Flexible Stainless Steel Flue Liner Installation: Sizing, Sealing and HETAS Requirements

Quick Answer: Flexible stainless steel flue liners must be 904 or 316 grade stainless steel, sized to the appliance output using BS EN 1856-2 flue tables. All liner installations for solid fuel appliances must be carried out by a HETAS-registered engineer. A CO alarm is mandatory within 1–3 metres of the appliance in England under the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022.

Summary

A flexible stainless steel flue liner is the most common solution for relining an existing masonry chimney to serve a wood-burning stove, multi-fuel stove, or open fire. Chimneys in older properties were built for coal fires at much higher temperatures and with larger cross-sections than modern appliances require — an unlined or oversized flue causes poor draw, condensate problems, and tar deposition that can lead to chimney fires.

The liner installation must be done correctly from the outset. A incorrectly sized, poorly sealed, or badly installed liner is both a fire hazard and a source of carbon monoxide risk. HETAS (Heating Equipment Testing and Approval Scheme) is the competent person scheme for solid fuel and biomass heating in the UK, equivalent to Gas Safe for gas. HETAS-registered engineers can self-certify their work under Building Regulations.

This article covers liner grade selection, sizing from BS EN 1856-2 tables, the complete installation procedure from chimney investigation through to final smoke test, and the fire safety and CO alarm requirements.

Key Facts

Quick Reference Table

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Appliance Output Minimum Liner Diameter (circular) Minimum Area (non-circular) Notes
Up to 5kW 125mm (5") 12,272mm² Small stoves, open fires
5–10kW 150mm (6") 17,671mm² Most domestic wood stoves
10–20kW 175mm (7") 24,053mm² Larger stoves, range cookers
20–30kW 200mm (8") 31,416mm² Commercial stoves, large appliances
Open fire 200mm minimum Variable Often 250mm or existing flue

These are minimum values from BS EN 1856-2; always check appliance manufacturer specification — some appliances require a larger flue collar than the minimum area dictates.

Detailed Guidance

Pre-Installation Chimney Investigation

Before ordering a liner, the chimney must be thoroughly assessed:

CCTV survey:

Sweep first:

Measure the flue:

Check for offsets:

Liner Grade Selection

Grade Alloy Composition Suitable Applications Not Suitable For
904L stainless Ni 25%, Cr 20%, Mo 4.5% All solid fuel, wood, biomass Nothing — this is the premium grade
316 stainless Ni 12%, Cr 17%, Mo 2.5% Gas boilers, light wood use High-sulphur coal, heavy wood use
316Ti stainless As 316 plus titanium Gas boilers with sulphurous flue Solid fuel
Aluminium N/A Gas only Never for solid fuel

Wood smoke contains acetic acid and other organic acids from the combustion of resins and cellulose. These are highly corrosive to 316 grade stainless over time. For any wood-burning stove installation, 904 grade is the correct specification.

Installation Procedure: Top-Down Method

The standard installation method feeds the liner from the top of the chimney downward:

Step 1: Access the chimney top

Step 2: Attach nose cone

Step 3: Feed liner from top

Step 4: Position the liner

Step 5: Fit closure/register plate

Step 6: Insulation (for open flue chimneys)

Step 7: Top plate and rain cowl

Step 8: Connect appliance

Sealing and Sealant Selection

All joints in a flue system must be gas-tight. Standard silicone sealant is not suitable — it degrades above 200°C. Use:

Chimney Pot and Cowl Selection

Cowl Type Pros Cons Use Case
Anti-downdraught (cage) Excellent draw in all winds Bird nesting risk if cage blocked Standard installation
H-pot Very effective; traditional look Larger; more expensive Listed/conservation areas
Bird guard (simple mesh) Cheap; preserves draw Does not address downdraught Low exposure sites only
Rotating wind cowl Self-orients; weatherproof Can stick in frost Windy hilltop locations

Never fit a cowl that restricts the flue area to less than 80% of the liner bore. A restricted cowl causes back-pressure and can force combustion gases into the room.

CO Alarm Requirements

Under the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England):

Frequently Asked Questions

What chimney height is needed for a wood-burning stove?

The minimum effective flue height is 4.5m from the stove grate to the top of the chimney pot (Building Regulations Part J guidance). Below this height, natural draw may be insufficient, particularly in cold weather or with unsealed houses. The chimney must also terminate at least 1m above the highest point of contact with the roof, and at least 600mm above the ridge if within 600mm horizontal distance.

Can I install a flue liner without HETAS registration?

You can install a flue liner without HETAS registration, but you cannot self-certify the work under Building Regulations. Without self-certification, Building Control must be notified before work starts, and an inspector must verify and certify the installation. In practice, most building insurers require HETAS certification for solid fuel installations; without it, your customer's insurance may be invalid. Obtaining HETAS registration is strongly recommended for anyone doing regular stove and flue work.

Does the liner need to be insulated?

For open fireplaces: insulation in the void between liner and chimney wall is strongly recommended. It improves draw by keeping flue gases hot from base to top, and reduces condensation and tar deposition. For stoves connected to a fully enclosed chimney: insulation is beneficial but less critical if the stove is regularly used and the flue warms up quickly. Isowool (refractory ceramic fibre blanket) is the standard insulation material.

How long does a 904 stainless liner last?

904 grade stainless steel is extremely durable in solid fuel applications. Most manufacturers offer 10-year warranties on 904 liners; real-world service life is typically 20–30 years with correct maintenance (annual sweep, proper operation at correct temperatures). Running a stove too slowly (smouldering) causes tar buildup that is corrosive even to 904 grade. Running at correct operating temperature (stove glass staying clear) extends liner life.

What is a bird guard and is it required?

A bird guard is a mesh cage fitted to the chimney terminal to prevent birds (especially jackdaws) from nesting in the flue. Nests are a fire hazard and cause flue blockage. Bird guards are not legally required but are strongly recommended. The mesh must be coarse enough not to accumulate soot and restrict draw — 25mm × 25mm mesh is typical. Check the bird guard annually and after the nesting season (June–August) for blockage.

Regulations & Standards