These are a supply unit comprising a damper, filter, heater and fan and an extract unit consisting of a fan or fan and damper. When the air-handling unit is inoperative the damper should be closed in order to protect the heating coil from frost and to prevent back draught through the system. Additional protection of the coil may be provided by the installation of an electric heater element within the unit.
The heating capacity should be sufficient to warm the incoming air to the desired temperature during the heating season. The supply fan draws in fresh air from the outside and distributes it through the ducting system; the extract fan removes the stale air from the ventilated area and discharges it from the outside.
Units may also be provided with an air mixing section so as to enable re-circulation or partial re-circulation of the air, if required.