Heat is lost from a building or building part by transmission or ventilation, as a result of a positive temperature difference between the indoor and outdoor (or other) environment. However, if the indoor temperature of the considered building or building part is lower than the temperature of the other environment (outdoor environment or another building part), the heat loss becomes negative. Therefore the more neutral term “heat transfer” was introduced from the 2007 versions of the EPBD related standards on. The downside of this is, that now it must be explicitly stated which direction has the positive sign. By definition, if the heat flows from the considered building or building part to the other environment, it has a positive sign.
NOTE: a negative heat loss should not be confused with “heat gain”. The term heat gain (see e.g. EN ISO 13790, energy use for heating and cooling) is used for solar and internal gains which are not a function of the indoor-outdoor temperature difference: if the indoor temperature rises, e.g. due to excess gains, the heat transfer by transmission and ventilation will automatically rise proportionally, but the solar and internal heat gains remain the same (compare the electric equivalent: a current through a resistor versus a current source).
Question submitted by: Dick van Dijk (TNO, The Netherlands)
Anwered by: Dick van Dijk, TNO (The Netherlands) and Brian Anderson, BRE (UK)