Conclusions And Recommendations; Determine The System Combination Ratio (Cr) - LG Multi IV Engineering Manual

EQUIPMENT SELECTION PROCEDURE
The corrected cooling capacity ratio (CCR% [clg]) should never
exceed 100% plus building diversity. If it does, increase the size of
the outdoor unit or change the system design by moving some of
the building load and associated indoor unit(s) to another Multi V
system.
The outdoor unit should also be large enough to offset the sum
of the building's space heating loads without considering building
diversity. In the heating season, it is typical that all spaces served
by the system will peak simultaneously in the early morning, thus
building diversity should never be considered. If the corrected heat-
ing capacity ratio (CCR% [htg]) exceeds 100%, increase the size of
the outdoor unit or change the system design by moving some of the
building load to another Multi V system.

Determine the System Combination Ratio (CR)

The system's CR compares the nominal capacity of all connected
indoor units with the nominal capacity of the outdoor unit serving
them. Locate nominal capacity information for indoor and outdoor
units in the General Data Tables of their respective Engineering
Manuals.
For example,
If a VRF system has an outdoor unit with a nominal capacity of C
and four indoor units having nominal capacity ratings of W, X, Y, and
Z respectively, the CR would be determined as follows:
(
W + X + Y + Z
CR% =
The Multi V system will not commission, start or operate unless the
CR is between 50% and 130%.
If the CR is over 100%, the designer is under-sizing the outdoor unit
relative to the combined nominal capacity of the connected indoor
units. In some applications, under-sizing of the outdoor unit is
prudent as it reduces the initial equipment investment and will
properly perform as long as the designer:
1. Knows the indoor unit(s) are oversized relative to the actual
load(s) in the spaces served.
2. Knows the space loads will peak at different times of the day (i.e.,
building has "load diversity").
In some designs, over-sized indoor units may be unavoidable in the
case where the smallest size indoor unit available from LG is larger
than what is necessary to satisfy the space load. This scenario may
occur when an indoor unit selection one size down from the selected
unit is slightly short of fulfilling the design load requirements, and the
designer must choose the next largest size unit.
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x 100
C
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If the outdoor unit is properly sized to offset the building's total
cooling block load and the system's combination ratio is above
130%, indoor units are likely oversized. In applications where all
indoor units are "right-sized" and there is no building diversity, the
If the CR is above 130%, review the indoor unit choices and
downsize, or select a larger outdoor unit. Consider moving indoor
units to another Multi V, Multi F, or single-split system if the
outdoor unit size cannot be increased.
If the CR falls below 50%, select a smaller outdoor unit or consider
adding more or larger indoor unit(s) to the system. This situation
is common on multi-phase projects where the design calls for the
majority of indoor units be added to the system at a later date. To
raise the CR above the minimum 50% requirement:
3. Consider including additional indoor units on the first phase
4. Design two smaller systems in lieu of a single larger system.
Connect all "first phase" indoor units to the outdoor unit being
installed on the first phase, and delay the installation of the
additional outdoor unit until a later date.

Conclusions and Recommendations

• Always use LATS Multi V system design software to check a
design.
• Validate that each indoor unit is appropriately sized. Before
validating, if the indoor units have been properly sized, the outdoor
unit's size must be temporarily adjusted to make the system's CR
• Using the indoor unit's corrected capacity for cooling and heating
provided by LATS and apply a correction factor for altitude if
appropriate.
• Verify that the outdoor unit selection for each system is properly
sized. Verify that the corrected capacity for cooling and heating
provided by LATS is sufficient to offset the block building space
load after applying additional correction factors for capacity and
frost accumulation, if appropriate.
• For each Multi V system, calculate the cooling and heating design
days
1. Corrected Capacity Ratio (CCR).
2. Combination Ratio (CR).
After these system checks are complete and design limitations are
adhered, the system's indoor and outdoor components should be
properly sized and the system's performance should now be
optimized. The VRF system component size selections should be
acceptable.
At any time, if further system design assistance is needed or you
have a unique application you would like to discuss, contact your LG
applied equipment representative for assistance.