Water (especially hort) smells with heating oil (shower, cold, filter)

[BusyMeAK]

We have System-2000, it also heats our water. Recently, I noticed that the hot water smells with heating oil. I can't drink hot water at all. Cold water also smells but way less, so I filter it and the smell disappears. I feel the smell when I shower. It must be pretty strong id even I can sense it (I'm almost smell-blind).

Question: Where can be a hole that lets heating oil from System-2000 get into the house
water system?