Log Burner Installation: Part J Air Supply, Hearth Dimensions and HETAS Registration

Summary

A log burner (solid fuel stove) is one of the most popular home improvement projects in the UK, but it is also one of the most heavily regulated. Approved Document J covers all combustion appliances: gas, oil and solid fuel. For solid fuel stoves, HETAS (Heating Equipment Testing and Approvals Scheme) is the relevant competent person scheme.

Installing a log burner without notifying Building Control is a criminal offence under the Building Act 1984. The consequences go beyond legal penalties — household insurance can be invalidated and the installation will need retrospective approval when selling the property, often requiring costly remediation.

The most common installation failures involve inadequate air supply, incorrect flue sizing, hearths that are too small or made from combustible materials, and flue terminals in prohibited positions. All of these are covered by Approved Document J and HETAS guidance.

Key Facts

• Competent person scheme — HETAS covers solid fuel. Gas Safe for gas. OFTEC for oil. Each issues a commissioning certificate.
• Building Control notification required — Either through HETAS registered installer (self-cert) or Building Notice before work starts.
• Smoke Control Area restrictions — In designated Smoke Control Areas, only DEFRA-exempt appliances may be used with solid fuel. Burning unapproved fuel is a fixed penalty (£175–£1,000). Check your local authority.
• Air supply sizing — Appliances above 5kW require dedicated ventilation: 550 mm² free area per kW above 5kW. E.g., 8kW stove needs 550 × 3 = 1,650 mm² vent.
• Flue diameter minimum — 150mm for up to 20kW output. 175mm for 20–30kW.
• Flue height minimum — 4.5m from appliance outlet for natural draught.
• Twin-wall flue — BS EN 1856-1 double-skin insulated flue must be used for external runs and through walls/ceilings.
• Single-skin liner — 316 or 904 stainless steel flexible liner for lining existing chimneys: BS EN 1856-2.
• Hearth size minimum — 840mm × 840mm, projecting at least 300mm in front of stove opening, 150mm each side.
• Hearth thickness minimum — 125mm solid non-combustible material (or 250mm if base is combustible).
• Combustible material distance — All combustible materials must be at least 150mm from the stove body, 300mm from door openings.
• Flue terminal position — Cannot be within 600mm horizontally of an opening window or door. Must be at least 600mm above the ridge if within 600mm of it.
• Carbon monoxide alarm — Mandatory in England (The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022) in all rooms with solid fuel appliances.
• Wood moisture content — Ready to Burn certified wood should be ≤20% moisture content. Higher moisture creates creosote and PM2.5 emissions.

Quick Reference Table

Detailed Guidance

Air Supply Requirements

Approved Document J requires all combustion appliances to have adequate air. The calculation method:

• Appliances ≤5kW output — Natural ventilation via room volume is usually sufficient in a normally-ventilated room.
• Appliances >5kW — Dedicated ventilation must be provided: 550 mm² free area per kW above 5kW.
• Example: 10kW stove = (10 – 5) × 550 = 2,750 mm² free area. This equates to one 100mm diameter sleeve vent (7,854 mm² gross — roughly 2,000 mm² free after mesh).
• The vent should be non-closeable if it is the only ventilation for the appliance.
• Air bricks through external walls count. Internal ducts from void spaces also work if the void is ventilated.
• Modern airtight homes often need larger vents. Pressure testing can verify whether natural infiltration is adequate.

Hearth Construction

The hearth serves two purposes: containing fallen embers and providing thermal insulation between the hot base of the stove and any combustible floor.

Standard hearth (over non-combustible base):

• Minimum 125mm solid non-combustible material (concrete, tile on sand:cement bed, natural stone)
• 840mm × 840mm minimum footprint
• 300mm projection in front of appliance door
• 150mm on each side of appliance

Hearth over combustible floor (timber joists/boards):

• Minimum 250mm non-combustible depth
• Or: non-combustible layer plus 50mm air gap above combustible — the air gap must be maintained

Pre-fabricated hearths: Proprietary hearth systems are available from manufacturers — these come with declared performance and are typically 125mm or 75mm with a certified thermal break. Always check the certificate covers your appliance output.

Flue and Liner Requirements

New flue installation (twin-wall):

• Use BS EN 1856-1 insulated twin-wall systems for all external runs and through rooms
• Minimum 50mm clearance to combustible materials (check manufacturer — may be more)
• Fix at centres not exceeding 1.5m vertically; every 2m horizontally
• Chimney height above flat roof: 1m minimum, or 600mm above any structure within 2.3m
• On pitched roofs: 600mm above roof surface if within 600mm of ridge; level with ridge if further away

Lining an existing chimney:

• Use 316 stainless steel flexible liner only if burning gas — for wood/solid fuel use 904L grade
• Liner must be continuous with no joints hidden in the structure
• Debris must be cleared before insertion
• Top plate and insulation blanket should fill the top of the chimney around the liner
• Pot must be replaced with a terminal cap — bird mesh included

Flue gas test: After commissioning, a spillage test (Antistatic smoke pellet) checks that combustion products are properly drawn up the flue and not spilling into the room.

HETAS Registration and Building Control

If HETAS registered:

1. Installer carries out work
2. Issues a BS 8303/HETAS commissioning certificate
3. Notifies HETAS within 30 days
4. HETAS notifies Building Control on installer's behalf
5. Building Control issues a completion certificate to homeowner

If NOT HETAS registered:

1. Building Notice must be submitted to Local Authority Building Control before work starts
2. LABC inspects work at stages: hearth, flue route, final commissioning
3. Full Plans application is an alternative

Retrospective regularisation: If a log burner was installed without any notification, a regularisation certificate can be applied for. This requires an inspection by Building Control and may require flue gas testing and visual access to all flue joints. Without a certificate, house sale can be problematic.

Smoke Control Areas

In a Smoke Control Area (London, most city centres, many residential areas), you cannot burn wood, coal or solid fuel in a standard stove. You have two options:

1. Use a DEFRA-exempt appliance — these pass a certified burn test and can legally be used in SCAs with permitted fuels
2. Use gas or electric

Check whether your property is in a SCA at smokecontrol.defra.gov.uk. Local councils can also confirm. Burning non-permitted fuel carries a fixed penalty notice of £175–£1,000.

Frequently Asked Questions

Does a log burner need planning permission?

Usually not for internal installations. However, if the flue exits through a side or front wall and is visible from a highway, permitted development rights may not apply and planning may be needed. In conservation areas and on listed buildings, consent is almost always required for new flue penetrations.

Can I install a log burner myself without being HETAS registered?

Yes — but you must submit a Building Notice to your Local Authority Building Control before starting work. LABC will charge an inspection fee and will visit at stages. Many LA Building Control teams are less familiar with HETAS standards than a registered installer, so this route can be slower and more uncertain. A HETAS registered installer is strongly recommended.

What fuel is best for a new log burner?

Ready to Burn certified logs (≤20% moisture) produce fewer PM2.5 emissions and less creosote than wet/unseasoned wood. Anthracite and dry manufactured fuels work in stoves rated for solid mineral fuel. Multi-fuel stoves can burn both wood and solid mineral fuel; wood-only stoves (with a flat grate) should not be used for coal.

How often does the flue need sweeping?

The National Association of Chimney Sweeps (NACS) recommends sweeping at least once a year for wood-burning stoves, and twice a year for coal or multi-fuel use. Insurers may require annual sweeping certificates.

What size log burner do I need?

As a rough guide: 1kW per 14m³ room volume for a well-insulated room; 1kW per 10m³ for a poorly insulated one. Most domestic living rooms suit a 5–8kW stove. Oversized stoves are a common problem — a stove that's too large can't be run at low output without smouldering and creosote formation.

Regulations & Standards

• Approved Document J (2010 with 2013 amendments) — Combustion appliances: flue sizing, air supply, hearths, shielding

• BS EN 1856-1:2009 — Requirements for metallic flue systems: twin-wall flue products

• BS EN 1856-2:2009 — Requirements for metallic flue systems: flexible liner products

• BS 8303:1994 (withdrawn but still referenced) — Installation of domestic heating and cooking appliances burning solid mineral fuels

• The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 — CO alarm mandatory for solid fuel appliances

• Clean Air Act 1993 — Smoke Control Area powers

• HETAS — Competent person scheme for solid fuel and biomass

• DEFRA Exempt Appliances Register — defra.gov.uk/exempt-appliances — check before buying a stove for SCA use

• HETAS Technical Guide for Solid Fuel Appliances — industry guidance

• Approved Document J — HM Government

• DEFRA Exempt Appliances List

• NACS — Sweeping frequency guidance

• carbon monoxide — CO alarm requirements in detail

• part j combustion — Full Approved Document J overview

• hot works — Hot works permit requirements

• biomass boilers — Biomass boiler installation (similar HETAS registration)