Flue Inspection and Testing: Smoke Test, Mirror Inspection, CCTV Survey and Reporting Defects to Customer

Summary

Inspecting a flue is not optional box-ticking — it is the core professional duty of a chimney sweep. A blocked or cracked flue can allow carbon monoxide to enter the living space, create conditions for a chimney fire, or cause incomplete combustion that damages the appliance. The sweep's inspection is often the only check a flue receives between installation and the next sweep, which may be 12 months away. Getting it right matters.

Many householders assume that a sweep is just someone with a brush. In practice, a qualified sweep is conducting a condition survey of a complex built structure that includes the firebox, smoke chamber, register plate, flue liner, chimney stack, and terminal. Each of these can fail independently, and each failure mode presents a different risk. Understanding the full inspection sequence — visual, smoke test, and where needed CCTV — is what separates a professional sweep from a domestic brush-wield.

A common misconception is that a smoke test passing means the flue is structurally sound. A smoke test checks draw and air-tightness to a practical level; it cannot identify cracks in clay liner tiles, deteriorated mortar joints deep inside the flue, or partial collapses within a lined or unlined stack. For those, a CCTV survey is the only reliable diagnostic. Knowing when to recommend one — and being able to articulate why — is part of the sweep's professional obligation.

Key Facts

• Visual inspection uses a torch and inspection mirror; the sweep checks the firebox, throat, and as far into the flue as visibility allows
• Mirror inspection is conducted from the fireplace opening looking up — in a straight flue on a bright day you should be able to see daylight at the top
• Smoke test uses a smoke match or smoke pellet placed in the firebox to check draw and spillage — smoke should travel up the flue, not spill into the room
• Draw is the negative pressure that pulls combustion gases up the flue; poor draw can indicate a blockage, too-short a flue, or a cold flue that needs warming
• Spillage test checks that smoke does not escape the appliance into the habitable space — critical for CO risk
• Open fire smoke test: light a smoke match or pellet in the hearth with the damper open; observe smoke movement — it should immediately draw upward
• Stove smoke test: introduce smoke with the stove door open slightly and the damper open; for stoves with glass doors, do not close the door during smoke testing — smoke must not escape the glass seal before draw establishes
• CCTV chimney survey uses a purpose-built camera on a flexible or rigid pole, lowered from the top of the stack or fed from the firebox upward
• CCTV is recommended before a property purchase, after any chimney fire, when a structural fault is suspected, or when a liner type cannot be confirmed
• Defect grading: advisory (monitor, no immediate risk) vs. immediate action required (appliance must not be used)
• Condemned appliance: sweep must advise the customer in writing that the appliance must not be used and document the specific defect; failure to do so exposes the sweep to liability
• CCTV surveys: no regulatory body controls who may carry them out; HETAS-registered sweeps and Guild of Master Chimney Sweeps members with appropriate camera equipment are the recognised practitioners
• Structural survey: if the chimney breast or external stack is suspected of movement or serious structural compromise, recommend a qualified structural engineer or surveyor — this is outside the sweep's scope
• Sweep liability: a sweep who fails to identify a visible or testable defect can be held liable if that defect causes harm; clear written records and a signed sweep certificate are the primary defence
• Defect reporting on the sweep certificate must include: location of defect, description, severity grade, and recommended action
• Fee for CCTV survey is typically £80–£150 additional on top of the sweep, depending on flue length and equipment used; always quote separately and explain why it is needed

Quick Reference Table

Detailed Guidance

Visual Inspection: Mirror, Torch, and What to Look For

Start at the firebox. Examine the firebox brickwork or chamber lining for spalling (brick face breaking off), cracks, crumbling mortar joints, and any dark staining beyond the firebox aperture that could indicate smoke tracking. Check the throat or gathering point — this is the narrowing above the fire that accelerates gases into the flue. Debris, bird nests, and displaced brickwork concentrate here.

With a torch and inspection mirror angled upward, examine the lower section of the flue above the throat. Look for white efflorescence (salt deposits leaching through wet brickwork), cracks in flue tiles, missing mortar, and debris shelves where combustible material may have lodged. In a straight flue on a bright day, light at the top is a useful initial check — no light suggests a partial or full blockage.

If the property has a lined flue, identify the liner type from the householder or inspection: flexible stainless steel (twin-wall or single-wall), rigid stainless, or original clay or terracotta flue tiles. This affects what defects to expect and how to assess them visually.

Smoke Test: Procedure for Open Fires and Stoves

For an open fire, ensure the damper or throat restrictor is fully open. Place a lit smoke match or smoke pellet on the hearth within the fireplace surround. Good draw is immediate — smoke should travel swiftly upward within a few seconds. Sluggish draw, smoke hovering, or smoke spilling into the room indicates a problem: check for blockage, cold flue (the flue may need warming with rolled newspaper before testing in winter), or an air pressure issue caused by extractor fans or trickle vents closed in an airtight house.

For a wood burning stove, open the door and the primary air control. Place the smoke source at the threshold of the door. Smoke should begin drawing into the stove almost immediately. With stoves that have glass doors, do not seal the door during a cold smoke test — if there is no draw, smoke will escape around the glass seal and fill the room. Establish draw first (with the door open), then close it gradually. The smoke test checks that the rope seal and door glass are not allowing cold air infiltration.

Note: the direction of travel is always up. If smoke drifts out horizontally, the draw is absent or reversed (downdraught). A short-term downdraught on a cold windy day can be distinguished from a chronic draw problem by repeating the test after warming the flue.

CCTV Chimney Survey: When and How

A CCTV survey is the only way to definitively assess the internal condition of a flue that cannot be seen by mirror and torch. The camera is typically lowered on a flexible rod from the chimney pot downward, or fed upward from the firebox where headroom allows. Modern systems display real-time video on a tablet or monitor, allowing the sweep to show the householder what the camera sees and record footage for the report.

A CCTV survey is strongly recommended — and should be offered, not just mentioned — in these circumstances:

• Pre-purchase inspection: the buyer wants confirmation of the flue's condition before exchanging contracts. A cracked clay liner in an unlined 1930s chimney can cost £3,000–£8,000 to reline. This information has direct financial impact.
• After a chimney fire: even a short, low-temperature chimney fire can crack clay liner tiles. The flue must not be used again until a CCTV survey confirms or rules out liner damage.
• Suspected structural fault: where the smoke test suggests a leak path but the source cannot be identified visually, CCTV will find mortar failure or a crack in the liner.
• Unknown liner type: in older properties where the liner type or installation quality cannot be confirmed, CCTV establishes the baseline.
• Intermittent draw problems: if the householder reports draw issues that are not explained by the standard sweep inspection, CCTV may reveal a partial collapse or debris shelf mid-flue.

Charge for a CCTV survey separately and explain the reason before carrying it out. Do not include it in the sweep price — it is a distinct service with distinct equipment and skill requirements.

Reporting Defects: Verbal, Written, and Grading Severity

Every defect identified during a sweep must be communicated to the customer. Verbal communication is a minimum courtesy but is not sufficient on its own — verbal reports are not recorded, may be forgotten, and do not protect the sweep legally.

Written reporting on the sweep certificate (or a separate defect sheet) should include:

• Location of the defect (e.g. "rope door seal, left-hand side", "clay liner tile at approximately 1.5 m above firebox")
• Description of the defect (e.g. "cracked tile — crack runs approximately 200 mm across tile face, some mortar missing at joint")
• Severity grade: advisory (no immediate risk, monitor or repair at next convenient opportunity) or immediate action required (appliance must not be used until remediated)
• Recommended action (e.g. "recommend CCTV survey to assess extent of cracking", "rope seal replacement required before next use")

Keep a copy of every sweep certificate, signed by the customer or photographed before leaving the property. This record is your primary defence if a liability claim arises later.

Condemning an Appliance: Procedure and Liability

If a defect presents an immediate risk — smoke spillage that cannot be resolved, a confirmed flue collapse, a missing baffle plate in a stove, or significant liner damage — the sweep must advise the householder that the appliance should not be used until the defect is remediated. This should be stated clearly in plain language: "I am advising you not to use this fireplace until [specific repair] has been carried out. I have noted this on your sweep certificate."

The sweep cannot physically prevent the householder from using the appliance. However, documenting the condemnation in writing — and ideally having the householder sign or countersign that they have received the advice — transfers moral and legal responsibility. A sweep who identifies a dangerous defect but does not document it in writing is exposed to liability if that defect subsequently causes harm, even if they delivered a verbal warning.

Do not overstep: condemning an appliance is not a permanent judgment. It is an instruction not to use it until a specified remediation has been confirmed. When the repair is done, the appliance may be retested and reinstated.

When to Recommend a Full Structural Survey

A chimney sweep's inspection is a condition survey of the flue and its immediate surrounds — it is not a structural engineering assessment. If the sweep observes any of the following, they should recommend the householder engage a qualified structural engineer or chartered building surveyor:

• Visible lean or movement in the external chimney stack
• Significant cracking in the chimney breast within the habitable room (particularly diagonal or stepped cracks in the masonry)
• Evidence of the chimney breast having been partially removed without adequate support
• Damp or efflorescence on the chimney breast that suggests the stack is not weathertight and water is tracking down inside the structure
• Any recent changes to the building structure adjacent to the chimney

These are outside the sweep's competence and professional indemnity. Recommend a professional, document your recommendation on the sweep certificate, and do not attempt a structural diagnosis yourself.

Frequently Asked Questions

Does a smoke test mean the flue is safe to use?

No — a smoke test confirms that draw is present and that there are no obvious large air-leakage paths at the level detectable by smoke. It cannot identify cracked liner tiles, deteriorating mortar joints deep inside the flue, or partial collapses within the stack. If the smoke test passes but there are other indicators of concern (age of property, history of chimney fires, unexplained draw problems), recommend a CCTV survey before clearing the flue for use.

Can a chimney sweep legally condemn an appliance?

A sweep has no statutory power to issue a legal prohibition notice — that authority rests with building control or local authorities. However, a sweep has a professional and moral duty to advise a householder in writing when they believe an appliance is unsafe to operate. The householder makes the final decision; the sweep's job is to document the advice clearly and ensure the householder cannot later claim they were unaware of the risk.

Who can carry out a CCTV chimney survey?

There is currently no statutory qualification required to carry out a chimney CCTV survey in the UK. HETAS-registered sweeps and members of the Guild of Master Chimney Sweeps with appropriate camera equipment are well-placed to provide this service. When commissioning or recommending a CCTV survey, look for documented qualifications and check that the operator can produce a written report with footage.

What should I do if I find a defect I cannot fully assess?

Document what you can observe and grade it conservatively — if you cannot determine the severity, grade it as requiring immediate action until proven otherwise. Advise the householder to commission a CCTV survey or structural assessment by a qualified professional. Never guess at structural severity when lives may depend on it.

How do I handle a customer who refuses to accept a defect finding?

Document the defect and your recommendation on the sweep certificate regardless. If the customer disputes your finding and insists the appliance is used, note their objection on the certificate and ensure they sign that they have received your written advisory. Your job is to provide accurate information and protect the householder — their decision to act on it (or not) is theirs to make as a competent adult.

Regulations & Standards

• Building Regulations Part J (England & Wales) — combustion appliances and fuel storage systems; includes requirements for flue construction and lining

• BS EN 15287 — design, installation, and commissioning of chimneys; includes performance requirements

• BS 8303 — installation of domestic heating and cooking appliances burning solid mineral fuels [verify current edition applicability]

• HETAS Technical Manual — inspection and testing guidance for sweeps; published and maintained by HETAS Ltd

• Guild of Master Chimney Sweeps Code of Practice — professional standards for sweep inspection, reporting, and certification

• HSE Carbon Monoxide Awareness guidance — context for why flue integrity is a safety-critical matter

• HETAS Ltd — www.hetas.co.uk — technical guidance, sweep standards, approved appliances

• Guild of Master Chimney Sweeps — www.guildofmasterchimneysweeps.co.uk — code of practice and sweep certification

• HSE — Carbon Monoxide: causes, symptoms, and prevention — www.hse.gov.uk/gas/domestic/co.htm

• UK Government — Building Regulations Part J: Combustion Appliances and Fuel Storage Systems — www.gov.uk/guidance/building-regulations-combustion-appliances

• chimney sweep certificate — What goes on the sweep certificate, frequency, insurance implications

• carbon monoxide safety — CO alarms, sweep duties, testing requirements

• chimney liner installation — Flexible and rigid liners, sizing, Part J compliance

• creosote and tar deposits — Identifying Stage 2 and 3 deposits that warrant CCTV follow-up