Experimental study of heated horizontal rectangular fin array (HRFA) under natural and assisting mode of mixed convection is presented in this paper. Mainly two type of flow patterns are encountered in case of HRFA namely: (i) single chimney flow pattern (L/H <= 5) and spacing about 8-10 mm, (ii) sliding chimney flow pattern L/H > 5 and small spacing less than 6 mm or so. It was envisaged to use mixed convection in its assisting mode to convert sliding chimney to single chimney flow pattern and take advantage of large area (A), due to small S and favorable flow to augment heat dissipation rate from the fin array. The mixed convection heat transfer has been investigated for a wide range of Rayleigh numbers and different fin spacings and flow velocities. The constant heat flux boundary condition is present with air as surrounding fluid. Experiments are conducted for Rayleigh numbers 4 x 10(4) <= Ra <= 3 x 10(5), Reynolds numbers 80 <= Re <= 1500 and Richardson number 0.08 <= Ri <= 5. Dimensionless fin spacing to height (S/N) ratio is varied from 0.05 to 0.3 and L/H = 5 is kept constant. A fan developed upward velocity in the range between 0.05 <= V <= 0.5 m/s. Assisting mode results of mixed convection are compared with natural convection for various heater inputs and Ri numbers. The average heat transfer coefficient has increased up to 69.46% at cost of very small energy input (0.3-0.65 W). At small spacing (S = 2-5 mm), a large surface area is available to dissipate. The optimum spacing region becomes wider (S approximate to 4-6 mm) under mixed convection as compared to natural convection (S approximate to 8-10 mm). An empirical equation has been developed to correlate the average Nusselt number as a function of Re, Gr and S/H within uncertainty of +/- 10%. (c) 2014 Elsevier Ltd. All rights reserved.