Buderus Logamax plus GB162 Applications Manual page 8

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Possible scenarios of the LLH:
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Both primary and secondary flow rates are
identical.
Fig. 4 Three possible flow patterns with a LLH
LLH are equipped with a well for a system strategy sensor to
measure supply water temperature.
A LLH is oversized to a point that it can handle the maximum
possible flow rates on either side without significant pressure
drop.
In the center of the LLH is the point of equilibrium and zero
pressure. It allows for a variety of flow rates to occur on both
sides of the system without influencing the other side.
A common case from retrofit applications: The system pumps
are too strong and remove more water from the LLH than
delivered by the boiler. With the consequence that none of
the zones gets up to temperature. Required measure: reduce
system flow rates.
For system redundancy, energy optimization, or combination
of different fuel types, different boiler types or models may be
combined in systems. Based on the load this allows turning
off complete appliances resulting in higher efficiencies of the
remaining system.
When a larger heat demand occurs additional boilers can be
enabled one by one. The changing flow rates are balanced by
the LLH and all boilers and zones in operation are continued to
be supplied with the necessary volumes.
To ensure efficiency while also providing redundancy, each
boiler must have its own boiler pump.
In particular when an existing system is retrofit with a new boiler
it is necessary to evaluate the boiler and system flow rates. A
reduction of boiler output can in many cases reach 40% and
completely change system hydraulics.
Technical specifi cations are subject to change without prior notice
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The boiler flow rate is greater than the
system flow rate and hot supply water is
returning to the boiler.
GB162 Applications Manual
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The system flow rate is greater than the
boiler flow rate and cold return water is sent
back out to the system.
Example:
Old boiler output 480 MBH (140 kW)
New boiler output: 340 MBH (100 kW)
The new system requires a system flow rate of 19 gpm (4300
liters/hr) at 36°F (20°C) delta T.
As older systems are typically equipped with oversized pumps,
the retrofit system can be expected to use pumps that may
be half the size of the old ones. In this case only a LLH allows
balancing of the primary and secondary flow rates.
It is recommended to use multi speed or ECM pumps.
Sizing a Low Loss Header
Size the pumps and LLH by the greater of the two volumes that
is circulating: boiler circuit or system circuit.
.
Note: It is recommended to set the minimum delta T between
boiler supply and boiler return to 20 to 25°F (10 to 15°C) by
adjusting the speed of the circulator or installing flow setters.
For sizing a LLH at maximum throughput the flow rate should
not exceed 0.67 feet/second (0,2 m/s). This makes the LLH
the area of slowest velocity and allows it to be used as an air
separator and dirt separator.
Over sizing a LLH however does not have negative
consequences on the system. In fact a buffer tank can be used
instead of a LLH.
Logamax plus GB162 applications manual | 03.2013
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