All freezers should be designed with two plates on opposite walls of the cabinet. Holdover plate surface area must be increased over that of the refrigerated space.  A spillover system consists of two holdover plates in the freezer section of the divided box - a vent at the top allowing air to circulate between the two. The box is divided by an air tight insulated wall, with a self-contained temperature control air damper (thermodamper) fitted at the bottom automatically controlling the amount of cold air flowing between the two boxes. A return air port is fitted at the top of the divider wall. In a large cabinet, or if the freezer section is not large enough to fit plates to cool both refrigerator and freezer boxes, a series system will be needed involving t wo plates in the freezer and one plate in the refrigerator. In a single valve set up, all three plates are pipes in series. A twin valve system is used where the freezer is not always needed, where the freezer needs more running time than the refrigerator, or where the cabinets are not adjacent to each other. Two zones are created and the freezer, refrigerator or both may be operated. Electric twin valve systems have two separate thermostats.