Harold Kestenholz
They key words here are ‘add-on’. Adding to a steam system later is difficult. The new steam boiler has to be sized to include the new steam radiation as the boiler has to be able to fill the future system completely or the system will have problems. An oversized steam boiler has short-cycle problems. The future steam system must have the proper pitches in supply and return to return the condensate. Unless you know the exact dimensions and floor plan of the addition, you will guess at the proper steam piping plan. Otherwise, new steam boilers are quite efficient. It is just that the heating of all pipes to 212F on every cycle in all weather is inefficient.
It is not unusual to have an oversized hot water system boiler work OK, so future expansion to the future load of the new boiler is uncomplicated. The hot water is in a sealed system with no requirement for pitching pipes for returning water, water is everywhere in the system. It is easy to zone the hot water system. The comfort and quiet operation is always superior to a warm air or steam system.
