I've got a Vulcan ducted too, about 9 years old, worked great for the first 7 years ! Started having intermittent not lighting faults, cleaned and then replaced the flame sensor, this seemed to help for a while then on the coldest few mornings in winter it just wouldn't go till the frost cleared... I was working OS and had to get a repair guy in before I got divorced.. They claim the processor board was faulty, and I must admit it hasn't played up once since.
The symptom with mine was, the exhaust fan would start, the clicker would start clicking, gas would flow, unit would light, then extinguish after about a half second. Then repeat after a minute or so.