Page 23
Step 1. Determine the desired
D
T
Measure entering air tempera-
ture using dry bulb (
A
) and wet bulb (
B
).
D
T
is the intersecting value
of
A
and
B
in the table (see triangle).
Step 2. Find temperature drop across coil
Measure the coil’s dry
bulb entering and leaving air temperatures (
A
and
C
). Temperature
Drop Formula: (
T
Drop
) =
A
minus
C
.
Step 3. Determine if fan needs adjustment
If the difference be-
tween the measured
T
Drop
and the desired
D
T
(T
Drop
–
D
T)
is within
+3º, no adjustment is needed. See examples: Assume
D
T
= 15 and
A
temp. = 72º, these
C
temperatures would necessitate stated ac-
tions:
Cº T
Drop
–
D
T
=
ºF
ACTION
53º
19
–
15
=
4
Increase the airflow
58º
14
–
15
=
−1
(3º range) no change
62º
10
–
15
=
−5
Decrease the airflow
Step 4. Adjust the fan speed
See indoor unit instructions to in-
crease/decrease fan speed.
Changing air flow affects all temperatures; recheck temperatures to
confirm that the temperature drop and
D
T are 3º.
D
T
80
24 24 24 23 23 22 22 22 20 19 18 17 16 15
78
23 23 23 22 22 21 21 20 19 18 17 16 15 14
76
22 22 22 21 21 20 19 19 18 17 16 15 14 13
74
21 21 21 20 19 19 18 17 16 16 15 14 13 12
72
20 20 19 18 17 17 16 15 15 14 13 12 11 10
70
19 19 18 18 17 17 16 15 15 14 13 12 11 10
57 58 59 60 61 62 63 64 65 66 67 68 69 70
Temp.
of air
entering
indoor
coil ºF
INDOOR
COIL
DRY BULB
DRY
BULB
WET
BULB
B
T
Drop
19º
A
Dry−bulb
Wet−bulb ºF
A
72º
B
64º
C
53º
air flow
air flow
All temperatures are
expressed in ºF
Figure 23. Checking Indoor Airflow over Evaporator Coil using Delta−T Chart
WEIGH IN
1. Check Liquid and suction line pressures
2. Compare unit pressures with tables 4 and
10,
Normal Operating Pressures
.
3. Conduct leak check; evacuate as
previously outlined.
4. Weigh in the unit nameplate charge plus
any charge required for line set
differences over feet.
Liquid Line
Set Diameter
Ounces per 5 feet (g per 1.5 m)
adjust from 15 feet (4.6 m) line set*
3/8" (9.5 mm)
3 ounce per 5’ (85 g per 1.5 m)
NOTE − *If line length is greater than 15 ft. (4.6 m), add this
amount. If line length is less than 15 ft. (4.6 m), subtract this
amount.
Refrigerant Charge per Line Set Length
This nameplate is for illustration
purposes only. Go to actual nameplate
on outdoor unit for charge information.
Figure 24. Using HFC−410A Weigh In Method
1
Check the airflow as illustrated in figure 23 to be sure the indoor airflow is as required. (Make any air
flow adjustments before continuing with the following procedure.)
2
Measure outdoor ambient temperature; determine whether to use
cooling
mode
or
heating mode
to
check charge.
3
Connect gauge set.
4
Check liquid and vapor line pressures. Compare pressures with either heat or cooling mode normal
operating pressures in tables 4 and 10 (second stage − high capacity),
NOTE − The reference table is a general guide. Expect minor pressure variations. Significant differences may
mean improper charge or other system problem.
5
Set thermostat for heat/cool demand, depending on mode being used:
Using cooling mode
When the outdoor ambient temperature is 60°F (15°C) and above. Target
subcooling values (second stage − high capacity) in tables 4 are based on 70 to 80°F (21−27°C) indoor
return air temperature; if necessary, operate heating to reach that temperature range; then set thermostat
to cooling mode setpoint to 68ºF (20ºC) which should call for second stage (high capacity) cooling. When
pressures have stabilized, continue with step 6.
Using heating mode
When the outdoor ambient temperature is below 60°F (15°C). Target
subcooling values (second stage − high capacity) in tables 4 and 10 are based on 65−75°F (18−24°C) indoor
return air temperature; if necessary, operate cooling to reach that temperature range; then set thermostat
to heating mode setpoint to 77ºF (25ºC) which should call for second stage (high capacity) heating. When
pressures have stabilized, continue with step 6.
6
Read the liquid line temperature; record in the LIQº space.
7
Read the liquid line pressure; then find its corresponding temperature in the temperature/ pressure chart
listed in table 15 and record it in the SATº space.
8
Subtract LIQº temperature from SATº temperature to determine subcooling; record it in SCº space.
9
Compare SCº results with tables 6 through 8, being sure to note any additional charge for line set and/or
match−up.
10 If subcooling value is greater than shown in tables 6 through 8 for the applicable unit, remove refrigerant;
if less than shown, add refrigerant.
11 If refrigerant is added or removed, repeat steps 4 through 10 to verify charge.
12 Disconnect gauge set and re−install both the liquid and suction service valve caps.
USE COOLING
MODE
USE HEATING
MODE
60ºF (15º)
SATº
LIQº
–
SCº
=
SUBCOOLING
Figure 25. Using HFC−410A Subcooling Method Second Stage (High Capacity)
