Heat loss prediction models for parabolic trough receivers do not consider the thermal effect of a secondary mirror. As an extension a Thermal Resistance Model (TRM) has been developed at Fraunhofer ISE for the prediction of the heat loss of three different Linear Fresnel Collector (LFC) receiver configurations. In previous investigations we have found the energy balance of a LFC receiver to be strongly influenced by the amount of solar radiation absorbed by the secondary mirror. This absorption provokes an increase of temperature of the secondary mirror and hence a decrease in the total amount of heat loss of a LFC. The size of this effect depends on the receiver geometry and diverse ambient parameters. Investigated parameters are wind velocity, ambient temperature and Direct Normal Irradiance (DNI). This dependency and its effect on heat loss and secondary mirror temperatures are analyzed for three different LFC receiver configurations. As the radiation absorbed by the secondary mirror is affected by the aperture area of the LFC, studies are performed for small-scale and for large-scale collectors.