If your propane tank is old, and the propane drops below 15 percent, it can cause extra strong odor. The smell they add to propane actually can settle to the bottom of the tank over years, so once the level gets low, the smell will be stronger. I have experienced this only a few times over the years, but have been told from our local propane supplier that this is the case.

A restriction in the heatexchanger can cause the system to soot up giving it a strong acid type smell. You will notice black soot around the exhaust of the system.