The traditional boiler will often feature something called a sight glass, which is designed to allow plumbers and engineers to check the water levels in a heating system without having to go into the boiler itself. Keeping water levels in the boiler above the heat source reduces the risk of fire or overheating, and for those trying to maintain an older boiler a working sight glass is an important part of any maintenance kit. The boiler sight glass should be checked regularly for any problems such as cracking or loss of water pressure, as these can indicate that the tube needs replacing.
The Boiler Sight Glass
Most domestic sight glasses will consist of a glass or plastic tube which connects to the top and bottom of the tank. This connection allows the glass to reproduce the water level inside the boiler. There will probably be marks along the side of the container telling you how much water there is inside the boiler. Most tubes are contained within a retaining sheet of glass, which holds the tube on to the outside of the boiler. Both the tube and the glass can become damaged with time. Regular checks of the glass should give you early indications that the glass is about to fail.
Discovering Cracks in the Sight Glass
The glass on the boiler should be checked regularly and examined for signs of wear and tear. Problems with the sight glass can be caused by a number of problems, including excessive solids build-up in the glass, and an excessive pressure. Most sight glasses should not be used with boilers with a high-pressure as this can cause the glass to erode. Check the top and bottom edges of the tube for cracks, as this is where weakness can first appear. You should also make sure that the crack is actually on the glass itself, and not simply on the outside of the retaining panel. Once you have discovered a crack in the glass, the option is usually replacement.
Replacing the Glass
The sight glass can be removed from the boiler in a few simple steps. First, unscrew the retaining glass, and lay that down close to the boiler. Then disconnect the ends of the tube which connect it to the top and bottom of the boiler. Pull the tube away. You will need to wear protective gloves during this procedure, as the cracked glass may shatter suddenly as you are removing it. You can replace your sight glass with a similar tube, available at home improvement stores, or you may decide that your boiler is too high-pressure for a standard tube, in which case purchase a prismatic gauge glass. This is very thick glass, held in place by a metal housing. The toughness of the metal and glass will ensure that it resists stronger pressure than conventional sight glasses. Connect the tube back into the boiler, and then replace the retaining glass, or screw the new metal casing into the sides. Your gauge should now be working correctly again.