AAON M1-011 - Manual (Page 29)

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5. If the discharge superheat measured does not
agree with the values in Table 7.2, then adjust the
charge (see heading “Charging” in this section).
If the superheat is 5
°F higher than shown, then the
system is undercharged.
If the superheat is 5
°F lower than shown, then the
system is overcharged.
Table 7.2, R-22 Discharge Superheat Temperatures, °F
Saturated Condensing Temperature Saturated
Suction
Temp. 100 105 110 115 120 125 130
34 64.2 67.6 70.9 74.0 77.0 79.7 82.1
36 62.4 65.7 68.9 72.0 75.0 77.7 80.1
38 60.7 63.8 67.0 70.1 73.0 75.7 78.2
40 59.0 62.0 65.1 68.1 71.0 73.8 76.3
42 57.3 60.3 63.3 66.3 69.1 71.9 74.4
44 55.7 58.6 61.5 64.4 67.3 70.0 72.6
46 54.0 56.8 59.7 62.6 65.5 68.2 70.8
48 52.4 55.2 58.0 60.8 63.7 66.4 69.1
50 50.9 53.5 56.3 59.1 61.9 64.7 67.3
52 49.3 51.8 54.5 57.3 60.1 62.9 65.6
54 47.7 50.2 52.8 55.6 58.4 61.2 63.9
Charging
The following charging methods apply to TXV systems
only as manufactured and sold by AAON.
The system should be charged during warm weather,
however there are times when systems must be
charged during colder weather. Charging in cold
weather, while not as simple as it would otherwise be,
is possible to accomplish contrary to popular belief.
Warm Weather Charging
If you are charging in warm, or hot weather, above
outdoor ambient temperatures of 65
°F, then use the
standard superheat charging method by first
determining the superheat as described in the
previous section, “Refrigerant Cycle”. If the system is
undercharged, add refrigerant according to Table 7.2.
If the system is overcharged, then recover refrigerant
according to Table 7.2.
Cold Weather Charging
If you are charging in cool, or cold weather, below
outdoor ambient temperatures of 65
°F, then you
should “weigh in” the refrigerant charge. Therefore,
you must have a refrigerant scale in order to charge
during cold weather, and if you are complying with all
of the U.S. Environmental Protection Agency’s laws
governing the use of refrigerants, then you will have a
refrigerant scale available to use.
First, recover all refrigerant from the system, and
evacuate to .05 microns. Next, create a false load on
the system by running the
building heat long enough
to hold an indoor temperature between 70 °F and 80°F
for the charging period. Using a scale, weigh in the
refrigerant nameplate charge. In AAON condensing
units, the nameplate charge is enough refrigerant for
the condenser, and 15 feet of line set. If you are not
using an AAON condensing unit, then you must check
that manufacturer’s literature for refrigerant charge
specifications.
Additional line length, beyond 15 feet with AAON
condensers, will require more refrigerant. Use Table
7.3 to determine how much refrigerant should be
weighed into the system based on line set diameters,
and lengths.
In extremely cold outdoor temperatures, it is helpful to
warm up the refrigerant drum. Electric heating pads,
an electric blanket, or simply a bucket of hot water can
be used.
Table 7.3, Weight of R-22 in Type L Copper Tubing
(Pounds per 100 Feet)
Line Size (In.) Liquid at 100 °F Suction at 40 °F
3/8 3.8 .08
1/2 7.4 .16
5/8 11.9 .26
7/8 23.9 .51
1 1/8 40.7 .87
1 3/8 60.1 1.29
1 5/8 - 1.82
Remember that as you add, or take away refrigerant,
system pressures must stabilize to obtain accurate
readings. The time required for proper stabilization
increases with the length, and overall size of the line
set, and can vary by the amount of refrigerant in the
system. Patience is required to properly charge any
system.
If you are charging the system for the first time after
installation, and start-up, then you are advised to
return after a full week of cooling operation, as outlined
in “Maintenance Schedule” of this section, in order to
check that refrigerant is cycling properly throughout
the system.