The scalar transport downstream of a localized blowing is analyzed through direct numerical simulations, in a fully developed low-Reynolds number turbulent channel flow subject to constant wall temperature. The Karman number is 180 and the Prandtl number is 1. The streamwise extend of the blowing spanwise slot is varied in a large range, going from the half channel height to particularly small values that are roughly twice the viscous sublayer thickness, while past published experimental and numerical results focused on significantly larger slots. Detailed data on the flow and temperature fields are reported and discussed. It is shown that, the severity parameter defined as the ratio of injected mass to the mean flow rate considered as the main similarity parameter so far, is no more valid when the streamwise extends of localized wall intervention becomes comparable to the buffer layer thickness. The Reynolds analogy breaks-up and the local wall transfer is significantly enhanced under these conditions. (C) 2011 Elsevier Ltd. All rights reserved.