Linear-Temp Space Heating Applications; Linear-Temp Space Heating Installations - A.O. Smith DB/DW-720-1810 User Manual

BYPASS BALANCING
With systems where water temperature can be expected to drop
appreciably due to long standby periods, or heavy draw down, a
bypass pipe of at least 1" size with a balancing cock should be
installed between the boiler inlet and outlet (see fig. 10). When
the system first starts, the valve should be slowly opened until the
condensing ceases. This adjustment remains at a permanent
setting to establish required temperature rise across the boiler.

LINEAR-TEMP SPACE HEATING APPLICATIONS

Controlling of these systems is decided mainly by the type of
building system controlling that is desired. A single boiler
installation might be controlled directly from space temperature
thermostat(s). Multiple boiler installations are more effective when
the boilers are sequenced in and out of operation by some form of
main water temperature controller. With one or two boilers,
individual control set at progressive temperature may be used.
For more than two boilers, electronic sequencing controlling is
recommended.
Individual controls, or the separate stages of a step controller,
should fire a boiler and also start the boiler loop circulator whenever
that boiler is fired. Some large installations may require the firing
of more than one boiler per stage.
The system or primary circulator may or may not be controlled by
the boiler sequencer. When this pump is operated through the
first switch of any type of step controller, care should be taken to
determine if a motor starter is needed due to insufficient switch
capacity.
If the primary pump is controlled by a manual switch or any other
controllers, the electric current supply to the boiler group should
be through the primary pump controller. The fast response of
A.O. Smith boilers eliminates any need to maintain boiler
temperature when the system is satisfied. Wiring should always
prevent firing of boiler(s) when there is no water flow in the mains.
Installation diagrams show safety flow switches in the outlet piping
from each boiler as good protection against any boiler being fired
when the boiler loop circulator is not in operation.
These safety flow switches will also provide some protection if
there is a loss of water.
LINEAR-TEMP multiple boiler installations are especially adapted
to the use of outdoor reset for main water temperatures. This
feature is not mandatory but offers smooth, efficient operation of
a modern system.
Outdoor reset systems should utilize an automatic shutdown
control if there is a continuous recirculating main and/or if the
entire building is not under control of space temperature
thermostats. A single bulb outdoor sensing control will serve this
requirement. This precaution will prevent overheating of halls,
stairways or other uncontrolled areas. There are occasions when
outdoor temperatures are temporarily too warm for even a
moderate amount of heating in these areas.
Space temperature controlling can be varied to meet the building
requirements. Either the single thermostat, as shown, or multiple
zone thermostats should control a common relay. This relay
controls electric power to the system primary circulator and to the
main water temperature controller. This provides for water
movement in the system before the main temperature controller
can start the secondary circulating pump or fire the boiler.
Figure 11 shows a typical field wiring diagram for a single stage
boiler LINEAR-TEMP installation. The boiler may be controlled
by a main temperature controller as shown or may include outdoor
reset if desired.
The following fig's. 11 thru 13 are shown as layouts for various
choices of controls often found in commercial heating. These
layouts are not intended to be wiring diagrams and only show the
relation of one device to another in the system.
Figure 11 is a typical layout of controls for two boilers with two
circulators and thermal balancers plus optional outdoor reset
control.
Figure 12 is shown as a typical control group for various multiple
boiler installations, and is not intended to imply that this is the
only arrangement to be considered. Commercial size installations
are always best when designed to individual building requirements.

LINEAR-TEMP SPACE HEATING INSTALLATIONS

A. New Installation
The A.O. Smith LINEAR-TEMP system has been designed to
provide efficient, trouble-free operation of these boilers with any
of the following conditions:
• unknown system flow rate.
• varying flow rates as with zoned systems and 3-way valve
system.
• multiple boiler installations.
Figure 14 shows piping and accessory arrangement for 1 or more
boilers pumped independently of the primary system mains. Pipe
sizing and boiler loop pump selection data, are shown in table 4 for
several different temperature rises across various boiler sizes.
Total heating requirements for the building can be supplied by a
series of boiler loops all connecting to a common pipe joining the
system supply and return mains. The supply and return branches
of each boiler loop must join the common pipe only a short nipple
length apart. The different sets of branches should be installed
reasonably close together, but not necessarily to the short nipple
length as required for the supply and return of each set. These
branches may be made with tees or with welded connections.
The installer is reminded that the total boiler flow rates do not
have to match the system flow rate. Flow rates through heat
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FIGURE 13