1.   Before draining any water, first check the sight glass to make sure that there is water in the boiler.

2.   Check the sight glass weekly and add water whenever the level drops.

3.   If, however, the sight glass reads low and the furnace does not shut off automatically, then a service technician should check out the cutoff valve.

4.   The sight glass is usually mounted on the outside of the burner-boiler cabinet.