Gas cookers and unflued gas heaters consume air and release products of combustion. They can make the room stuffy if there is inadequate ventilation to let in fresh air. Also, water vapour present in the combustion products can condense on cold surfaces and may lead to mould growth on walls and ceilings where there is poor ventilation. Never seal doors, windows, skylights or ventilation grilles while an unflued appliance is being used.

Unflued gas appliances produce small amounts of oxides of nitrogen, which may irritate the respiratory system. For this reason, unflued gas heaters are not recommended where there are people in the house who suffer from respiratory problems.

If an appliance has deteriorated, or if the wrong gas type is being burnt, harmful carbon monoxide may be produced.

To avoid these problems, appliances should be properly maintained, and the correct gas type must be used. Adequate ventilation will reduce the concentration of any harmful products of combustion produced by a faulty appliance.

If having an unflued heater installed, make sure it is the correct size for the room - if in doubt check with a licensed gas fitter or equipment supplier. Ventilation must be installed in the room at the rate of 10cm² per megajoule per hour of appliance rating at both high and low level.