Domestic water heater
As with hot-water boilers, steam boilers can also be equipped for generating domestic hot water. When it produces domestic hot water, the boiler must be fired all year long. When heat is not required, the boiler-water temperature is controlled by an aquastat. The aquastat activates the burner when the boiler-water temperature drops below a preset figure and shuts off the burner when the boiler-water temperature rises to about 200° F. When steam heat is required, the burner is activated by the thermostat. In this case, the thermostat overrides the aquastat control so that steam is produced.
In a forced-hot-water heating system that also produces domestic hot water, a flow-control valve is needed to prevent the boiler water from circulating as a gravity system during those months when heat is not required. In a comparable steam system, no flow-control valve or equivalent is necessary because the boiler water is not heated sufficiently to produce steam.
Advantages and disadvantages
The advantages of a steam heating system can be appreciated more in a large building than in a residential structure. It is a relatively simple system that does not require a pump or fan for circulation of the steam. Since there is no water in the pipes when the system is not operating, there is no problem of the pipes freezing and bursting. If a repair or replacement is needed to a section of pipe or a fitting, it is not necessary to drain the system. Also, a steam leak would result in very little water accumulation.
On the other hand, a steam system is slow in responding to an initial rapid change in heat demand because the boiler-water temperature must be brought up to 212° F before steam circulation begins. Unless the condensate is returned to the boiler by means of a pump, the boiler must be located below the radiator in the lowest rooms.
Hybrid heating systems
Depending on the extent to which the heating system has been modified or expanded, you may find a hybrid heating system. A hybrid system is composed of two separate heating systems working together. I have seen the following systems.
This system functioned as a two-zone system. The main portion of the house was heated by a steam system, and the remaining portion of the house (the lower sections) was heated by a forced-hot-water system. The distribution pipe for the hot-water system was tapped directly off the portion of the steam boiler that was below the water level, and a pump was used to circulate the hot water. The thermostat for this zone controlled the pump and the burner. There was also an aquastat temperature control for the boiler water to keep it from generating steam when the main zone was not calling for heat.
Hot water–warm air
In this system, the house was heated by forced warm air. The furnace, however, did not have its own burner for heating the air. The furnace was heated by distribution piping from a forced-hot-water heating system. The only function of this hot-water system was to generate domestic hot water and to provide the heat source for the furnace. Since the boiler water was hot all the time because of the domestic hot water, the house thermostat controlled only the circulating pump from the hot-water system. The pump in turn forced hot water to circulate through the furnace, heating it, which in turn activated the fan. If you find a hybrid heating system, have it analyzed by a professional.