## 5 Conclusions  
In this paper we have compared quantitatively the operative temperature induced by different types of heat emitters, in the search for the most performing, or \"ideal\", heater for thermal comfort. We considered several configurations of practical interest, with analytical and numerical calculations of \(t_{op}\) performed in a test room with a standard size defined by updated European Standards. We first addressed panel radiators of 10- and 21-type installed on the cold wall, for a variety of sizes and surface temperatures. Compared with an ideal convector providing the same output \(\sim\)134 W, we found the 10-type to be the most performing radiator, and accordingly the \"ideal\" radiator for the setup considered in this paper. By means of our analytical calculations, we were able to draw several considerations, proving for instance that the thermal comfort performance of radiators is more sensitive to the height than to the width. For underfloor UFH and ceiling heater strips, we identified the occurrence of non-trivial global maxima, corresponding to the highest temperature sensed by a person sitting in the middle of the room. Furthermore, compared to typical radiator sizes with height 0.6 m, the UFH provides 0.25 K-0.3 K higher \(t_{op}\) relative to the 10-type and 0.35 K relative to the 21-type. In general, we found the convector and the 21-type panel radiator to be fairly underperforming. Specifically, the former shows the worst performance: the op.temp. is lower by 0.55 K when compared to UFH. Let us remark anyway that the air and operative temperature differences calculated in this study should not be directly applied for energy saving assessment, because they are valid at the outdoor temperature \(-\)15\({}^{\circ}\)C, which is much lower than the average heating season value. In conclusion, the investigation presented in this paper constitutes a good starting point for several improvements in the search for an ideal heater. Here we have addressed the operative temperature as the sole parameter that is responsible for thermal comfort; we therefore do not claim by any means to have provided an ultimate method for determining an ideal heater. Taken as a whole, the phenomenology of heat transfer and thermal comfort inside an enclosure depends indeed on several factors that are very difficult to track simultaneously. For these reasons, there definitely exists some ground for future work. As remarked in the Introduction, our method can be easily applied to HVAC as a way to fine-tune the room ventilation. Also, we can investigate how any quantity of interest (such as the op.t.) is affected by e.g., the emitters position, the location and size of the windows, the emissivity of materials and so on. Furthermore, parametric studies on the relationship between view factors, room and geometry of the emitter might show a more general pattern, whose impact on the whole energy demand could be quantified with e.g., annual simulations. Conceptualization, A.F. and J.K.; methodology, A.F. and J.K.; software, A.F. and K-V.V. ; validation, K-V.V. ; formal analysis, A.F. ; investigation, A.F. ; writing--original draft preparation, A.F. ; writing--review and editing, A.F., K-V.V. and J.K.; supervision, J.K.; funding acquisition, J.K. The research was supported by the Estonian Research Council with Institutional research funding grant IUT1-15. The authors are also grateful to the Estonian Centre of Excellence in Zero Energy and Resource Efficient Smart Buildings and Districts, ZEBE, grant 2014-2020.4.01.15-0016 funded by the European Regional Development Fund. The authors declare no conflict of interest.