I had just assumed that people realized servicing fuel burning appliances is necessary. However, several cases of service neglect have come across the workbench recently. And, in one case, an owner was surprised to learn that service is called for in the manual.
These things burn fuel; sometimes dirty fuel. One of the most common question about these types of heaters and cook tops is the use of kerosene versus ordinary diesel fuel. It is perceived that kerosene burns cleaner, but the penalty is a short shelf life. Regular pump diesel has a longer shelf life, but the best of both worlds may be #1 winter blend.
The picture at the top is from a high hour unit burning diesel from the boat's main tank, which in this case is large. Sometimes algae grows in the fuel and can lead to deposits like this. The issue with this over the long term are the hot spots created in the burn pot from the burning residue, as evidenced by the failed thermo-couple.
All types of common diesel heaters have service intervals that should be followed for the best performance possible.