Thermal radiation from bulk disorderly placed nonresonant emitters is incoherent broadband and isotropic. In an external magnetic field the thermal radiation from any source is circularly polarized. Here we propose a thermal radiation source which emits circularly polarized radiation and which is not placed in a magnetic field. The thermal source consists of a slab waveguide with etched chiral metasurface of fourfold rotational symmetry. Using the Fourier modal method we analyze the eigenmodes of the structure and the emissivity spectra. We demonstrate that due to the absence of a mirror symmetry of the metasurface its eigenmodes as well as the thermally generated electromagnetic waves become circularly polarized. In this paper we discuss the origin of this phenomenon in detail. We demonstrate that the degree of circular polarization in an optimized structure can be as high as 0.87.