Shevi'is, Chapter 8, Mishnah 11. This mishnah too deals with the sanctity of shevi'is, and specifically with the question of how it affects things that are not fit for eating.
"Merchatz shehusak b'teven uv'kash shel shevi'is":
A bathhouse that was heated with straw or stubble of the shemittah year - "mutar lirchotz bah" - one is permitted to bathe in it. The Kesef Mishneh writes that one may even use these materials at the outset, since this use is for a human need.
The permission stems from the fact that straw and stubble are not fit for eating. As we learned earlier in the tractate, the rule is that something not fit for human consumption may be used for other purposes, such as making a medicine or a bandage, provided that it is not fit for eating. The same applies here: using it to heat the bathhouse is permitted.
"V'im mitchashev hu - harei zeh lo yirchotz":
A prominent person, someone of standing, may not bathe in such a bathhouse, and it is forbidden to him even after the fact. Two approaches are offered to explain this stringency:
Since he is a significant person, the sort of person whose conduct others learn from, it is incumbent upon him to be stringent with himself.
Some understand that the reason is that because of his importance, people are liable to honor him by adding materials to heat the bathhouse in order to give him a pleasant fragrance, and among them things that are fit for eating - and thereby a violation of the sanctity of shevi'is would occur.
This understanding resolves a difficulty: in light of the words of the Kesef Mishneh that one may use the stubble even at the outset, it was hard to understand why we would be stringent with a prominent person regarding something entirely permitted. But once it is explained that there is a possibility that the bathhouse owner will add, in his honor, materials that are fit for eating, it is understandable why a prominent person would not bathe in a bathhouse heated with straw and stubble of shevi'is.
In summary: A bathhouse heated with straw and stubble of shevi'is - it is permitted, even at the outset, to bathe in it, since these are not things fit for human consumption. A prominent person, however, refrains from this: because his conduct is learned from and he must be stringent with himself, and because of the concern that materials fit for eating will be added in his honor and the sanctity of shevi'is will be violated.