A dehumidifier comprising - a housing provided with a process air inlet and a process air outlet; - a drying device provided with moisture adsorption means; - a fan adapted to bring process air entering through the process air inlet to pass the drying device and, subsequent to having been dehumidified, depart through the process air outlet; - means for heating regeneration air, wherein the regeneration air, during operation, passes through the drying device for regeneration of the moisture adsorption means; - a regeneration air outlet; and - a heat-exchanger with a first inlet and a second inlet adapted for effecting an exchange of heat between air entering through the first inlet and air entering through the second inlet; characterized by - a valve switchable between ∘ a first position, wherein the drying device and the regeneration air outlet are in fluid connection, allowing wet regeneration air to exit through the regeneration air outlet; and ∘ a second position, wherein the drying device and the second inlet of the heat-exchanger are in fluid connection, allowing an exchange of heat between wet regeneration air having passed through the drying device and air entered through the first inlet of the heat-exchanger, wherein the heat exchanger is configured so that wet regeneration air will be cooled and water vapour of the wet regeneration air will condense. - a housing provided with a process air inlet and a process air outlet; - a drying device provided with moisture adsorption means; - a fan adapted to bring process air entering through the process air inlet to pass the drying device and, subsequent to having been dehumidified, depart through the process air outlet; - means for heating regeneration air, wherein the regeneration air, during operation, passes through the drying device for regeneration of the moisture adsorption means; - a regeneration air outlet; and - a heat-exchanger with a first inlet and a second inlet adapted for effecting an - a valve switchable between ∘ a first position, wherein the drying device and the regeneration air outlet are in fluid connection, allowing wet regeneration air to exit through the regeneration air outlet; and ∘ a second position, wherein the drying device and the second inlet of the heat-exchanger are in fluid connection, allowing an exchange of heat between wet regeneration air having passed through the drying device and air entered through the first inlet of the heat-exchanger, wherein the heat exchanger is configured so that wet regeneration air will be cooled and water vapour of the wet regeneration air will condense. ∘ a first position, wherein the drying device and the regeneration air outlet are in fluid connection, allowing wet regeneration air to exit through the regeneration air outlet; and ∘ a second position, wherein the drying device and the second inlet of the heat-exchanger are in fluid connection, allowing an exchange of heat between wet regeneration air having passed through the drying device and air entered through the first inlet of the heat-exchanger, wherein the heat exchanger is configured so that wet regeneration air will be cooled and water vapour of the wet regeneration air will condense.