Is it reasonable to calculate the thermal peak load using acausal models and short time steps?

Heating and cooling load estimation is based on the heat balance method. Although this method has strong physical basis, it yields an acausal system when the heat capacity of the indoor space is non-negligible. New emergent problems in low energy buildings require time steps significantly smaller than 1 hour, which is usually used by simulation software. Therefore, when the time step tends to zero, the calculated load tends to infinity when the indoor air temperature has an incremental variation. This paper demonstrates that the heat balance method results in an acausal system and analyses the variation of load estimation with the size of the simulation time step.