This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
As less radiating surface is required for steam heating, there is an advantage due to the reduction in first cost. This is of considerable importance, as a heating system must be designed of such dimensions as to be capable of warming a building in the coldest weather, and this involves the expenditure of a considerable amount for radiating surfaces, which are needed only at rare intervals. A combination system of hot water and steam heating requires, first, a heater or boiler which will answer for either purpose; second, a system of piping which will permit the circulation of either steam or hot water; and third, the use of radiators which are adapted to both kinds of heating. These requirements will be met by using a steam boiler provided with all the fittings required for steam heating, but so arranged that the damper regulator may be closed by means of valves when the system is to be used for hot water heating. The addition of an expansion tank is required, which must be so arranged that it can be shut off when the system is used for steam heating. The system of piping shown in Fig. 32 is best adapted for a combination system, although an overhead distribution as shown in Fig. 35 may be used, by shutting off the vent and overflow pipes, and placing air valves on the radiators.
While this system has many advantages in the way of cost over the complete hot water system, yet the labor of changing from steam to hot water will in some cases be troublesome, and should the connections to the expansion tank not be opened, serious results would follow.