Building Design and Construction Handbook - Merritt - Ventech!

13.52 SECTION THIRTEEN
Allow for expansion and contraction of pipe without causing undue stresses.
All supply and return piping should be insulated.
In very high buildings, the static pressure on the boiler may be too great. To
prevent this, heat exchangers may be installed as indicated in Fig. 13.11. The boiler
temperature and lowest zone would be designed for 200�F supply water and 180�F
return. The second lowest zone and heat exchanger can be designed for 170�F
supply water and 150�F return, etc.
FIGURE 13.11 Piping layout for tall building,
including heat exchangers.
Control of Hot-Water Systems. The
control system is usually arranged as
follows: An immersion thermostat in the
heater controls the heat source, such as
an oil burner or gas solenoid valve. The
thermostat is set to maintain design
heater water temperature (usually about
180�F). When the room thermostat calls
for heat, it starts the circulator. Thus, an
immediate supply of hot water is available
for the radiators. A low-limit immersion
stat, usually placed in the boiler
and wired in series with the room stat
and the pump, is arranged to shut off the
circulator in the event that the water
temperature drops below 70�F. This is
an economy measure; if there is a flame
failure, water will not be circulated unless
it is warm enough to do some good.
If the boiler is used to supply domestic
hot water via an instantaneous coil or
storage tank, hot water will always be
available for that purpose. It should be
kept in mind that the boiler must be
sized for the heating load plus the probable
domestic hot-water demand.
High-Temperature, High-Pressure Hot-
Water Systems. Some commercial and
industrial building complexes have installed
hot-water heating systems in
which the water temperature is maintained
well above 212�F. This is made
possible by subjecting the system to a
pressure well above the saturation pressure
of the water at the design temper-
ature. Such high-temperature hot-water systems present some inherent hazards.
Most important is the danger of a leak, because then the water will flash into steam.
Another serious condition occurs when a pump’s suction strainer becomes partly
clogged, creating a pressure drop. This may cause steam to flash in the circulating
pump casing and vapor bind the pump.
These systems are not generally used for heating with radiators. They are mostly
used in conjunction with air-conditioning installations in which the air-handling
units contain a heating coil for winter heating. The advantage of high-temperature