Many works have been performed on heated vertical rectangular open-ended channels. Whilst in most cases, thermal fields are quite well predicted or reproducible, there were often large unexplained variations in the experimental flow rate for apparently the same conditions. An experimental and theoretical investigation has therefore been carried out to identify the effect of external thermal stratifications on the flow rate. Four values of steady and uniform heat flux, equivalent to Rayleigh numbers between 2.9×104 and 1.6×108 were imposed experimentally either on one or both sides of a vertical rectangular channel, with various ambient thermal gradients external to the channel. It was observed that the mass flow rate was significantly reduced as the positive upward, external thermal gradient increased. A theoretical model of the phenomenon was also developed. There is an excellent agreement between the theoretically predicted and experimentally measured mass flow rates. This clearly highlights that external temperature distributions are key driving factors and their influence is accurately quantified in this work.