Solid-State SCR Control - When temperature must be controlled precisely, or for larger kW heaters, built-in SCR’s are recommended. They are furnished with field-selected inputs of 2200 or 135 ohms, 0-10 VDC, or 4-20 mA. SCR's have zero-cross firing to eliminate radio frequency interference. To meet FM and CSA requirements, multi-unit designs (up to four heating units in series) also have controls set at 80°F (27°C) to limit the inlet air temperature to all but the inlet unit. These limit controls prevent excessive temperatures at the heater outlet as the inlet air temperature rises.