When designing or replacing a low pressure steam boiler, throw out everything you know about hydronic boilers. A steam system is closer in operation to an air conditioning unit than it is to a hydronic boiler. If you design a steam system using hydronic design methods, it will not operate properly. A hydronic system uses sensible heat while a steam system uses both sensible and latent heat. Sensible heat can be measured with a thermometer while latent heat causes a change of state and cannot be measured accurately with a thermometer. The water inside the boiler will change state from a liquid to a vapor and then back to a liquid during the heating process. Steam systems are more holistic than hydronic ones because a problem in one area could affect the entire system. The following are some of the steps I take when designing a replacement steam system.