The natural convection in horizontal concentric annulus with isothermal walls is studied numerically by using the recently-proposed variable property-based lattice Boltzmann flux solver (VPLBFS). The widely-concerned constant property solution (CPS) and the real solution (VPS) in which the total variation in fluid properties is considered, are obtained by using the simplified version and the standard version of the VPLBFS, respectively. The integral effect of the total variation in fluid properties on flow patterns and heat transfer rates is discussed, and the deviation of the CPS from the real solution is quantified in a wide range of temperature difference ratio. Then, several commonly-reported solutions with partial variation in fluid properties (P-VPSs) are obtained by using various simplified versions of the VPLBFS. The individual effects of partial variation in fluid properties are discussed through the comparison between the P-VPSs and the CPS. It is found that the individual effects depend on individuals, but none of which can be regarded as a good approximation to the integral effect of the total variation in fluid properties as a large temperature difference ratio is imposed. The present work gives the real solution for the natural convection heat transfer in horizontal concentric annulus and demonstrates the necessity of considering the total variation in fluid properties in the study about thermal flow problems in which the Boussinesq approximation is invalid. (C) 2017 Elsevier Ltd. All rights reserved.