An Air Handling Unit for a ventilation system in a building has supply and extract air channels, each with an inlet and an outlet for guiding air to and from a building. Each air channel includes a damper controlling air flow. The air channels are in a heat exchanging relation to each other. The AHU also includes a fan and an Electronic Control Unit. The ECU outputs a defrost cycle initiation signal to defrost the heat recovery arrangement. The ECU outputs a defrost cycle termination signal to end the defrost cycle upon indication of the defrosting being completed. To optimize the defrost cycle time the ECU changes the first or second criterion for defrost cycle termination signal depending on the time for performing the defrost cycle such that the time period of the defrost cycle is prolonged/shortened when the defrost cycle time period is shorter/longer than preferred.