875—93.9 (89) Washout openings. Miniature boilers exceeding 12 inches internal diameter or having more than ten square feet of heating surface shall be fitted with not less than three brass washout plugs of 1-inch pipe size that shall be screwed into openings in the shell near the bottom. In miniature boilers of the closed type system heated by removable internal electric heating elements, the openings for these elements when suitable for cleaning purposes may be substituted for washout openings. Boilers not exceeding 12 inches internal diameter and having less than ten square feet of heating surface need not have more than two 1-inch openings for cleanouts, one of which may be used for the attachment of the blowoff valve; these openings shall be opposite each other where possible. All threaded openings shall be opposite each other where possible. All threaded openings in the boiler shall be provided with a riveted or welded reinforcement to give four full threads therein.
Electric boilers of a design employing a removable top cover flange for inspection and cleaning need not be fitted with washout openings.