Air Source Heat Pump Pipe Insulation Guide

July 25, 2026

Introduction

Air source heat pumps (ASHPs) rely on correctly insulated pipework to deliver their designed efficiency, prevent condensation, protect outdoor services, and comply with UK installation requirements. Because a heat pump operates across several different temperature ranges, no single insulation material or thickness is suitable for every pipe on the system.

For a complete overview of heat pump pipe insulation, visit our Heat Pump Pipe Insulation Hub.

This guide explains how to specify insulation for the heating flow and return, domestic hot water pipework, refrigerant lines, outdoor connections, valves, fittings, and building penetrations on an air source heat pump installation.

How an Air Source Heat Pump System Is Arranged

An ASHP extracts low-grade heat from outdoor air and transfers it into the building through a refrigerant cycle. Depending on the system design, the outdoor unit may connect to the building using refrigerant lines or water-filled primary flow and return pipework.

Typical pipework on an ASHP installation includes:

Heating flow and return between the heat pump and the heating distribution system
Refrigerant suction and discharge lines on split systems
Domestic hot water pipework to and from the cylinder
Outdoor pipe runs between the unit and the building
Short connections at valves, strainers, pumps, buffers, and hydraulic components

Heating Flow and Return Pipework

Most ASHP heating circuits operate at lower temperatures than conventional boiler systems, typically 35–55°C. The lower operating temperature does not remove the need for insulation. Heat loss from poorly insulated primary pipework reduces system efficiency, increases running time, and can prevent the heat pump from achieving its design coefficient of performance.

Specification: Use insulation that meets the required BS 5422 thickness for the actual pipe outside diameter, operating temperature, ambient condition, and thermal conductivity of the chosen material. Closed-cell elastomeric foam, polyethylene foam, mineral wool, or phenolic sections may all be suitable where correctly specified for the location and service.

For most domestic ASHP heating circuits, 25mm wall thickness is a common starting point on smaller pipe sizes, but the final selection must be checked against BS 5422 and the product datasheet. See: What Thickness Pipe Insulation Do I Need?

Refrigerant Lines on Split Systems

Split ASHP systems use refrigerant pipework between the outdoor and indoor units. The suction line operates below ambient temperature and presents a significant condensation risk, while the discharge line operates at a much higher temperature and must be insulated to control heat loss and protect against hot surfaces.

Suction line: Use closed-cell elastomeric foam with an integral vapour barrier, such as Armaflex EVO or K-Flex ST. All longitudinal seams, butt joints, bends, valves, and service connections must be fully sealed.

Discharge line: Use insulation with a service temperature rating above the maximum refrigerant discharge temperature. Standard Armaflex EVO is suitable for many domestic systems, while Armaflex HT may be required where temperatures exceed the standard grade limit.

Always follow the heat pump manufacturer's installation manual for the specified minimum insulation thickness. See: Heat Pump Refrigerant Line Insulation.

Outdoor Pipework

The pipe run between the outdoor heat pump unit and the building is exposed to sunlight, rain, frost, wind, and repeated temperature cycling. Standard indoor insulation will deteriorate if it is installed outdoors without UV and weather protection.

Preferred specification: Use a UV-resistant closed-cell elastomeric product such as Armaflex HT Solar or K-Flex Solar HT. Where the installation may be exposed to mechanical damage, use a pre-jacketed product such as Armaflex Tuffcoat or add suitable aluminium or PVC cladding.

Standard black elastomeric foam should only be used outdoors where it is protected by a manufacturer-approved coating or weatherproof cladding. See: Outdoor Pipe Insulation for Heat Pumps.

Domestic Hot Water Pipework

Where the ASHP heats a domestic hot water cylinder, the cylinder primary pipework and hot water distribution pipework must be insulated to the relevant BS 5422 and Part L requirements. The same principles apply as for a conventional heating system: determine the service temperature, pipe outside diameter, insulation thermal conductivity, and installation location before selecting the nearest available thickness that meets or exceeds the requirement.

Pay particular attention to cylinder connections, secondary circulation pipework, and pipework passing through garages, lofts, service voids, or other unheated spaces.

Valves, Fittings and Unit Connections

Leaving valves, bends, strainers, pumps, flexible connectors, or short pipe tails uninsulated creates thermal bridges and condensation points. These small exposed areas can undermine an otherwise compliant installation.

Insulate bends and tees with accurately cut and bonded sections
Cover valves and strainers with removable insulation where future maintenance access is required
Continue insulation as close as practicable to the heat pump casing
Seal all vapour barriers around supports, brackets, and service penetrations
Use compatible adhesive and tape recommended by the insulation manufacturer

BS 5422, Part L and MCS Requirements

Heating and hot water pipework on a heat pump installation must be insulated in accordance with the applicable Building Regulations and BS 5422. MCS-certified installations must also follow the heat pump manufacturer's installation instructions and the relevant MCS installation standard.

For refrigerant lines, the manufacturer's minimum insulation specification is critical. For outdoor sections, the insulation must also be protected from UV degradation and weather exposure. See: Heat Pump Pipe Insulation — BS 5422 & MCS Requirements and the BS 5422 Resource Centre.

Indicative Insulation Thicknesses

The following values are indicative only and must be checked against BS 5422, the insulation manufacturer's thermal conductivity data, and the heat pump installation manual.

Pipework	Typical Material	Indicative Thickness
Heating flow and return up to 35mm OD	Elastomeric or polyethylene foam	25mm
Larger heating primary pipework	Elastomeric, phenolic or mineral wool	25–32mm+
Refrigerant suction line up to 22mm OD	Closed-cell elastomeric foam	19–25mm
Refrigerant discharge line up to 22mm OD	Temperature-rated elastomeric foam	13–19mm
Outdoor pipework	UV-resistant elastomeric foam	As required by service and manufacturer

Common Installation Mistakes

Using indoor insulation outdoors
Standard foam exposed to UV radiation will crack and lose its vapour barrier. Use a UV-resistant grade or protective cladding.

Leaving the final connections uninsulated
Short exposed connections at the heat pump, cylinder, buffer, or valves create measurable heat loss and condensation risk.

Using polyethylene foam on refrigerant suction lines
Refrigerant suction lines require a continuous closed-cell vapour barrier. Use elastomeric foam specified for condensation control.

Selecting thickness by nominal pipe size
BS 5422 uses pipe outside diameter and the insulation's thermal conductivity. Confirm both before selecting a thickness.

Failing to seal joints
On cold services, every seam and butt joint must be bonded. A small vapour barrier breach can lead to hidden moisture and corrosion.

Frequently Asked Questions

What is the best insulation for air source heat pump pipes?

For indoor heating flow and return, several insulation types can be suitable when correctly sized to BS 5422. For refrigerant suction lines, use closed-cell elastomeric foam. For outdoor sections, use UV-resistant elastomeric insulation or a correctly weatherproofed and clad system.

Do air source heat pump pipes need thicker insulation than boiler pipes?

Not automatically. The required thickness depends on pipe OD, operating temperature, insulation thermal conductivity, ambient conditions, and whether the objective is heat-loss control or condensation prevention. Heat pump manufacturers may also specify minimum thicknesses for refrigerant lines.

Can Climaflex or Tubolit be used on an ASHP?

Polyethylene foam may be suitable for some indoor heating flow and return pipework where it meets the required thermal performance. It should not be used on refrigerant suction lines or other cold services that require an integral vapour barrier.

Does outdoor heat pump insulation need cladding?

Not where a purpose-designed UV-resistant product such as Armaflex HT Solar is installed in accordance with the manufacturer's instructions. Cladding may still be required where mechanical protection is needed.

Who is responsible for heat pump pipe insulation compliance?

The installer is responsible for following Building Regulations, MCS requirements, the heat pump manufacturer's installation manual, and the specified insulation system. Product and thickness records should be retained as part of the installation documentation.