Chemical smell when lowering heat temp?

[usmarinedelta]

I normally have my heat temp at high and fan speed at 1. When I went to lower the heat to mid 70's there is this awful chemical smell, doesn't smell like coolant or anything, just like burning plastic. I tried this both on fresh air and re-circulating air. The kicker is when I raise the heat back to high, the smell goes away. Anyone have ideas? Thanks!