Optimizing System Refrigerant Charge
This section provides instructions on optimizing the system charge. This section includes:
SOptimizing procedure
SAdjusting indoor airflow
SUsing subcooling method
SApproved matched components, targeted subcooling (SC) values and add charge values
SNormal operating pressures
STemperature pressures
OPTIMIZING PROCEDURE
1.Move the low−side manifold gauge hose from the vapor line service valve to the true suction port (see figure 21).
2.Set the thermostat for either cooling or heating demand. Turn on power to the indoor unit and close the outdoor unit disconnect switch to start the unit.
3.Allow unit to run for five minutes to allow pressures to stabilize.
4.Check the airflow as instructed under Adjusting Indoor Airflow to verify or adjust indoor airflow for maximum
efficiency. Make any air flow adjustments before continuing with the optimizing procedure.
5.Use subcooling method to optimize the system charge (see figure 24). Adjust charge as necessary.
ADJUSTING INDOOR AIRFLOW
Heating Mode Indoor Airflow Check
(Only use when indoor unit has electric heat)
Indoor blower airflow (CFM) may be calculated by energizing electric heat and measuring:
STemperature rise between the return air and supply air temperatures at the indoor coil blower unit,
SMeasuring voltage supplied to the unit,
SMeasuring amperage being drawn by the heat unit(s).
Then, apply the measurements taken in the following formula to determine CFM:
Amps x Volts x 3.41
CFM =
1.08 x Temperature rise (F)
Cooling Mode Indoor Airflow Check
Check airflow using the Delta−T (DT) process using figure 23.
ADJUSTING INDOOR AIRFLOW
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| of air |
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| 80 | 24 | 24 | 24 | 23 | 23 | 22 | 22 |
| 22 | 20 | 19 | 18 | 17 | 16 | 15 | |
| entering | Dry−bulb | 78 | 23 | 23 | 23 | 22 | 22 | 21 | 21 |
| 20 | 19 | 18 | 17 | 16 | 15 | 14 | ||
| indoor |
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| 76 | 22 | 22 | 22 | 21 | 21 | 20 | 19 |
| 19 | 18 | 17 | 16 | 15 | 14 | 13 | ||||
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| coil ºF |
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| 74 | 21 | 21 | 21 | 20 | 19 | 19 | 18 |
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| A |
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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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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 23. Checking Airflow over Indoor Coil Using Delta−T Formula
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