Indoor climate and heat demand of surface-heated rooms considering the radiatively participating air

Radiation absorption by water vapor in indoor air is already a well-known phenomenon. However, the radiative heat transfer between surfaces and air is frequently incorrectly deemed negligible in building energetics and HVAC (Heating, Ventilation, and Air Conditioning) simulations. Thus, we demonstrate a model for numerically determining the radiative heat transfer between the room envelope – containing a heated surface – and the indoor air. Considering a broad range of boundary conditions, the effect of radiative heat transfer between surface and air on the indoor climate and the heat demand of a room is analyzed in a Monte-Carlo based sensitivity analysis.It is found that considering radiation absorption by the room air can significantly reduce the spread between the mean radiant temperature and the air temperature. Extreme deviations occur with large room sizes, high air exchange rates, high relative humidities, low thermal transmittances and low outdoor temperatures. It is found that neglecting radiation absorption by the indoor air leads to an underestimation of the heat demand if the ratio of ventilation to transmission heat losses increases. Within the cases investigated, the room heat demand increases by up to 30% if radiation absorption of water vapor is considered in the room energy balance.