A new conduction calibration apparatus has been designed to deliver heat fluxes up to a maximum of 100 kW/m2 with an established goal of ± 5 % precision. This system will provide a close to purely diffusive (as opposed to radiative) heat flux boundary condition and, when compared to the gauge’s response in the National Institute of Standards and Technology (NIST) radiative calibration facility, act as a check on the sensitivity of a heat flux gauge to the mode of heat transfer. A platinum-plated copper block heated electrically with 2 kW power is designed to produce uniform temperatures up to 750 K across its face. A cold plate will be maintained around 290 K through pool boiling using a liquid refrigerant and a remote condenser. A 1 mm wide helium filled gap between the hot plate and the sensing surface of a cooled heat flux gauge will provide the high conductive fluxes desired (while limiting radiation to a few per cent and avoiding the uncertainties associated with contact resistance). Detailed numerical modeling of the device is being used to identify limitations and evaluate alternatives in the design, and to analyze the level of uncertainty associated with the facility. A description of the apparatus and the results of preliminary modeling are reported.