Air Source Heat Pump Installation: Siting, Refrigerant Pipe Runs, Electrical Supply and Commissioning Steps

Quick Answer: An ASHP outdoor unit requires: a firm, level base (concrete slab or anti-vibration feet on a frame); minimum clearances of 300mm at the rear and sides (more for front discharge units), clear airflow in the prevailing wind direction; not enclosed on more than two sides. Refrigerant pipe runs from the outdoor unit to the indoor unit (monobloc: water pipes only; split: refrigerant pipes) should be as short and straight as possible. A dedicated electrical supply (typically 6mm² to a 20–32A RCBO) is required. Commissioning requires hydraulic filling and flushing, pressure test, controls setup, and a first-run COP check.

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

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:

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

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:

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:

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:

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:

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