## 1 Introduction  
The energy performance of heat emitters is a key factor in the energy demand of the building sector, which is primarily determined by space heating [1; 2; 3]. Such devices can be of very different type (panel radiators, connectors, ceiling and underfloor heating...), each determining the energy demand in a specific way [4; 5; 6; 7; 8]. For these reasons, several studies have investigated the emitters' performance on both the experimental and theoretical viewpoint [9; 10; 11], focusing especially on the design, specific type and room placement of panel radiators (e.g., close to a window or slightly detached from a wall) [4; 12; 13; 14; 15; 16; 17; 18]. For instance, measurements have shown a better performance of low temperature panel radiators [19], and a sensibly different outcome for serial and parallel connected radiators [9]. Despite such recent advances, this kind of investigation seems to be very involved, for a variety of reasons. Contrasting results also exist: an experimental investigation of a connector, a radiant and a baseboard heater showed a lower energy consumption by the connector [7], in contrast with the classic work by [PERSON] et al. [4], written in the early 80s. While it was concluded in [7] that the cause was probably the improved flow outlet design of the newer convector, an older study already considering this improvement [13] agreed instead with [4]. Previous research regarding the energy performance of heat emitters has largely dealt with matters of emission efficiency and losses within the system. In such a framework, one typically addresses the emitted power and energy usage by the system under predefined boundary conditions [20; 21]. So far, this approach has been effective enough for product characterisation. However, the methodology fails to take into account the different levels of thermal comfort provided by the heaters at equal air temperature (which is the target value that is kept constant throughout product comparison). Such a difference in the thermal environment can affect negatively even the perceived productivity in the workplace, with a seizable economical impact [22; 23]. More importantly, accounting for this phenomenon could allow for some direct energy savings if different set-points within the heating systems could be used. Thermal comfort within an enclosure can be quantified with the so-called operative temperature (hereinafter op.t. or \(t_{op}\)), which is finding an increasing use recently [9; 13; 18; 24]. This is defined as the uniform temperature of an enclosure in which an occupant would exchange the same amount of heat by radiation and convection as in the existing non-uniform environment [25]. Unfortunately, such a formulation generates a fundamental problem for product comparison, as it does not provide one clear \"ideal\" emitter to benchmark others against. For ordinary systems where the air temperature is controlled, such an ideal heater is generally described as a dimensionless point (Figure 1). Heat is transferred by convection to the thermal node of indoor air, and by radiation to the surrounding surfaces. A higher convective transfer fraction induces lower surface temperatures on the surrounding surfaces with minimal heat loss, as in Figure 1. To reach the desired air temperature set-point \(t_{air}\), an emitter with convective fraction of 1 (pure convection) thus requires the lowest possible heat output. In this sense, it represents an \"ideal\" heater. By the definition of operative temperature however, \(t_{op}\) is inversely proportional to the surrounding surfaces' temperatures. In this respect, the \"ideal\" device described above should now exhibit a lower performance, as it heats the surrounding surfaces only minimally. As we will illustrate in the following, preliminary simulations with the software IDA ICE [26] confirm indeed that several real emitter configurations can outperform the point heater (or convector).