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Minimum Air Change Requirements
Avoid over-sizing indoor units in an attempt to increase the air
exchange rate in the space. VRF systems are designed for
minimum airflow over the coil to maximize latent capacity while
cooling, maintain a comfortable, consistent discharge air
temperature while heating, and minimize fan motor power
consumption. In extreme cases, over-sizing indoor units may
compromise the outdoor unit's ability to effectively match the space
Check the Indoor Unit Selection(s)
Verify the sensible (and total) corrected cooling capacity. For each
indoor unit the corrected capacity must be at least equal to the sum
of the appropriate cooling design day space load(s) (plus ventilation
load, if applicable) for the space(s) served by the indoor unit.
Verify the corrected heating capacity. For each indoor unit, the
corrected capacity must be at least equal to the sum of the
heating design day space load (plus ventilation load, if applicable)
for all spaces served by the indoor unit.
Select the Outdoor Unit
After all indoor units are properly sized to offset the applicable loads
in each space, begin the selection of the outdoor unit by choosing a
size that meets both the block load cooling requirement and offsets
the sum of the peak heating load.
After making an outdoor unit selection, look up the outdoor unit's
corrected cooling and heating capacity at the specified ambient
design conditions. Use values reported by LATS or find it in the
tables provided in the separate Multi V IV Performance Data Manual
In LATS always run the Auto-Pipe and System Check features
following any change in the outdoor unit selection to verify the
system design is acceptable.
Capacity Correction
For air-cooled systems operating in cooling mode, a capacity
correction factor may apply to account for the length of the system's
liquid pipe and elevation difference between the outdoor unit and the
indoor unit(s). If the outdoor units corrected cooling capacity was
derived from the LATS report, the elevation difference correction
factor has already been applied. If the corrected cooling capacity
was found using corrected capacity tables found in the separate
Multi V IV Performance Data Manual on, apply the
appropriate elevation difference factor also found in the Performance
Data Manual (choice of table depends on the architecture of the
system design). Multiply the outdoor unit corrected cooling capacity
by the elevation difference correction factor. For high altitude
locations, locally accepted altitude correction factors must be
manually applied to outdoor unit capacity. LATS will not de-rate
outdoor unit capacity for altitude.
Frost Accumulation
The outdoor unit heating capacity may need to be adjusted for frost
accumulation on air-cooled systems. If design day conditions are
below the dew-point of the surrounding air, frost may not be a
problem and no correction factor is needed. In certain weather
conditions, however, frost may form and accumulate on the air-
cooled outdoor unit coil and impact the coils ability to transfer heat.
If significant frost accumulates on the outdoor unit coil, a defrost
algorithm will start automatically. The timing between defrost periods
is determined by the system's ability to achieve a target head
pressure value.
LATS will automatically apply a frost accumulation factor if the check
box labeled "Defrost Factor" in the outdoor unit selection dialog box
is marked. The dialog box can be accessed by double-clicking on
the outdoor unit image. If checked, the corrected outdoor unit
capacity provided by the LATS report and displayed on the tree
mode piping diagram will already be adjusted for outdoor unit coil
frost accumulation.
If the corrected heating capacity was found using the Performance
Data tables, the frost accumulation factor must be applied. (For
correction factors, see the separate Multi V IV Performance Data
Manual on
Check the Indoor Unit Selection(s)
After applying the appropriate correction factors to the outdoor unit,
verify the corrected cooling capacity is at least equal to the total
building load (considering building diversity, if applicable), and the
corrected heating capacity is at least equal to the sum of the peak
heating loads for all spaces and/or thermal zones served by the
System Sizing Checks
Calculate the Corrected Capacity Ratio (CCR)
The system's CCR is defined as the sum of the space loads divided
by the outdoor unit corrected capacity after all applicable correction
factors are applied. Calculate this ratio for both the cooling and
heating design days.
Total Cooling Block Load
Actual Corrected Outdoor
Unit Cooling Capacity
Heating Peak Load
Actual Corrected Outdoor
Unit Heating Capacity
The outdoor unit selected should be large enough to offset the total
block cooling load for all spaces served by the VRF system during
the peak cooling load hour on the cooling design day (account for
ventilation air cooling load if the ventilation air has not been
pretreated to room neutral conditions).
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