As I mentioned I see quite a lot in a given week. They do get very dirty at times, especially if they have laminate floors and pets. It is the way they operate. Combustion ventilation comes from the open room, so the boiler is constantly drawing in air which pulls in dust and pet hairs with it. Unless this is extremely excessive, it is generally nothing to worry about if it is serviced every year. Make sure their is a CO detector fitted and make sure the vent is not covered. Take the vent cover out and check inside. Covering the vent will make the boiler work harder for its air, drawing in more crap.
