Whether steam or hot water is used as the heat transfer medium depends on
the building’s heating requirements. Heat transfer is simply the passage of
thermal energy from a hot to a colder body. Hot water systems are usually
more efficient and less susceptible to corrosion than steam systems. Steam sys-
tems are typically used in situations where large amounts of heat are required,
such as centralized heating plants, but they require much more maintenance
than hot water systems.
Boilers are available in two main categories: conventional units and con-
densing units. Condensing units allow the water vapor produced during the
combustion of whatever hydrocarbon fuel is used to produce heat to condense.
Condensing units typically have efficiencies of over 90% and are more energy
efficient than conventional units. Enough heat can be extracted from condens-
ing units that the exhaust gases are typically cool enough to be pumped
through PVC piping.
Conventional boiler units are typically made of materials that cannot handle
the corrosive properties of the condensing gases, and therefore that heat becomes
waste. Conventional boilers can be retrofitted with a stack gas economizer, a
device that captures some of the exhaust heat from the combustion gases and
transfers it to the incoming water to the boiler, thereby raising the boiler’s
efficiency.
Boilers are also categorized in terms of heating methods: fire tube and water
tube. Fire tube boilers transfer heat from combustion gases to the water using
a series of straight tubes surrounded by water. The hot gases flow through the
tubes and transfer heat to the surrounding water. Whereas, water tube boilers
Figure 3.1 Commercial boiler.
Heating, Ventilating, and Air Conditioning Systems 33