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| of air |
| 80 | 24 | 24 | 24 | 23 | 23 | 22 | 22 |
| 22 | 20 | 19 | 18 | 17 | 16 | 15 | |
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| Dry−bulb | 78 | 23 | 23 | 23 | 22 | 22 | 21 | 21 |
| 20 | 19 | 18 | 17 | 16 | 15 | 14 | ||
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| 76 | 22 | 22 | 22 | 21 | 21 | 20 | 19 |
| 19 | 18 | 17 | 16 | 15 | 14 | 13 | |||
| coil ºF |
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| 74 | 21 | 21 | 21 | 20 | 19 | 19 | 18 |
| 17 | 16 | 16 | 15 | 14 | 13 | 12 | |||
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| 72 | 20 | 20 | 19 | 18 | 17 | 17 | 16 |
| 15 | 15 | 14 | 13 | 12 | 11 | 10 |
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7019 19 18 18 17 17 16 15 15 14 13 12 11 10
Wet−bulb ºF 57 58 59 60 61 62 63 64 65 66 67 68 69 70
1. Determine the desired DT
ature using dry bulb (A) and wet bulb (B). DT is the intersect- ing value of A and B in the table (see triangle).
2. Find temperature drop across coil
bulb entering and leaving air temperatures (A and C). Tem- perature Drop Formula: (TDrop) = A minus C.
ference between
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C | TDrop |
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53º | 19º |
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| air flow | air flow | B |
DRY |
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| 64º |
BULB |
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All temperatures are | INDOOR |
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COIL |
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expressed in ºF |
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A 
72º
DRY
BULB
WET
BULB
the measured TDrop and the desired DT
15 and A temp. = 72º, these C temperatures would necessi- tate stated actions:
Cº | TDrop | – | DT | = | ºF | ACTION |
53º | 19 | – | 15 | = | 4 | Increase the airflow |
58º | 14 | – | 15 | = | −1 | (within +3º range) no change |
62º | 10 | – | 15 | = | −5 | Decrease the airflow |
4.Adjust the fan speed crease/decrease fan speed.
Changing air flow affects all temperatures; recheck tempera- tures to confirm that the temperature drop and DT are within +3º.
Figure 22. Checking Airflow over Indoor Coil Using Delta−T Formula
USE
COOLING
MODE
60ºF
(15ºC)
USE
HEATING
MODE
SATº
LIQº –
SCº =
1.Check liquid and vapor line pressures. Compare pressures with either second−stage heat or cooling mode normal operating pressures listed in table 4. Table 4 is a general guide and expect minor pressures variations. Significant pressure differences may indicate improper charge or other system problem.
2.Decide whether to use cooling or heating mode based on current outdoor ambient temperature:
AUse COOLING MODE when:
SOutdoor ambient temperature is 60°F (15.5°C) and above.
SIndoor return air temperature range is between 70 to 80°F (21−27°C). This temperature range is what the target subcooling values are base upon in table 5.
If indoor return air temperature is not within reference range, set thermostat to cooling mode and a setpoint of 68ºF (20ºC). This should place the outdoor unit into second−stage (high−capacity) cooling mode. When operating and temperature pressures have stabilized, continue to step 3.
BUse HEATING MODE when:
SOutdoor ambient temperature is 59°F (15.0°C) and below.
SIndoor return air temperature range is between 65−75°F (18−24°C). This temperature range is what the target subcooling values are base upon in table 5.
If indoor return air temperature is not within reference range, set thermostat to heating mode and a setpoint of 77ºF (25ºC). This should place the outdoor unit into second−stage (high−capacity) heating mode. When operating and temperature pressures have stabilized, continue to step 3.
3.Read the liquid line pressure; then find its corresponding temperature pressure listed in table 3 and record it in the SATº space to the left.
4.Read the liquid line temperature; record in the LIQº space to the left.
5.Subtract LIQº temperature from SATº temperature to determine subcooling; record it in SCº space to the left.
6.Compare SCº results with table 5 (either Heating or Cooling mode column), also consider any additional charge required for line set lengths longer than 15 feet and/or unit matched component combinations (Add Charge column).
7.If subcooling value is:
AGREATER than shown for the applicable unit match component, REMOVE refrigerant; BLESS than shown for the applicable unit match component, ADD refrigerant.
8.If refrigerant is added or removed, repeat steps 3 through 6 to verify charge.
9.Close all manifold gauge set valves and disconnect gauge set from outdoor unit.
10.Replace the stem and service port caps and tighten as specified in Operating Service Valves on page .
11.Recheck voltage while the unit is running. Power must be within range shown on the nameplate.
Figure 23. Using HFC−410A Subcooling Method tage (High Capacity)
506728−01 | Page 26 |
