I have a combi boiler in a property who's flue vents out of the wall onto a public pathway at about 2.3 m off the ground. The fitters who put it in have just done a sevice 12 months and state that the flue ( plastic ) should be covered with a metal grill. If this the case any why. They fitted it 12 months ago but did not put a gril over it then. Regards Peter