You may have what is commonly refered to as Dirty Socks Syndrome. You can Google it and find scads of related stories. It is not the heat strips, it is more likely on your coil. There is a spray on cleaner that works for a little while or you may want to investigate a UV light that treats the coil, not the ducts. Recently installed one in our house and no more smell. Ours was never very bad though, just at the very beginning of the heating season. I added a pretty informative link. Hope this helps.

Here is a link that might be useful: HVAC Training Center

Not a dirty socks smell and it was there from when the system was brand new. Definately chemical and definately from the heat strips which have since been replaced.

However I found your link very interesting. For the past three years I've noticed a very occasional faint smell which may be something like that. After all the other hassles we've had with the system I suppose I pushed this little detail out of my mind. The smell reminds me of the smell around old drink refrigerators or icecream freezers in milk bars (what I think you call soda shops or ice cream parlours). It has only appeared in the last three years as I said. So maybe it's time to find out if we can get one of those UV lights. I don't remember that being an option when we had the system installed. Sounds like a good idea. Thank you.