Airflow Pre Post Heater for DV250 DV400 700W 990 322 220 The SRD100T runs with 70% The SRD100T runs with 70% less noise than conventional fans and is designed so that the fan will operate for an adjustable period of 2 to 30mins These units work on a calendar timer basis and simply changing the filters will still present the same issue as the heat recovery unit has to be told by the user when the filters have been changed The optional additional heater (700W) can be used pre or post the heat recovery unit (a pre heater must be used if Passive House conformance for the heat recovery unit is required) They are used to temper incoming supply air through the heat recovery unit