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

Quick Answer: A thorough flue inspection combines a visual check with a mirror and torch, a smoke test to verify draw and check for spillage, and — where structural defects are suspected — a CCTV camera survey. There is no single BS standard that mandates a specific inspection sequence, but HETAS guidance and the Guild of Master Chimney Sweeps recommend these checks as best practice before and after every sweep. Defects must be reported to the householder in writing, and a condemned appliance should not be operated until remediated.

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

Quick Reference Table

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Inspection Method What It Checks When to Use
Mirror and torch from hearth Firebox condition, throat, visible lower flue Every sweep
Torch from above stack Pot condition, liner visible at top Every sweep (if safe roof access)
Smoke match / pellet Draw, spillage, air-tightness to practical level Every sweep
Smoke pellet (higher volume) Large open fireplaces, double-check of poor-draw flues As needed
CCTV survey (camera on pole) Full structural condition of liner, mortar, any collapse Pre-purchase, post chimney fire, suspected fault
Structural engineer assessment Chimney breast movement, stack lean, major cracking Sweep identifies signs of movement
Defect Grade Action
Minor soiling on register plate Advisory Monitor
Small mortar gap at throat (visible) Advisory Recommend repointing
Cracked clay liner tile (confirmed by CCTV) Immediate action Do not use — reline or repair
Collapsed section of unlined flue (CCTV) Immediate action Do not use — structural repair required
Rope seal worn on stove door Advisory Replace before next use
Baffle plate missing or collapsed Immediate action Do not use until replaced
Smoke spillage during smoke test Immediate action Do not use — identify cause

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:

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:

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:

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