In this study, two approaches to developing simple correlations have been described and analysed. The recent ‘effective thermal diffusivity’ model has been benchmarked and the advantages of the approach illustrated. The ‘uniqueness’ of this kind of approach has been discussed. The second approach, which is new and called the ‘effective velocity’ approach, has been described and tested, against the limited data sets available. It has been demonstrated that by preserving the first order effect (i.e. the convection effect) in the heating equation, the second approach gives an opportunity to correlate with good accuracy the experimental data (whether it is generated by CFD or field measurement). Here the essential features of the natural convection driven process is captured well. The predicted circulation velocity level matches well with the previous CFD simulations for the two dimensional situations, supporting the validity of the approach. More in depth and quantitative study is required before any of these models can be used in practice.