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

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

• 904 grade stainless steel — austenitic high-grade stainless; recommended for wood and solid fuel applications; higher corrosion resistance than 316
• 316 grade stainless steel — adequate for gas applications; lower resistance to sulphurous and chloride corrosion from wood smoke; not recommended for solid fuel
• BS EN 1856-2 — European standard for metal chimneys; Part 2 covers metal liners; includes flue sizing tables
• Flue sizing — liner diameter must match or exceed the appliance flue collar; minimum cross-sectional area from BS EN 1856-2 tables based on appliance output and chimney height
• HETAS registration — mandatory for installation of solid fuel and biomass appliances including flue liners; enables self-certification under Building Regulations Part J
• CO alarm — mandatory in rooms with solid fuel appliances in England (Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022); within 1–3m of appliance, 1m from ceiling
• Nose cone — flexible tapered end fitting on the liner; guides liner into the chimney and past bends; removed after lining
• Closure plate — a sealing plate at the fireplace opening that seals the gap between liner and chimney breast; supports the liner and prevents cold air downdraft
• Register plate — the closure plate, sometimes called a register plate; should be 1.5mm minimum steel, sealed with fireproof sealant
• Insulation blanket — Isowool (refractory ceramic fibre) or vermiculite insulation wrapped around the liner in the chimney void; improves draw and reduces condensation in open fireplaces
• Top plate — sealing plate at the chimney top; weatherproofs the chimney and holds the rain cap
• Rain cowl — prevents rain ingress; bird-excluding mesh types are standard; must not restrict flue draw
• Minimum height — chimney must reach at least 1m above the highest point of contact with the roof, and 2.3m above any roof window within 2.3m horizontally

Quick Reference Table

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:

• A CCTV drain camera (or dedicated chimney camera) is lowered from the top of the chimney
• Check for: cracks in the flue brickwork, collapsed flue liners (clay pots), blockages, mortar joints that have failed, unauthorized openings, and proximity to combustibles
• A chimney with significant cracks or structural defects may require full pointing or rebuilding before lining

Sweep first:

• The chimney must be swept before any survey or liner installation
• Accumulated tar deposits are a fire risk during installation and if not removed, continue burning when the liner is in service

Measure the flue:

• Lower a measuring tape from the chimney top to the fireplace opening
• Add 500mm at each end for connection allowances
• Order liner 500mm longer than the measured chimney height to allow for trimming

Check for offsets:

• Many chimneys have bends (offsets) where the flue deviates from vertical
• Flexible liner can negotiate offsets up to approximately 45° without significant performance loss
• Offsets over 45° require specific flexible liner grades designed for severe bends, or a twin-wall rigid liner in the straight sections

Liner Grade Selection

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

• Erect scaffolding or use roof access equipment; work safely at height
• Ensure the chimney stack is structurally sound before accessing the top

Step 2: Attach nose cone

• Fit the tapered nose cone to the leading end of the liner
• The nose cone guides the liner past offsets and prevents the liner catching on brickwork

Step 3: Feed liner from top

• Have an assistant at the fireplace opening to guide the liner as it descends
• Feed slowly, rotating the liner to ease it around offsets
• Do not kink the liner — a kink permanently reduces the effective bore

Step 4: Position the liner

• The liner should project 25–50mm below the chimney pot at the top
• At the base, the liner should project 200–300mm into the fireplace recess

Step 5: Fit closure/register plate

• Cut a steel plate (1.5mm minimum) to the dimensions of the fireplace opening
• Cut a hole in the plate to accept the liner collar
• Seal the plate to the fireplace surround with fire-resistant silicone or fire-rated sealant
• Ensure the plate is firmly supported — it carries the weight of the liner assembly

Step 6: Insulation (for open flue chimneys)

• For open fireplaces: thread Isowool insulation blanket into the void between liner and chimney walls from the top
• For connecting to a wood stove: insulation is less critical but recommended in cold climates to improve draw

Step 7: Top plate and rain cowl

• Slide the top plate over the liner at the chimney top
• Seal with fire-resistant sealant between the top plate and chimney pot/stack
• Fit the rain cowl to the projecting liner; fix with the provided locking ring

Step 8: Connect appliance

• Connect the stove's flue collar to the liner using a twin-wall flue connector or flexible liner connector
• Seal all joints with fire-rated stove rope sealant (not silicone — silicone degrades above 180°C)
• Ensure the appliance is level and the flue connection is gas-tight

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:

• High-temperature fire-rated sealant (e.g. Vitcas, Stove Bright black fire cement): rated to 1,250°C; suitable for all joints at the stove connection and register plate
• Fire-resistant silicone: rated to 280–300°C; suitable for the closure plate seal to the chimney breast (lower temperature)
• Stove rope (ceramic fibre rope): used with fire cement for door gaskets and stove pipe joints; compressible seal

Chimney Pot and Cowl Selection

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):

• A carbon monoxide alarm must be fitted in any room that contains a fixed combustion appliance (excluding gas cookers)
• The alarm must be placed within 1–3 metres of the appliance
• The alarm must be positioned 1m below ceiling level (CO mixes with air; ceiling mounting is a common but incorrect installation)
• Alarms must be tested by the installer and handed to the occupant
• Battery or mains-powered alarms are both acceptable; the regulations do not mandate interlinked alarms for CO (but interlinked systems are available)

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

• Building Regulations Part J (2010) — combustion appliances and fuel storage systems; chimney and flue installation requirements

• BS EN 1856-2:2009 — chimneys, requirements for metal chimneys; Part 2 metal liners and flue pipes; sizing tables

• HETAS Installation Standards — HETAS technical installation guidance for solid fuel and biomass appliances

• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (SI 2022/1150) — mandatory CO alarms in rooms with combustion appliances

• CIGA (Chimney and Fireplace Industry Alliance) — guidance on chimney construction and maintenance

• HETAS Technical Guidance — HETAS solid fuel installation standards

• Building Regulations Approved Document J — HM Government

• BSI BS EN 1856-2 — British Standards Institution

• CIGA: Chimney Installation Guide — Chimney and Fireplace Industry Alliance

• Carbon Monoxide-Be Alarmed Campaign — CO safety guidance and alarm requirements

• gas pipework installation — gas pipe sizing, materials, and pressure testing

• efficiency calculations — boiler and heat pump efficiency comparisons

• co alarms — CO alarm placement and testing guidance