I have a Glow worm fuelsaver mk2 boiler In a small Cupboard under the stairs that is over 10 years old it has never been any trouble but recently It has started to give off combustion smells from the flu I have installed a visual carbon monoxide CO2 gas detector but this has not changed colour. the odour is strong inside and outside the house. The flue doesn't appear to be blocked can anyone advise on this.