fitted.

Ib ≤ In ≤ Iz

Design current must not exceed the protective device rating, which must not exceed the corrected current-carrying capacity of the cable. Correction factors apply to Iz.

Voltage drop limits

BS 7671 guidance limits voltage drop to 3 % for lighting and 5 % for other circuits. Long runs to outdoor units often require a larger conductor.

RCD type selection

Inverter-driven heat pumps may produce DC residual current. Manufacturers commonly specify Type A or Type B RCDs; always follow the manufacturer's instructions and check DC leakage guidance.

Outdoor unit isolator

Fit a lockable, weatherproof local isolator within sight of the outdoor unit, plus isolation at the consumer unit, and label the circuit clearly.

Sensor cable separation

Use manufacturer-specified screened cable for sensors and communication buses, keep sensor leads away from power cables, and observe polarity on bus or 0-10 V connections.

Part P notification

Electrical work for heat pumps is notifiable in England and Wales. Issue an Electrical Installation Certificate or Minor Works Certificate and a Building Regulations compliance certificate.

Earthing and bonding checks

Confirm the means of earthing and main protective bonding of incoming metallic services. Bond extraneous conductive parts at the outdoor unit where present.

Continuous load sizing

Because heat pumps run for long periods at full input, protective devices and cables must be rated for continuous duty, not intermittent boiler-style loads.

Annual service scope

Check refrigerant charge and leaks, clean coils, inspect fan and electrical connections, test safety devices, and record flow temperatures, electrical input and calculated COP.

F-gas leak check intervals

F-gas rules require certificated engineers and periodic leak checks based on CO2-equivalent charge, with records of charge added or recovered kept available for inspection.

Legionella control cycle

Raise stored hot water to at least 60 °C periodically, typically weekly, with distribution reaching 55 °C. Record the cycle and temperatures as part of the risk assessment.

Water quality checks

At service, check inhibitor concentration, pH (typically 6.5-8.5) and glycol strength, top up as needed, replace filter elements and clean strainers.

MCS and Building Regulations

MCS certification supports grant eligibility and sign-off. Installations must comply with Part L for energy efficiency and Part P for electrical work.

High-pressure trip causes

High-pressure faults usually indicate restricted airflow, fouled coil, overcharge or low water flow. Check fans, coils and flow rate before adjusting charge.

Low-pressure trip causes

Low-pressure faults typically indicate refrigerant leak, restriction in the pipework or a faulty sensor. Leak-test and verify charge before resetting.

Frequent defrost diagnosis

Repeated defrost cycles point to iced coils, faulty defrost sensors, blocked drainage or low refrigerant charge. Check sensors and drain paths before parts replacement.

Performance trend review

Compare measured SPF and electricity use with design. Falling COP with rising flow temperature suggests fouling, low charge, air in the system or undersized emitters.