Strange odor related to HVAC running
I have been having a strange odor now for a long time. Most prominent when my forced air heater is running. I smell it throughout the house. It smells smoky/sulfury/cinammonlike. It is very hard to characterize. It gets so strong that I smell it with every breath. I get burning eyes and a headache and feel nauseated. My HVAC guy has checked my furnace multiple times and my system is only a few years old. He said nothing is wrong with it. I change the filter with the best type I can get every month. I bought a very expensive air purifier and even that doesn't get rid of the odor. If I don't have the heater running, after a few days the odor usually goes away. It is intermittent--I'll have the smell for days, and then just like that it vanishes. My HVAC is a closed system, so no real fresh air gets in. I am at my wit's end trying to figure out what it is. My neighbors smoke, but we have a cinder block wall between our townhouses, and it's not really a cigarette smell, nor is it a creosote smell (and my chimney was recently cleaned). I am losing my mind with this awful odor. I've tried tracking temperature/humidity/barometric pressure/wind. It seems to often start in damp conditions but not always. Help!