Closed Water Systems; Thermal Expansion; Vent Valves; Manifold Headers - A.O. Smith HW 300 Instruction Manual

CLOSED WATER SYSTEMS

Water supply systems may, because of code requirements
or such conditions as high line pressure, among others, have
installed devices such as pressure reducing valves, check
valves, and back flow preventers. Devices such as these cause
the water system to be a closed system.

THERMAL ExPANSION

As water is heated, it expands (thermal expansion). In a closed
system the volume of water will grow when it is heated. As the
volume of water grows there will be a corresponding increase
in water pressure due to thermal expansion. Thermal expansion
can cause premature tank failure (leakage). This type of failure
is not covered under the limited warranty. Thermal expansion
can also cause intermittent Temperature-Pressure Relief Valve
operation: water discharged from the valve due to excessive
pressure build up. This condition is not covered under the limited
warranty. The Temperature-Pressure Relief Valve is not intended
for the constant relief of thermal expansion.
A properly sized thermal expansion tank must be installed on
all closed systems to control the harmful effects of thermal
expansion. Contact a local plumbing service agency to have a
thermal expansion tank installed. An expansion tank or a similar
device may be required in the inlet supply line between the boiler
and the meter or valve to compensate for the thermal expansion
of water under supply pressure, see Piping Diagrams section on
Page 47.
An air separator as shown in the piping diagrams is recommended
especially for modern commercial hydronic systems.

VENT VALVES

It is recommended that automatic, loose key or screwdriver type
vent valves be installed at each convector or radiator.

MANIFOLD HEADERS

Split systems with individual supply and return lines from the boiler
room should normally have this piping connected to supply and
return manifold headers near the boiler. To achieve good water
distribution with maximum pressure drop for several circuits,
manifolds of at least 2-1/2" (64 mm) diameter are suggested on
HW-399, HW-420, HW-520 and HW-670 units. HW-300 units
should have 1-1/2" (38 mm) diameter manifolds.
The circuits should be spaced on the header at a minimum of
3" (76 mm) center to center. Install a balancing cock in each
return line.
Manifold headers are recommended for split systems with
or without zone valves and also those installations with zone
circulating pumps. If the system is to be split at remote points,
good practice requires special attention be given to main pipe
sizing to allow balancing of water flow.
The boiler piping system of a hot water boiler connected to
heating coils located in air handling units where they may be
exposed to refrigerated air circulation must be equipped with
flow control valves or other automatic means to prevent gravity
circulation of the boiler water during the cooling cycle.

COOLING PIPING

When the boiler is used in conjunction with a refrigeration system
it must be installed so that the chilled medium is piped in parallel
with the boiler with appropriate valves to prevent the chilled
medium from entering the boiler, see Figure 7.
Water temperature in the heating system must be reduced to less
than 100 F (38°C) before cooling system is started, or damage to
0
the chiller unit may occur.
FIGURE 7. SCHEMATIC SHOWING PROPER PIPING
ISOLATION OF THE BOILER FROM THE CHILLER
If the boiler is connected to chilled water piping or its heating coils
are exposed to refrigerated air, the boiler piping system must be
equipped with flow valves or other automatic means to prevent
gravity circulation through the boiler during the cooling cycle.

CIRCULATING PUMP

Constant circulating pump operation of the boiler voids the
warranty. Constant water flow through the unit will "wash" away
the copper's natural protective coating. This is called velocity
erosion. This erosion is not as great a problem when intermittent
circulating operation is used per the recommended installation
procedure. Constant circulation of water through the building's
system main is permissible as long as the water does not
constantly flow through the boiler. Only all bronze or stainless
steel circulating pumps are to be used with the unit when it is
installed in hot water supply systems.
FIGURE 8. A TYPICAL CIRCULATING PUMP
Although each circulating pump that requires oiling is oiled and
operated by the manufacturer, It must be oiled again before
operated. See Figure 8 above.
Refer to the pump manufacturer's instructions for lubrication
requirements.
13