Water Level
The proper water level in a boiler sight glass should show that the tank is at least half full, but not more than three-quarters full. On some boilers, you need to open the drain valve on the boiler every two weeks to allow the sediment on the bottom of the water tank to drain out. If the sediment is left in the tank, the sight glass may show you that the water level is at the correct level, but the actual amount of water could be lower if the sediment is deep on the bottom of the tank.
Even if you have a boiler with an automatic water feeder, you still must check the water level in the sight glass because the feeder may not maintain the correct water level in the boiler.
Type of Water
The type of water you use in a hot water boiler is specified in most boiler manuals. The water must have a pH of 7.0 to 8.5 parts per million (ppm). The amount of chlorine in the water must not exceed 200 ppm. Generally, tap water has less than 5 ppm. You cannot fill the boiler with water that runs through a water softener. Water that runs through the water softener can cause the interior of the boiler and pipes to corrode.
Water Pressure
The water pressure in a hot water boiler should read between 12 to 20 pounds per square inch (psi). Steam boilers should have a pressure reading of 1 to 2 psi and should never go past 4 psi.
Water Temperature
Both the steam boiler and hot water boilers have temperature gauges so you can watch the temperature of the water. The water temperature should not exceed 200 degrees Fahrenheit, but the ideal range is between 140 and 180 degrees Fahrenheit.