Holding tanks in RVs comprises of two tanks: one for gray water and one for black. Tank capacities generally increase in proportion to the RVs size. Sinks and showers generate more than toilets, the gray-water tank may be the larger of the two, but some RVs have tanks of equal capacities. Holding tanks in some RVs are in an enclosed compartment under the floor and receive heat from the furnace, thereby preventing winter freeze-ups. Holding tanks aren't enclosed and heated are susceptible to freezing in cold weather, especially during travel. Discharging the contents of holding tanks is controlled by a gate valve installed in the pipe from each tank. Beyond the valves, the pipes are connected to a T or Y fittings so the two tanks empty through a common pipe. On the end of this pipe is a fitting for attaching the sewer hose; when connected, the valves can be opened to empty the tanks. Most RVs with holding tanks have a monitor panel with lights that indicate the liquid level of each tank.