The effect of the gap width between two symmetrically heated vertical, parallel, isothermal plates on intensity of natural convective heat transfer in a gas (Pr = 0.71) was experimentally studied using the balance and gradient methods. In the former method heat fluxes were determined based on measurements of the voltage and electric current supplying the heaters placed inside the walls. In the latter, heat fluxes were calculated from the temperature distribution in the air in the plane perpendicular to the surface of the heating plates. Temperature fields were visualised using a thermal imaging camera. The analysis was conducted on two parallel vertical plates of height H = 0.5 m and width B = 0.25 m with the heated surfaces facing each other. Vertical planes with peripherally open channels and three different distances s = 0.045, 0.08 and 0.18 m were created this way. The surface temperature of the heating plates t(W) was changed every 5 K and set at t(W) = 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 and 80 degrees C, while the ambient temperature range was from 18 to 25 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.