When a boiler flue fails a gas safety check, the engineer classes the installation as At Risk (AR) or Immediately Dangerous (ID) and attaches a Danger Do Not Use warning label. With your permission the gas to the boiler is turned off, and it stays off until a Gas Safe registered engineer has put the fault right. Using a gas appliance you know to be unsafe, or reconnecting one that was isolated on safety grounds, is a criminal offence.
What does an engineer check on the boiler flue during a gas safety check?
A flue check begins with a visual inspection of the whole flue system and its seals, before any combustion readings are taken. Manufacturer servicing instructions set out what that covers: the integrity of the flue system and flue seals, the fall of the flue back towards the boiler, and a terminal that is undamaged and not blocked or restricted. The terminal is the visible outlet where the products of combustion leave the building. Where a plume management kit is fitted, which redirects the visible vapour away from a window or a path, it is checked for damage.
The duty behind the check sits in regulation 26(9) of the Gas Safety (Installation and Use) Regulations 1998 as amended, which applies in England. Anyone who works on a gas appliance must immediately afterwards examine the effectiveness of any flue, the supply of combustion air and the operating pressure or heat input. Regulation 27 adds that an appliance must not be connected to a flue that is unsuitable, or not in a proper condition, for safe operation of the appliance.
Combustion readings follow the visual work. The engineer uses a calibrated flue gas analyser, runs the boiler at its maximum rate, then compares the carbon monoxide reading and the combustion ratio with the figures printed in the manual for that exact model. Those figures vary between models, so there is no single pass number for all boilers. All of this is work for a Gas Safe registered engineer, including anything behind the casing or inside the combustion circuit.
Flue At Risk or Immediately Dangerous: what the two gas safety codes mean
Both codes come from the Gas Industry Unsafe Situations Procedure, the document engineers follow when they find something unsafe that they cannot put right on the spot. At Risk and Immediately Dangerous both result in the same Danger Do Not Use label on the appliance.
| What it covers | At Risk (AR) | Immediately Dangerous (ID) |
|---|---|---|
| Why the code is given | The installation is not an immediate danger, but a flue fault or a flue that cannot be verified could make it dangerous in use | The flue fault is a danger to life or property if the boiler is used in the state it is in |
| Label left on the appliance | Danger Do Not Use | Danger Do Not Use, the same label and wording as At Risk |
| Reporting beyond your home | Recorded on the paperwork with the defect found and the action taken | Also reported by the registered engineer where the situation could cause death, unconsciousness or a hospital stay |
| A flue example | A concealed flue in a ceiling void that cannot be seen or inspected because there are no hatches | The more serious case, such as a flue fault that could let products of combustion reach the living space |
Neither code is a prediction about your boiler, and neither is advice you can weigh up. Both mean the appliance must not be used until a Gas Safe registered engineer has corrected the fault. The wording of the codes and the readings written next to them are set out in our guide to what Immediately Dangerous and At Risk mean on a gas safety check.
Can the engineer turn off the boiler if the flue fails, and can you turn it back on?
An engineer who finds an unsafe flue will ask for your permission to disconnect the boiler, and HSE is clear that giving that permission is in your own interest. Registered engineers are instructed to disconnect appliances and installations that would be dangerous to life if they were used. If a Gas Safe registered engineer has turned your boiler off, it is because the appliance is unsafe and must not be used.
An appliance isolated or disconnected on safety grounds must not be reconnected under any circumstances until the fault has been put right. It is a criminal offence to use, or to let someone else use, a gas appliance that you know to be unsafe.
The codes, ticks and readings that appear next to a failed flue on the paperwork are explained in our guide to ID, AR and NCS on a gas safety record. What you can still do in the house is limited to the safe householder checks: boiler pressure, thermostat and programmer settings, the reset button and bleeding radiators. None of those will clear a flue classification, and pressing reset on a boiler that has been failed is not a repair.
Why a boiler flue hidden in a ceiling void is classed At Risk
A flue that runs concealed in a ceiling or wall void, with no way of seeing it, is classed At Risk under the industry procedure, and the gas to the boiler is turned off with the owner's agreement so the appliance cannot be used. HSE raised this because certain flues on gas central heating boilers, mainly in flats and apartments built from around 2000, may have been installed poorly or may have deteriorated. If a flue fault coincides with a boiler that is not working correctly, dangerous amounts of carbon monoxide can enter the living space.
Technical guidance asks for inspection hatches wherever the flue is concealed and cannot be seen, and the deadline for flat owners to arrange them was 31 December 2012. In certain cases a carbon monoxide void monitoring safety shutdown system is accepted instead of hatches. Where no hatches exist yet, the registered engineer carries out a risk assessment: looking for signs of leakage along the flue route, taking flue gas and combustion readings, and checking whether suitable audible carbon monoxide alarms are present.
HSE states plainly that owners should not try to inspect the flue themselves unless they are Gas Safe registered, because they are more likely to cause damage and increase the risk. Once hatches are in place, the engineer can establish whether the flue was installed safely to the relevant standards and the manufacturer's instructions. What the hatches have to allow and where they usually go is covered in our guide to a boiler flue in a ceiling void and the inspection hatch.
What a landlord has to do when the flue fails the annual gas safety check
Flues are part of the annual landlord gas safety check in their own right, not just the appliances they serve. In England, under the Gas Safety (Installation and Use) Regulations 1998 as amended, the record of the check must describe each appliance or flue checked and its location, name the engineer with their registration number and signature, and give the date, the property address and the landlord's or agent's details. It must also record any safety defect identified and any remedial action taken, so a failed flue appears on the paperwork with what was done about it.
Existing tenants must be given a copy within 28 days of the check, and new tenants before they move in. Records are kept until two further checks have been carried out. Where the heating appliance has been disconnected, the landlord has to provide alternative heating, and HSE points tenants and landlords to the local authority for further information while the repair is arranged with a Gas Safe registered engineer. The wider duties on checks, records and maintenance are set out in our guide to landlord gas safety in England.
Signs between checks that products of combustion may be reaching the room
Certain signs in the house point to incomplete combustion and possible carbon monoxide, and they are a reason to call a Gas Safe registered engineer rather than to investigate the appliance yourself. HSE lists yellow or orange flames instead of blue, soot or yellow-brown staining on or around an appliance, pilot lights that blow out often, and increased condensation on the inside of windows.
Carbon monoxide itself gives no warning. It cannot be seen, tasted or smelt, and it can kill quickly. Symptoms include headaches, dizziness, breathlessness, nausea, collapse, tiredness, chest or stomach pain, odd behaviour and visual problems, and early symptoms are easily mistaken for food poisoning, a viral infection, flu or simple tiredness. If you suspect carbon monoxide poisoning, seek medical advice urgently from a GP or A and E. HSE records that around seven people a year die from carbon monoxide poisoning caused by gas appliances and flues that were badly installed, poorly maintained or badly ventilated.
Does a failed flue mean a new flue, or a new boiler?
A failed flue is put right in one of two broad ways, and which applies depends on what the engineer found. Where the flue system itself is damaged, poorly supported, badly sealed or terminating in the wrong place, the flue work is the repair. Flue components are manufacturer specific: concentric flue systems and flue kits carry their own installation and testing instructions for that make of boiler. Where the appliance can no longer be verified as safe, or the flue route cannot be made to comply as it stands, the conversation moves to replacing the boiler and its flue together.
The situations that lead to a new flue with a replacement boiler, including a change of boiler type or a different position on the wall, are set out in our guide on whether you need a new flue with a new boiler. Where the outcome is a boiler replacement rather than a flue repair, you can arrange a free home survey and receive a written fixed price for a new boiler installation before any work begins.
Sources
- Domestic gas: frequently asked questions - HSE
- Gas boilers - flues in voids - HSE safety bulletin
- Gas Safety (Installation and Use) Regulations 1998, Part E
- Gas safety - carbon monoxide awareness FAQs - HSE
- Gas safety check records and what to keep - HSE
- Worcester Bosch Technical Bulletin TB 0127 A: flue gas plume