10
Location
slab, observe the following guidelines.
• If elevating a unit on a flat roof, use 4" x 4"
[10.2 cm x 10.2 cm] or equivalent stringers
positioned to distribute unit weight evenly and
prevent noise and vibration.
• Heat pump products will need to be elevated per
local climate and code requirements to provide
clearance above the estimated snowfall level to
ensure the unit will be protected from damage.
Failure to follow these instructions may result in
equipment damage and improper operation.
NOTICE: Do not block drain openings on bottom of unit.
• If unit must be elevated because of anticipated
snowfall, secure unit and elevating stand such that
unit and/or stand will not tip over or fall off. Keep in
mind that someone may try to climb on unit.
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
crankcase. This is especially true for long line lengths.
Variable speed systems present an additional challenge
due to the fact that the system operates at a significantly
reduced refrigerant flow rate for a significant 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 O
Cycle
Long line set applications can require a considerable
amount of additional refrigerant. This additional
refrigerant needs to be managed throughout the
entire 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.
Excessive build-up of refrigerant at the compressor
will result in poor reliability and noisy operation during
startup. Section 4.5.2 demonstrates the required unit
configuration for different applications.
The outdoor unit controls can energize a portion of
the compressor motor stator as needed during the off
cycle to prevent liquid refrigerant from accumulating
inside the compressor, similar to how a crankcase
heater functions on conventional systems.
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 liquid at
the metering device and stable control of superheat.
Be sure to account for vertical separation, elbows,
filter driers, solenoid valves, sight glasses, and check
valves when calculating liquid line pressure drop.
4.4.2.4 Liquid Line Refrigerant Flashing
Excessive pressure drop and heat gain in long liquid
lines can result in the refrigerant flashing into a vapor
before it reaches the expansion device which will
dramatically reduce the capacity and efficiency of
the system. For this reason, the liquid line must be
sized properly using Table 2 and must be insulated in
unconditioned spaces.
4.4.2.5 Oil Level Adjustment for Long Line
Set Applications
Additional oil may need to be added if refrigerant is
added during installation. If the system contains more
than 20 lbs [9 kg] of refrigerant charge, add 1 fluid
oz of POE oil for every 5 lbs [13 ml/kg] of refrigerant
charge over 20 lbs [9 kg].
4.4.2.6 Capacity Losses
Long line lengths can result in a reduction in capacity
due to vapor line pressure drop and heat gain or
loss. Refer to Table 2 for capacity loss multipliers
for various vapor line diameters and equipment line
lengths. This table does not account for any capacity
loss due to heat gain or loss from the environment.
It is extremely important not to oversize the vapor line
to minimize capacity loss at the expense of proper
oil return. If the table shows an “NR” for a particular
vapor line diameter and length, or, if a vapor line
diameter is not listed, oil return will not be adequate.
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