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Fujitsu FO20R Series Installation Instructions Manual page 11

Variable speed r-410a heat pump outdoor units
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4.4 Refrigerant Line Set Selection
4.4.1 Replacing Existing
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.
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
The following are special considerations that need
to be addressed when selecting and installing a line
• Additional refrigerant charge
• Fitting losses and maximum equivalent length
• Refrigerant migration during the off cycle
• Oil return to the compressor
• Capacity losses
• System oil level adjustment 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
vapor line sizes listed in Table 2 to assure proper
oil return. DO NOT oversize vapor line!
When replacing an
4.0 INSTALLATION Refrigerant Migration Dur-
ing Off Cycle
Long line set applications can require a consider-
able amount of additional refrigerant. This addition-
al refrigerant needs to be managed throughout the
entire ambient operating envelope that the sys-
tem 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. Excessive build-up of refrigerant at the
compressor will result in poor reliability and noisy
operation during startup. Section 4.5.2 demon-
The outdoor unit controls can energize a portion
of the compressor motor stator as needed during
the off cycle to prevent liquid refrigerant from ac-
cumulating inside the compressor, similar to how a
crankcase heater functions on conventional sys-
tems. Maximum Liquid Pressure
The total liquid line pressure drop must not exceed
50 psig [345 kPa] to assure a solid column of liquid
at the metering device and stable control of super-
heat. Be sure to account for vertical separation,
and check valves when calculating liquid line pres-
sure drop. Liquid Line Refrigerant
Excessive pressure drop and heat gain in long liq-
vapor before it reaches the expansion device which
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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