This paper presents a numerical study of coupled heat and mass transfers in an adsorber with external fluid heating. A one-dimensional model incorporating both heat transfer and Ergun flow of the adsorbate inside the adsorber is developed. Numerical investigations are made for two adsorbent-adsorbate pairs: zeolite 13X-water and 13X-ammonia, whose operating pressures are considerably different. It is shown that, for the 13X-water pair which has a low operating pressure and therefore a low gas density, the dynamics of the adsorber can be significantly limited by the mass transfer inside the adsorbent bed, when its dimension is large. The poor mass transfer can lead to very large pressure variations and small adsorbed or desorbed quantities, making the heat pump process ineffective. Conversely, the zeolite-ammonia pair operates at much higher pressures (several bars) and the corresponding pressure variations in the bed are, in general, negligible. In this case, the limitation by mass transfer can be neglected and the uniform-pressure assumption is acceptable.