We experimentally study the effect of the magnetic force from one or more block magnets on natural convection of paramagnetic liquid inside a partially-heated duct. The permanent magnet(s) is/are placed partially/fully behind the heated wall. For a single magnet, the local heat transfer coefficient is suppressed near the bottom edge of the magnet and enhanced near the top edge if the magnet edge is behind the heated wall. This effect depends on the elevation of the magnet relative to the heated wall. Using two block magnets with alternating magnetic poles enhances the force and corresponding effect at their junction because of an increase in magnetic flux density. (C) 2018 Elsevier Ltd. All rights reserved.