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4.0 INSTALLATION
4.4 Refrigerant Line Set Selection
4.4.1 Replacing Existing Systems
To prevent failure of a new unit, the existing line set
must be correctly sized for the new unit and must
be cleaned or replaced. Care must be taken so
the expansion device is not plugged. For new and
replacement units, a liquid line filter drier must be
installed and the line set must be properly sized. Test
the oil for acid. If it tests positive for acid, a suction
line filter drier is mandatory.
IMPORTANT: When replacing an R-22
unit with an R-410A unit, either replace the line set
or ensure that residual mineral oil is drained from
existing lines including oil trapped in low spots.
4.4.2 Line Set Application
Considerations
The following are special considerations that need
to be addressed when selecting and installing a line
set.
• Additional refrigerant charge
• Fitting losses and maximum equivalent length
considerations
• Refrigerant migration during the off cycle
• Oil return to the compressor
• Capacity losses
• System oil level adjustment
4.4.2.1 Oil Return to Compressor
Small amounts of compressor crankcase oil is
picked up and carried out of the compressor by
the moving refrigerant and is circulated through the
system along with the refrigerant before it returns to
the compressor crankcase. It is critical to the life
of the compressor for the oil to be able to return to
the compressor to maintain an adequate level of oil
in the compressor crankcase. Oversized vapor lines
result in inadequate refrigerant velocities to carry
the oil along with the refrigerant and will cause the
oil to accumulate in the low spots in the vapor line
instead of being returned to the compressor crank-
case. This is especially true for long line lengths.
Variable speed systems present an additional chal-
lenge due to the fact that the system operates at a
signicantly reduced refrigerant ow rate for a sig-
nicant percentage of operating time. Only use the
vapor line sizes listed in Table 2 to assure proper oil
return. DO NOT oversize vapor line!
4.4.2.2 Refrigerant Migration During Off
Cycle
Long line set applications can require a consider-
able amount of additional refrigerant. This additional
refrigerant needs to be managed throughout the en-
tire ambient operating envelope that the system will
go through during its life cycle. Off-Cycle migration
is where excess refrigerant condenses and migrates
to the coldest and/or lowest part of the system. Ex-
cessive build-up of refrigerant at the compressor
will result in poor reliability and noisy operation dur-
ing startup. Section 4.5.2 demonstrates the required
unit conguration for different applications.
4.4.2.3 Maximum Liquid Pressure Drop
The total liquid line pressure drop must not exceed
50 psig [345 kPa] to assure a solid column of liq-
uid at the metering device and stable control of su-
perheat. Be sure to account for vertical separation,
elbows, lter driers, solenoid valves, sight glasses,
and check valves when calculating liquid line pres-
sure drop.
4.4.2.4 Liquid Line Refrigerant Flashing
Excessive pressure drop and heat gain in long liq-
uid lines can result in the refrigerant ashing into a
vapor before it reaches the expansion device which
will dramatically reduce the capacity and efciency
of the system. For this reason, the liquid line must
be sized properly using Table 2 and must be insu-
lated in unconditioned spaces.
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