This Chimney Champion Fan Proving Switch for Gas/Oil Fuels is a safety feature that makes sure your fan is working properly before allowing your fireplace to be turned on. It is a simple switching device that will sense pressure in the duct (air flow). If there is now air flow from the fan because of some blockage or mechanical failure the Chimney Champion Fan Proving Switch for Gas/Oil Fuels will sense it and shut down the fireplace.