How often a boiler should be washed out and cleaned depends upon the quality of the water it uses, and varies from about once a week to once a month, according to whether bad and unpurified water or purified water is used.
The first thing to do is to draw the fire, leaving the chimney damper open and closing all the other dampers so that as little cold air as possible can get into the boiler, while the heat can pass away up the chimney.
Let the steam and water all remain in the boiler until there is a gauge pressure of about 5 lbs. in the boiler.
Then open the blow-off cock and let out the water. If the water is blown off under a high pressure, then after the waste is all out the iron is hot enough to dry up the scale, making it hard and very difficult to remove.
After all the water is blown off, take out all the mud plugs and the man-hole and hand-hole covers, and wash out the boiler under as much water pressure as can be had, directing the hose so to reach all parts of the boiler and tubes, and continuing the washing until the water leaves the boiler clean.
Then with a wooden hoe on a piece of gas-pipe of small diameter for a handle, and small enough to pass through the hand-hole, draw all the loose scale to the hand-hole and remove it, letting the water run slowly, so as to carry the small pieces of scale towards the hand-hole as fast as the hoe disturbs it.
Then get inside the boiler, and a few blows with a light ball-pened hammer will loosen the scale, and a steel scraper will remove more, which must be washed down and drawn out with a hoe.
After the cleaning and scaling are complete, the engineer, with lamp in hand, should carefully examine the interior of the boiler and of the fire box, paying especial attention to the stays to see that they are not broken.
The hammer test should also be applied. It consists of sounding the boiler by light blows given by a light ball-pened hand hammer, the sound indicating defective places.