[caminante]

I doubt it's the furnace, unless there's a serial defect/recall.

The problem is likely "between the keyboard and the chair." ;-)

[PaulDavisThe1st]

I had a gas furnace that wasn't properly maintained as far as cleaning. Result: insufficient air flow for full combustion. Secondary result: CO build up in basement space. Tertiary result: asthma-like symptoms for me.

[caminante]

I believe it.