The presence of calcium can build up over time in water heaters, on faucet heads and showerheads. The build-up looks similar to a white/grey/light brown "scale." The scale is formed when the heat energy inside a water heater separates calcium from the water. Calcium deposits will accumulate on the heat exchangers over time and, if left unchecked, will lower your water heater's efficiency and reduce pressure in hot water lines. The issue is more pronounced in tankless water heaters, where the heating elements work at a much higher temperature to provide instant heat to the water passing through the tank. In addition, the lack of a tank means that this build-up will be more likely to impact your system if not addressed every year with proper maintenance.