Thermal comfort and emission efficiency are important factors when choosing heat emission systems in low energy buildings. In this study, we consider the context of northern low-energy buildings, and compare the three most popular heat emission systems: radiator, underfloor and air heating systems with upper distribution. With an analysis based on laboratory measurements performed in Tallinn, Estonia (Northern European climate), we determine the vertical temperature gradient and quantify heat emission losses in a typical low energy building with mechanical heat recovery ventilation. Compared to a reference ideal heater, ventilation-supplied air heating with upper distribution induced a vertical temperature gradient of about 1 K/m, which together with operative temperature variation increased the energy use by 9.9%. Regarding radiator and underfloor heating, the losses amounted instead to 2.3% and to 5.5%–7.9% respectively. Using our air and surface temperature measurements, we also verified experimentally the operative temperature variations previously determined for radiator and floor heating, and extended this concept to air heating. In conclusion, it is proposed to add operative temperature variation to the heat emission standard in order to enable fair comparison of heat emitters. Both vertical temperature gradient and operative temperature variations determined in this study provide a solid extension of the EN 15316-2:2017 methodology.