Hilchos Shemittah v’Yoveil 5:19

Hay and straw of shemittah may not be used as fuel because they’re fit to use as animal feed. However, the refuse of shemittah olives and grapes may be used for this purpose.

Hilchos Shemittah v’Yoveil 5:20

If a bathhouse was heated using shemittah hay and straw as fuel, one is permitted to bathe there, even for a fee. An important person, however, may not do so. This is out of concern that the bathhouse operators might mix other sabbatical produce into the fuel in order to make it more fragrant for him. By doing so, they would be wasting shemittah produce.