Summary

Installing an air source heat pump is more complex than a conventional boiler swap. It involves outdoor unit siting (planning and noise considerations), refrigerant or hydraulic pipe runs, a dedicated electrical circuit, hydraulic system preparation (flushing, inhibitor, correct pH), and controls commissioning. Each stage has technical requirements from MCS 007, the manufacturer's installation manual, and BS 7671.

This article covers the installation sequence for a typical domestic ASHP, from site survey through to commissioning.

Key Facts

  • ASHP (Air Source Heat Pump) — extracts heat from outdoor air using a refrigerant vapour-compression cycle; operates down to approximately -20°C outdoor ambient (efficiency reduces significantly below 0°C)
  • Monobloc ASHP — all refrigerant components (compressor, evaporator, condenser) are in the outdoor unit; heat is transferred to the heating system via a water circuit (the installer only handles the water pipework, not refrigerant)
  • Split ASHP — the evaporator is outdoors; the condenser and hot water circuit are in an indoor unit; refrigerant pipes run between the two; F-Gas handling and REFCOM registration required for the refrigerant side
  • Outdoor unit placement — requires a stable base, clearances per manufacturer, and orientation to the prevailing wind; planning rules apply (see heat pump noise planning)
  • Refrigerant pipe run (split) — typically 3–15m for residential; pre-charged lineset or field-charged with vacuum pump and nitrogen purge; rated for R32 or R410A (check refrigerant type)
  • Flow and return (water circuit) — typically 28mm copper or DN25 plastic (MLCP or PE-RT) from the outdoor unit to the indoor heat pump unit and to the heating circuit; insulation required on all pipes (including outdoor)
  • Anti-vibration pads — the outdoor unit must be mounted on anti-vibration materials to reduce sound transmission through the building structure
  • Dedicated electrical circuit — ASHP outdoor unit typically requires: 6mm² single-phase, 20–32A MCB/RCBO (check manufacturer spec for the specific model); run from the consumer unit; the outdoor unit cable entry must be weatherproof
  • Permanent hardwiring — ASHP units are permanently wired (not plug-and-socket connection); connection box on the unit; live connection in the consumer unit
  • Commissioning record — MCS 007 requires a commissioning record signed by the installer; includes system pressure, flow rate, and initial flow/return temperatures
  • Inhibitor and pH — the heating system water must be treated with a suitable corrosion inhibitor (Fernox, Sentinel, or equivalent); pH must be within the heat pump manufacturer's specification (typically pH 6.5–8.5); check using a calibrated pH meter or test strips at commissioning

Quick Reference Table: ASHP Installation Key Parameters

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Parameter Typical Value Notes
Minimum rear clearance 300–500mm Per manufacturer; check spec
Minimum side clearance 300mm Both sides
Minimum front clearance 1–2m Unobstructed airflow required
Electrical supply 6mm², 20–32A RCBO Check manufacturer nameplate
Pipe size (water circuit) 28mm copper or DN25 MLCP Monobloc; sized to design flow rate
Refrigerant pipe (split) Typically 6.35mm liquid / 9.52mm suction Check manufacturer for model-specific sizes
System water pressure (cold fill) 1.0–1.5 bar Check manufacturer operating range
Inhibitor pH range pH 6.5–8.5 Manufacturer-specific; check specification
Commissioning flow rate Verify against manufacturer's minimum flow Often 0.3–0.8 l/s for 8–12kW units
Anti-vibration pads Rubber composite, rated for unit weight Required under all ASHP outdoor units

Detailed Guidance

Site Survey for Siting the Outdoor Unit

Before designing the installation, the site survey must determine the outdoor unit location:

Requirements:

  • A firm, level surface (concrete base, proprietary mounting frame with anti-vibration feet, or wall-mounted bracket)
  • Clear airflow in front of the unit: minimum 1–2m unobstructed space in front of the inlet; more for large units
  • Rear clearance: minimum 300mm (most manufacturers specify 300–500mm) between the rear of the unit and the wall
  • Side clearances: check manufacturer spec for units that discharge from the side or front
  • Not enclosed on three or four sides (heat pump requires airflow through the evaporator; an enclosed courtyard or tight alcove restricts airflow and reduces efficiency)
  • Accessible for servicing and annual maintenance: filter cleaning, coil cleaning, defrost drain inspection
  • Drain provision: the heat pump defrost cycle produces condensate/drainage; must drain to a suitable drain or soakaway (not a sealed, sealed area that could flood or freeze)
  • Neighbour and highway relationship: noise considerations; permitted development rules (see heat pump noise planning)

Foundation: A concrete slab of minimum 100mm thickness is the standard base for ground-mounted ASHP. The slab must be:

  • Slightly larger than the outdoor unit footprint (typically unit + 100mm each side)
  • Level (not sloped towards the wall)
  • Set on a stable, compacted sub-base
  • Anti-vibration pads or a rubber-composite mat between the unit and the concrete

For wall-mounted installations, a proprietary galvanised steel frame is secured to the wall; the unit is bolted to the frame via anti-vibration feet.

Refrigerant and Water Pipe Runs

Monobloc ASHP (water only): The pipe run from the outdoor unit to the indoor hydronic station (or to the heating circuit) carries hot water, not refrigerant. Typically:

  • 28mm copper or DN25 MLCP (multilayer composite pipe)
  • Insulated throughout, including the outdoor section (25–30mm wall insulation, UV-stable for exposed sections)
  • Flow and return clearly labelled
  • A service valve on both flow and return immediately adjacent to the outdoor unit connection
  • Draining arrangement (low point drain valve) if the outdoor unit may be isolated in winter

Split ASHP (refrigerant pipe run): The refrigerant pipe run between the outdoor evaporator unit and the indoor unit (containing the condenser/heat exchanger) carries the refrigerant:

  • Liquid line (small bore): typically 6.35mm (¼") or 9.52mm (⅜") copper, depending on unit size
  • Suction line (large bore): typically 9.52mm (⅜") or 12.7mm (½") copper; insulated with at least 25mm wall closed-cell insulation
  • Maximum length: most domestic splits allow up to 25–30m pipe run; longer runs require additional refrigerant charge
  • Height difference: check manufacturer specifications for allowable height difference between indoor and outdoor units (limits vary)

F-Gas requirement for split systems: Any work involving the refrigerant circuit (connecting, pressure testing, charging) requires the operative to hold F-Gas certification (REFCOM Category 1 or City & Guilds 2079). This includes pressure testing the refrigerant pipework with nitrogen and drawing a vacuum before charging. Unqualified personnel cannot legally handle the refrigerant circuit.

Flushing the Heating System

Before connecting the heat pump to the heating system, the existing system must be flushed:

Why: Residual iron oxide sludge, debris, and old inhibitor chemicals from the existing gas boiler system can damage the heat pump's plate heat exchanger and internal components. The heat pump manufacturer's warranty typically requires a clean system at installation.

Flushing procedure:

  1. Isolate and drain the heating system
  2. Fill with clean water and circulate while running the system pump
  3. Drain and refill; repeat until the drained water runs clear
  4. Power flush (for older, heavily sludged systems): connect a power flushing machine via a radiator connection; circulate with a flushing agent (Fernox F3 or equivalent) for 30–60 minutes; add a sludge loosener if required; drain; fill and flush to clear
  5. Add a chemical inhibitor (Fernox F1, Sentinel X100, or equivalent) at the manufacturer's recommended dosage
  6. Check pH (should be within the heat pump manufacturer's specification); adjust if required using pH adjustment chemical

Magnetic filter: Fit a magnetic in-line filter (Spirovent, Adey MagnaClean, or equivalent) on the return pipe to the heat pump. This traps magnetite particles before they reach the plate heat exchanger. Essential on systems with steel radiators.

Electrical Supply

Outdoor unit supply:

  • Confirm the electrical requirements from the manufacturer's nameplate or data sheet: voltage, maximum current, recommended cable size, MCB/RCBO rating
  • Typical: 230V single-phase, 6mm² twin-and-earth, 20–32A Type B RCBO
  • Larger units (12kW+) may require three-phase supply
  • Run the cable from the consumer unit to the outdoor unit in suitable conduit (UV-resistant conduit or SWA armoured cable for external runs)
  • Weatherproof the cable entry at the unit (suitable gland, sealed conduit entry)

Controls wiring: The heat pump controls (indoor unit, thermostat, weather compensation sensor, zone valve controls, hot water cylinder thermostat) typically require low-voltage controls wiring in addition to the main power circuit. Route controls wiring separately from power wiring; use screened cable where the manufacturer specifies it (outdoor temperature sensor cables in particular can pick up interference).

Part P notification: The new electrical circuit is notifiable under Building Regulations Part P. Self-certify if CPS-registered; otherwise notify local building control.

Commissioning

Step 1: Fill and pressure test (hydraulic circuit) Fill the heating system through the filling loop at 1.0–1.5 bar (as per manufacturer specification). Bleed all radiators and the heat pump indoor unit. Close the filling loop. Check for leaks throughout the pipework and connections. The system should hold pressure (pressure drop of <0.1 bar over 24 hours is acceptable; a higher drop indicates a leak to investigate).

Step 2: Flow rate verification Confirm the system flow rate meets the heat pump manufacturer's minimum requirement. Most heat pumps have a minimum flow requirement (typically 0.3–0.5 l/s for a 8kW unit). Check using a commissioning valve (if fitted) or a calibrated ultrasonic flow meter on the pipe.

Step 3: First run and controls setup Power on the heat pump. The initial startup sequence varies by manufacturer; typically:

  1. The heat pump performs a self-diagnostic routine
  2. Set the target flow temperature (linked to weather compensation curve — see heat pump controls setup)
  3. Verify the flow and return temperatures once the system reaches steady state

Step 4: DHW commissioning Set the DHW (domestic hot water) target temperature: typically 50–55°C for daily use; set up a legionella pasteurisation schedule (60°C for 1 hour, typically weekly) per L8 guidance.

Step 5: Record commissioning data Record on the MCS commissioning form:

  • System fill pressure (bar)
  • Flow and return temperatures at steady state (°C)
  • Flow rate (l/s)
  • Outdoor temperature at time of commissioning (°C)
  • Calculated COP at commissioning conditions (optional but demonstrates system performance)
  • Inhibitor type and dose; pH reading

Frequently Asked Questions

Can the outdoor unit be installed on an existing concrete path or patio?

Yes, provided the path/patio is structurally sound, level, and can accommodate the anti-vibration feet or a mounting mat. The existing surface should not crack or subside under the unit's weight (most ASHP units weigh 60–150kg). Fit anti-vibration pads or a mat between the unit and the surface.

The customer's house only has a 40A fuse on the main supply cutout. Can I still install an ASHP?

Potentially yes, but load assessment is critical. A 40A single-phase supply at 230V = 9.2kW maximum continuous. The heat pump startup current may be higher than the running current (check the unit's inrush current specification). If the total running load of the household (ASHP + other loads) approaches 40A continuously, load management or a DNO supply upgrade should be considered. Most ASHP units draw 2–4A running current for an 8kW heat pump — this is modest relative to a 40A fuse.

Does the outdoor unit need planning permission?

For most domestic properties in England: no, ASHP outdoor units are Permitted Development subject to conditions (noise threshold, setbacks from property boundary, not on listed building, not on principal elevation). See heat pump noise planning for full planning rules.

Regulations & Standards