In this paper, we present 3D finite element modeling and simulation of a CMOS MEMS single axis convective accelerometer. We describe the sensor architecture and present a sensor geometry model to be used in 3D FEM simulations. Differences between 3D and previously published 2D simulations are discussed. This work investigates 3D effects which give the opportunity to better predict not only sensor sensitivity but also power dissipation. Experimental sensitivity values and 3D FEM ones are compared for two different sensor geometries and two different heater temperatures. For a prototype having a heatercavity border distance of 340 mu m and a heater length of 230 mu m, maximum sensitivity point is obtained for detectors localized at a distance of 125 mu m from heater center. This distance should be moved to 90 mu m if a 50 mu m heater length is used. So, detectors should be placed closer to the heater than the usually used mid distance. Moreover, optimal detectors location shifts closer to the heater as heater length shrinks. We also show that if heater length is reduced by 80% (from 230 to 50 mu m), then both electrical power and sensitivity decrease by 63% and 55%, respectively. So, best efficiency is obtained for shorter heaters. In addition, detectors length decrease is found to have a significant effect on sensitivity, with an increase of 58% and 87% using heater lengths of 230 mu m and 50 mu m, respectively. Here, detector's length decreased from the total side bridge length to a fraction of this length equals to 2.5%. Optimal length is obtained when detectors are implemented on the same side bridge fraction as that used to implement the heater on the central bridge.