SP has developed a procedure for calibration of heat flux meters. This procedure, described in ISO/DIS 14932-2 (method 2), comprises a spherical black body cavity as a radiant heat source. The furnace has a downward facing opening with the heat flux meter fitted in the opening in a specially designed holder. The walls of the opening, and the holder, are water cooled. The upper part of the holder incorporates flanges that prevent radiation from being reflected onto the heat flux meter. The downward position of the furnace aperture minimises the effects of air flow or convection that could otherwise result in significant errors. The equipment at SP has been upgraded. This report presents the results of calibrations performed in the new furnace, calculations to estimate the convective part, and a heat balance study in order to investigate the possible non-linearities in the voltage output. Several comparisons have been carried out between heat flux meters calibrated in the old and the new furnace, and the results have been found to agree.