Almost all steam boilers are constructed of carbon steel, which rusts if exposed to oxygen and moisture The following steps will minimize this problem. (Please observe "shock & burn warnings."): The boiler's blowdown valve must be safely piped in accordance with state and local codes. Ensure that safe discharge is provided for 100 PSIG and 338 degrees F. Turn the boiler off and allow the pressure to drop to 5 psig. Open the blowdown valve for ten seconds, then close.