The return air box mainly returns air. It saves energy and keeps the cooling effect. Without it, the air at the ceiling above the terminal may not be conditioned, which wastes energy. Many boxes also carry a filter that removes dust and keeps the air clean.
The return air outlet must be designed. The box wraps only the fan and keeps dust and debris out. If the box itself has no filter, the return outlet sits in the ceiling and its neck connects to the box opening by a duct.
Air under the ceiling that is not drawn into the box is lost. So the box saves energy and keeps air clean, but the fan coil unit costs a little more.
FP-123456: product name, fan coil unit.
1) Rated airflow: national-standard type x 10 m³/h.
2) Construction: horizontal type W, vertical type L, cassette type K, wall-mounted type BG.
3) Mounting: concealed or exposed.
4) No return box; type with return box marked G.
5) Inlet direction: left inlet Z, right inlet Y.
6) Feature code: standard type has no suffix; static-pressure type shown as H plus a number.
Low-static-pressure units have an external residual pressure of 0 Pa, with no outlet or filter; residual pressure 12 Pa.
Cooling conditions: inlet dry-bulb 27°C, wet-bulb 19.5°C, inlet water 7°C, water temperature difference 5°C.
Heating conditions: inlet dry-bulb 21°C, inlet water 60°C, hot water flow same as cooling.