Updated
Black Marks Or Soot Around A Gas Fire
Sooty or yellow-brown staining on or above a gas fire means combustion products are getting into the room.
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What's happening?
Staining above or around a gas fire is deposited carbon. It means the fire is either burning incompletely, or its products are spilling into the room instead of going up the flue — and often both, because a blocked flue starves the burner of draught. HSE names soot and yellow-brown staining on or around appliances as one of the visible signs that incomplete combustion is occurring and carbon monoxide may be produced. Gas fires and back boilers are the appliances most often involved, because they sit on old chimneys that were built for coal and may never have been lined, swept or checked since. Marks that appear or spread over weeks are worse than an old stain from a previous owner, but neither should be left.
What usually causes it?
- A blocked chimney or flue — bird nest, soot fall, or debris from a decayed lining
- A chimney that has never been lined for a gas appliance, so it is cold, oversized and drawing badly
- A missing, blocked or filled-in air vent serving the room
- A powerful extractor fan or a nearby open-flue appliance pulling the fire's flue gases back into the room
- A fuel bed laid out wrongly after cleaning, with coals or pebbles blocking flame paths
- Burner ports or injectors fouled so the fire burns rich
- A closure plate or catchment space fitted incorrectly at installation
Is it dangerous?
Yes. Staining on a gas fire is the visible half of a problem whose invisible half is carbon monoxide, and an open-flued fire in a living room is one of the highest-risk configurations in a home. Stop using the fire, ventilate the room, and do not clean the marks away before an engineer has seen them — the pattern of the staining tells them where the spillage is coming from. Get the appliance checked before the next cold snap, not after it.
Can I fix it myself?
Nothing on the appliance or the flue. You can and should uncover any air vent or airbrick serving the room that has been blocked, covered by furniture, or filled in during draught-proofing, since starving the fire of air is a direct cause. You can also stop running a kitchen or bathroom extractor while the fire is lit, as a test. Everything else — the fuel bed, the burner, the closure plate, the flue — is Gas Safe work, and disturbing the fuel bed yourself can make the combustion worse.
When should you call a gas engineer?
Call a Gas Safe registered engineer promptly, and do not use the fire in the meantime. If a CO alarm has sounded, or anyone gets headaches, drowsiness or nausea in that room, treat it as an emergency and ring the National Gas Emergency Service first. Landlords have to have this checked annually in any case; owner-occupiers get the same benefit from an annual service on any open-flued appliance.
What will the gas engineer actually do?
- Carry out a flue flow test and a spillage test with the room in its worst-case condition, extractors running
- Inspect the chimney or flue for blockage, lining condition and correct termination
- Measure the purpose-provided ventilation to the room against the appliance's requirement
- Strip, clean and correctly lay the fuel bed to the manufacturer's layout diagram
- Take flue gas analyser readings and record CO and CO/CO2 ratio
- Classify the appliance under the industry unsafe-situations procedure and cap it if it fails, with your permission
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