Heat Pump Refrigerant Line Insulation

July 25, 2026

Introduction

The refrigerant lines on an air source heat pump are the most technically demanding pipework to insulate on any domestic or light commercial installation. The suction line operates at temperatures well below ambient — creating significant condensation risk — while the discharge line operates at high temperatures that can exceed the rating of standard insulation materials.

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

This guide covers the correct insulation specification for heat pump refrigerant lines — suction line, discharge line, and the connections at the indoor and outdoor units — with product recommendations, thickness guidance, and MCS compliance requirements.

Understanding the Refrigerant Circuit

An air source heat pump refrigerant circuit has four main components: the outdoor evaporator, the compressor, the indoor condenser (heat exchanger), and the expansion valve. The refrigerant pipework connecting the outdoor and indoor units consists of two lines:

Suction line — carries low-pressure refrigerant gas from the indoor unit back to the outdoor compressor. Operating temperature: typically −5°C to +10°C depending on outdoor conditions and system load.
Discharge line — carries high-pressure hot refrigerant gas from the compressor to the indoor heat exchanger. Operating temperature: typically 60–90°C.

Both lines require insulation, but for different reasons and with different priorities.

Suction Line Insulation

The suction line is the highest-priority insulation task on an ASHP installation. Operating at temperatures below ambient, it presents a significant condensation risk — and condensation on refrigerant pipework causes corrosion, insulation failure, and reduced system efficiency.

Why condensation forms: When the suction line surface temperature is below the dew point of the surrounding air, water vapour condenses on the pipe surface. In a typical UK plant room or utility room at 20°C and 60% relative humidity, the dew point is approximately 12°C — well above the suction line operating temperature. Without adequate insulation, condensation will form continuously. See: Vapour Barriers for Pipe Insulation.

Correct material: Closed-cell elastomeric foam with an integral vapour barrier — Armaflex EVO or K-Flex ST. Do not use polyethylene foam (Climaflex, Tubolit) — it is open-cell and provides no vapour barrier.

Minimum thickness: As specified by the heat pump manufacturer's installation manual. Typically 19–25mm for pipe ODs up to 22mm, 25–32mm for larger diameters. Always check the manufacturer's specification — it may exceed the BS 5422 minimum. See: BS 5422 Resource Centre.

Discharge Line Insulation

The discharge line carries high-pressure hot refrigerant gas at temperatures of 60–90°C. The primary risk is heat loss, which reduces system efficiency. A secondary risk is personnel contact with hot surfaces.

Correct material: Closed-cell elastomeric foam rated for the operating temperature. Standard Armaflex EVO is rated to +105°C and is suitable for most ASHP discharge lines. For systems with discharge temperatures above 105°C, use Armaflex HT (rated to +150°C).

Minimum thickness: As specified by the heat pump manufacturer's installation manual. Typically 13–19mm for most domestic ASHP discharge lines.

Outdoor Refrigerant Line Insulation

The section of refrigerant pipework between the outdoor unit and the building penetration is exposed to UV radiation, rain, and temperature cycling. Standard elastomeric foam without UV protection will degrade within 12–24 months outdoors. See: Outdoor Pipe Insulation for Heat Pumps.

Correct specification for outdoor refrigerant lines: Armaflex HT Solar (UV-stabilised, rated −50°C to +150°C) or K-Flex Solar HT. No additional weatherproofing required.

Connections at the Indoor and Outdoor Units

The connections at the indoor and outdoor units — where the refrigerant lines connect to the heat pump — are frequently left partially uninsulated. These short sections are a significant source of both condensation (suction line) and heat loss (discharge line).

Insulate all connections fully, including any flexible sections, service valves, and the section of pipework inside the outdoor unit casing where accessible. Use cut-and-bond elastomeric foam for awkward geometries.

Insulation Thickness Reference

The following thicknesses are indicative for Armaflex EVO (λ = 0.033 W/m·K). Always verify against the heat pump manufacturer's installation manual — the manufacturer's specification takes precedence.

Pipe OD	Suction Line (min)	Discharge Line (min)
10mm	19mm	13mm
12mm	19mm	13mm
16mm	19mm	13mm
22mm	25mm	19mm
28mm	25mm	19mm
35mm	32mm	25mm

MCS Compliance

MCS installation standards require refrigerant lines to be insulated in accordance with the heat pump manufacturer's installation manual. An MCS inspection will check that the suction line is insulated with closed-cell elastomeric foam at the manufacturer's specified minimum thickness, the discharge line is insulated at the manufacturer's specified minimum thickness, outdoor sections are UV-resistant, and all connections at the indoor and outdoor units are fully insulated. See: Heat Pump Pipe Insulation — BS 5422 & MCS Requirements.

Frequently Asked Questions

What is the difference between the suction line and discharge line on a heat pump?

The suction line carries low-pressure refrigerant gas back to the compressor at low temperatures (−5°C to +10°C) — condensation control is the primary concern. The discharge line carries high-pressure hot gas to the heat exchanger at high temperatures (60–90°C) — heat loss is the primary concern.

Can I use standard pipe lagging on heat pump refrigerant lines?

No — standard polyethylene foam (Climaflex, Tubolit) is open-cell and provides no vapour barrier. It will become waterlogged on the suction line and fail rapidly. Use closed-cell elastomeric foam only.

What thickness insulation do I need on the suction line?

Check the heat pump manufacturer's installation manual for the minimum thickness on your specific model. Typically 19–25mm for pipe ODs up to 22mm. The manufacturer's specification takes precedence over BS 5422 where it is more demanding.

Does Armaflex EVO work on heat pump discharge lines?

Yes — Armaflex EVO is rated to +105°C and is suitable for most domestic ASHP discharge lines. For systems with discharge temperatures above 105°C, use Armaflex HT (rated to +150°C).

How do I insulate the connections at the outdoor unit?

Use cut-and-bond elastomeric foam sections for the connections at the outdoor unit. Apply Armaflex 520 adhesive to all seams and joints. Insulate as close to the unit casing as possible — any uninsulated section is a condensation risk on the suction line.