An Air Handling Unit, AHU, for a ventilation system in a building, said AHU having a supply air channel comprising at least one supply air inlet for guiding supply air from the outdoor into a building through at least one supply air outlet and an extract air channel comprising at least one extract air inlet for guiding extract air from a building to the outside through at least one extract air outlet, said supply air channel comprising a supply air damper in order to control the air flow in the supply air channel and said extract air channel comprising an extract air damper in order to control the air flow in the extract air channel, said supply air channel and said extract air channel being in a heat exchanging relation to each other via a heat recovery arrangement, e.g. a heat exchanger and/or a heat pump, said AHU further comprising at least one fan (11a, 11b) in order to induce a flow in said air treatment system and an Electronic Control Unit, ECU, programmed to output a defrost cycle initiation signal including a control signal to a flow controller, such as an air damper (4a, 4b) or a fan (11a, 11b), or a heating element in order to change or heat the air flow to defrost said heat recovery arrangement when there is an indication of an undesired high level of ice in a portion of said heat recovery arrangement according to a first predefined criteria and a defrost cycle termination signal in order to end the defrost cycle when there is an indication of the defrosting being completed according to a second criteria, said AHU is further provided with a pressure indicating system connected to the ECU characterized in that said first and second criteria are pressure drops measured over a heat recovery arrangement, said ECU is further programmed to - change the first, higher pressure drop limit for output a defrost cycle initiation signal or the second, lower pressure drop limit for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that - the first, higher pressure drop limit is increased and/or the second, lower pressure drop limit is decreased when the defrost cycle is shorter than a first predefined time period such that the time period of the defrost cycle should have been increased if the changed first, higher pressure drop limit or changed second, lower pressure drop limit had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal, or - the first, higher pressure drop limit is decreased and/or the second, lower pressure drop limit is increased when the defrost cycle is longer than a second predefined time period such that the time period of the defrost cycle should have been decreased if the changed first, higher pressure drop limit or changed second, lower pressure drop limit had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal. - change the first, higher pressure drop limit for output a defrost cycle initiation signal or the second, lower pressure drop limit for output of a defrost cycle termination signal depending on the time for performing the defrost cycle such that - the first, higher pressure drop limit is increased and/or the second, lower pressure drop limit is decreased when the defrost cycle is shorter than a first predefined time period such that the time period of the defrost cycle should have been increased if the changed first, higher pressure drop limit or changed second, lower pressure drop limit had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal, or - the first, higher pressure drop limit is decreased and/or the second, lower pressure drop limit is increased when the defrost cycle is longer than a second predefined time period such that the time period of the defrost cycle should have been decreased if the changed first, higher pressure drop limit or changed second, lower pressure drop limit had been used for the defrost cycle initiation signal respectively the defrost cycle termination signal.