Domestic Hot Water (DHW) Pipe Insulation — Part L, BS 5422 & Legionella Guidance

Domestic hot water (DHW) pipe insulation has a more specific role than general hot-water lagging. On DHW distribution systems, insulation must limit heat loss while helping the system maintain the intended hot-water temperature through distribution pipework, secondary returns, calorifier connections and plant-room headers.

This guide focuses specifically on DHW systems, Part L, BS 5422 and Legionella risk management. For broader advice covering domestic hot-water pipes, material comparisons and general installation, see our general hot water pipe insulation guidance.

Why DHW Distribution Pipework Needs Its Own Insulation Strategy

Uninsulated DHW pipes lose heat continuously, increasing boiler or calorifier load and allowing water to cool as it travels through the system. In homes this means slower hot-water delivery and unnecessary energy loss. In commercial DHW systems, temperature control also forms part of Legionella risk management.

• Reduce distribution heat loss — insulation keeps more useful heat in the water between the heat source and outlets.
• Support Part L compliance — new and substantially refurbished systems require appropriate insulation on primary and secondary DHW distribution pipework.
• Maintain DHW temperatures — insulation reduces temperature drop in distribution and return pipework.
• Support Legionella control — commercial systems should be designed and operated so hot water remains at the required temperatures through the system.
• Improve hot-water availability — reduced heat loss helps shorten recovery and waiting time after draw-off.

Part L and BS 5422 for DHW Pipe Insulation

Part L of the Building Regulations requires heat losses from building-services pipework to be limited. For DHW services, insulation thickness should be selected using the applicable specification and the thermal conductivity of the insulation material rather than by pipe size alone.

For Armaflex EVO with a thermal conductivity of approximately 0.033 W/mK on DHW pipework around 60°C, the existing project guidance used on this site gives the following indicative wall thicknesses:

These figures are indicative only. Always verify the current BS 5422 requirement and the relevant Approved Document for the actual pipe diameter, operating temperature, insulation material and project location.

For specification support, use the BS 5422 Resource Centre and our BS 5422 pipe insulation guide.

Legionella Control in Commercial DHW Systems

In commercial buildings, DHW insulation is not only an energy-efficiency measure. It also supports the temperature-control strategy used to manage Legionella risk. The existing guidance for this site identifies 50°C or above through DHW distribution as the key temperature-control objective, with the system design and operation managed in accordance with the applicable Legionella guidance.

Insulation is particularly important on:

• Secondary circulation and return loops — these runs should retain heat continuously rather than losing temperature as water circulates back to the calorifier.
• Calorifier connections — flow, return and distribution connections are concentrated heat-loss points and should be insulated continuously.
• Plant-room headers and pump sets — exposed DHW distribution components can lose substantial heat in warm plant spaces.
• Long distribution mains — larger buildings need insulation continuity over the full run to minimise temperature decay.
• Low-flow sections — system design should avoid unnecessary dead legs; insulation does not replace correct hydraulic design or Legionella management.

Choosing Insulation for DHW Pipework

Armaflex EVO

Armaflex EVO is a closed-cell elastomeric insulation used across residential and commercial building services. The current product data used in this guide gives a service-temperature capability up to +105°C and a thermal conductivity of approximately 0.033 W/mK, making it suitable for standard DHW operating temperatures.

It is available in common copper-pipe sizes and multiple wall thicknesses, allowing the insulation specification to be matched to the required thermal performance.

Armaflex EVO Self-Seal

Armaflex EVO Self-Seal is useful where DHW pipework is already installed and connected. The factory-applied closure allows sections to be fitted around existing pipe runs without sliding a continuous tube over the pipe.

Kingspan Kooltherm Pipe Insulation

Kingspan Kooltherm pipe insulation is a phenolic option for DHW distribution where low thermal conductivity and reduced insulation thickness are important, particularly on larger commercial services or in congested plant areas.

Polyethylene Foam

Polyethylene foam can provide basic heat retention and frost protection on suitable domestic services, but its higher thermal conductivity means it should not automatically be assumed to satisfy a Part L or BS 5422 specification. Check the actual material performance and required thickness before use.

Calorifiers, Secondary Returns and Plant Rooms

DHW systems with calorifiers and secondary circulation should be treated as complete thermal systems rather than isolated straight pipe runs. Insulation should remain continuous across distribution mains, return loops, calorifier connections, valves, pumps and headers wherever practicable.

Gaps at clips, valves, tees and equipment connections create local heat-loss points. In commercial plant rooms, these losses accumulate quickly because pipe diameters are often larger and the DHW network may operate continuously.

Where space is restricted, insulation selection should be based on the required thermal performance and available installation space rather than simply choosing the thinnest product that fits.

Installation Requirements for DHW Insulation

• Maintain continuity — avoid gaps between adjacent sections and around fittings, valves, brackets and penetrations.
• Use the correct bore size — pipe-insulation sizes should match the pipe outside diameter. A 15mm copper pipe requires insulation sized for a 15mm outside diameter.
• Seal joints properly — elastomeric insulation butt joints should be installed and bonded in accordance with the insulation manufacturer's method. Armaflex 520 adhesive is available for compatible Armaflex installations.
• Insulate fittings — elbows, tees and valves should not be left as uninsulated thermal bridges.
• Protect exposed insulation — where the insulation is exposed to UV or weather, use the protection specified for the insulation system.

Selecting Pipe Size and Insulation Thickness

Measure the pipe by its outside diameter, then select the insulation wall thickness required by the project specification. Common domestic copper sizes include 15mm, 22mm and 28mm.

Use our pipe insulation thickness guide, measurement guide and copper pipe sizes guide before ordering.

Frequently Asked Questions

What is DHW pipework?

DHW means domestic hot water. It includes the pipework distributing heated potable water from a boiler, cylinder or calorifier to outlets and, on circulated systems, the secondary return pipework back to the heat source.

Should secondary DHW circulation loops be insulated?

Yes. Secondary circulation operates specifically to keep hot water available throughout the distribution system. Leaving the return loop uninsulated increases continuous heat loss and makes temperature control more difficult.

Why are calorifier connections important?

Calorifier connections, headers and nearby valves often carry hot water continuously and can become significant local heat-loss points. Insulating them helps maintain the designed DHW distribution temperature and reduces plant-room heat loss.

What insulation thickness should I use on DHW pipes?

The correct thickness depends on pipe diameter, operating temperature, insulation thermal conductivity and the applicable Part L or BS 5422 requirement. Do not select thickness solely from pipe diameter.

Does insulation alone make a DHW system Legionella compliant?

No. Insulation supports temperature retention, but Legionella control also depends on correct system design, operation, temperature monitoring, avoiding unnecessary dead legs and following the applicable water-hygiene management requirements.

Related Guides

• Hot Water Pipe Insulation Guide — broader domestic hot-water insulation, material and installation guidance
• Part L Compliance for Pipe Insulation
• BS 5422 Pipe Insulation Standards Guide
• BS 5422 Resource Centre
• What Thickness Pipe Insulation Do I Need?
• How to Measure Pipe Insulation Size
• Copper Pipe Sizes Explained

For product selection, browse the full pipe insulation range or the Armaflex range.