Heater types and where carbon monoxide risk comes from

Where the air comes from and the products go

• Room-sealed: isolates combustion from occupied room air. Its air comes from outdoors or a ventilated uninhabited space, and its products discharge outdoors. A balanced flue is one arrangement: the outdoor inlet sits beside the outlet so wind pressure acts similarly on both; a concentric system combines them at a terminal. Other room-sealed layouts exist. Sound case and flue seals keep the separation, so inspect them: the design can still leak.
• Open-flued: takes combustion air from the room and relies on the flue to carry products outside. It needs both a sound flue and adequate room ventilation, and it can spill if the flue underperforms.
• Flueless: releases all its products into the room, relying entirely on complete combustion and ventilation.

A flued fire has a discharge path to outside. That path lowers room exposure only while the flue is clear and sound and the appliance has enough combustion air. Obstructions, leaking joints and an extractor that upsets the draught can still expose occupants to combustion gases. Check the flue and air supply together; a flueless heater instead needs its specified room ventilation and safeguards.

How heat is delivered

• Radiant fire: glowing ceramic radiants send out heat felt directly by people and surfaces, almost at once. Broken or misplaced radiants disturb combustion.
• Convector: heats room air as it passes through the appliance; in a natural-convection model, cooler air enters low and warmed air rises out above. Furniture blocking the grilles restricts heat removal and can cause overheating or impair safe combustion. Keep the specified openings and clearances free.
• Decorative fuel-effect (DFE): chosen for the look of a real fire over coals or logs. It gives relatively little useful heat and is open-flued, typically needing a generous chimney or flue. In a fireplace recess with a throat, Approved Document J asks for a permanent vent of at least 100 cm², unless the fire is no more than 7 kW (net), clears no more than 70 m³/h of flue gas without spilling, and is in a dwelling with air permeability above 5.0 m³/(h·m²).
• Inset live fuel-effect (ILFE): intended to deliver useful room warmth as well as a flame picture. These fires come in open-flued and room-sealed versions; identify the model and follow its flue instructions.
• Gas-fired air heater: the exchanger separates burner exhaust from the air supplied to rooms. A breach may contaminate that air and carry CO along the ducts. Keep a heater with a compromised separation out of use and assess the actual fault; staining or surface rust does not prove there is a hole.

A gas-fired flame-effect fire is still a combustion appliance, and many gas fires stand on a non-combustible hearth. Never use an outdoor appliance, such as a patio heater or barbecue, in an enclosed room: its products can build up to dangerous CO levels.

For an ordinary gas fire requiring a hearth, its non-combustible, non-friable top layer must be at least 12 mm thick. This top-layer provision does not remove every constructional-base requirement: a back-boiler fireplace has additional hearth or base provisions. Check the appliance instructions and the requirements for that installation.

Where CO risk comes from

CO forms when fuel burns incompletely, for example after poor air supply, a disturbed fuel bed or burner faults. A blocked flue may also disturb combustion and cause spillage; a leaking flue can let products reach occupied rooms. Formation and escape are separate faults to check. CO cannot be seen, smelt or tasted, and a fire may keep heating while producing it.

Signs to act on:

• sooting or staining on the wall above or around the fire
• large, lazy yellow flames where blue ones are specified
• headaches that ease when people leave the room

Keep a suspect fire out of use and arrange investigation. If anyone has possible CO symptoms, get them into fresh air and obtain medical advice promptly; leave suspect appliances off and do not go back inside until told it is safe. Severe symptoms require emergency medical help. Turning the fire down or adding an alarm does not correct a fault. An alarm gives a backup warning, and regular servicing can identify combustion and flue problems.

Fitting a gas fire into an old solid-fuel chimney

Before a gas fire is commissioned on a chimney that last served a solid-fuel appliance, sweep it and confirm it is sound and clear. Old soot and debris left behind can fall and obstruct the new fire's flue, so a sweep's certificate does not replace the flue-flow and spillage tests that still follow.