General Data
CAB-PRC001-EN
17
Selecting the Correct
Modulating Valve Size
Modulating valves are available in any of
four port sizes: 0.7, 1.5, 2.5 or 4.0 Cv
(coefficient of flow values). The coefficient
of flow is defined as the volume of water
flow through a control valve in the fully
open position with a 1 psig (6.895 kPa)
differential across the valve. Calculate the
coefficient of flow using the formula:
Cv = Q/square root
∆
P where:
Cv = flow coefficient
Q = flow rate (gpm)
∆
P = pressure drop across the valve or
coil (psig).
For good control, the valve Cv should be
approximately equal to the Cv of the
water coil.
Modulating Valve Selection Example
Assume a size 06 vertical cabinet heater
is selected to operate at the following
conditions:
EWT = 180°F (82°C)
LWT = 150°F (66°C)
EAT = 70°F (21°C)
The coil selection is a four-row coil. Select
the best modulating valve size for this
unit.
1
Find the
∆
P across the water coil. Refer to
the ARI performance table to determine
the
∆
P across the water coil or use the
Trane Official Product Selection System,
TOPSS™, selection program. The water
pressure drop is found to be 5.7’ (17.0
kPa) of water at a flow rate of 3.59 gpm.
This converts to a pressure drop of 2.47
psig (1.0 feet of water = 0.4328 psig.)
2
Calculate the Cv of the water coil.
Cv = gpm/Square root
∆
P.
Cv = 3.59/Square root 2.47
Cv = 2.29
Therefore, select the 2.5 Cv valve
because it is closest to the water coil. The
following tables illustrate possible valve
selections at ARI conditions for horizontal
concealed units with a high static motor
and vertical cabinet units with a free
discharge motor. For other applications,
use TOPSS to determine flowrate and
make calculations using the formulas
above.
Table GD-3. Modulating valve selections for horizontal concealed units, high static
motor, 70°F (21°C) EAT, 180°F (82°C) EWT, 30°F (-1°C)
∆∆∆∆∆
T
unit
coil
coil
valve
size
coil
gpm (l/s)
wpd (kPa)
Cv
Cv
2-row
1.19 (0.08)
6.0 (17.8)
0.74
0.7
02
3-row
1.52 (.10)
13.8 (41.2)
0.62
0.7
4-row
1.59 (.10)
3.8 (11.5)
1.24
1.5
2-row
1.53 (.10)
10.3 (30.8)
0.72
0.7
03
3-row
1.82 (.11)
4.3 (12.8)
1.33
1.5
4-row
1.98 (.12)
6.2 (18.6)
1.21
1.5
2-row
1.73 (.11)
3.3 (9.8)
1.45
1.5
04
3-row
2.57 (.16)
9.1 (27.0)
1.29
1.5
4-row
2.81 (.18)
13.4 (39.9)
1.17
1.5
2-row
2.87 (.18)
9.9 (29.5)
1.39
1.5
06
3-row
3.96 (.25)
5.9 (17.7)
2.48
2.5
4-row
4.37 (.28)
8.2 (24.6)
2.32
2.5
2-row
3.71 (.23)
4.7 (14.2)
2.60
2.5
08
3-row
4.74 (.30)
9.1 (27.1)
2.39
2.5
4-row
5.22 (.33)
12.7 (37.8)
2.23
2.5
2-row
4.71 (.30)
8.1 (24.1)
2.52
2.5
10
3-row
6.50 (.41)
18.1 (54.0)
2.32
2.5
4-row
7.13 (.45)
25.3 (75.6)
2.15
2.5
2-row
5.48 (.35)
11.4 (34.0)
2.47
2.5
12
3-row
7.19 (.45)
14.5 (43.2)
2.87
2.5
4-row
7.83 (.51)
10.5 (31.4)
3.67
4.0
Table GD-4. Modulating valve selections for vertical cabinet units, free discharge motor,
70°F (21°C) EAT, 180°F (82°C) EWT, 30°F (-1°C)
∆∆∆∆∆
T
unit
coil
coil
valve
size
coil
gpm (l/s)
wpd (kPa)
Cv
Cv
2-row
1.06 (.07)
4.8 (14.4)
0.74
0.7
02
3-row
1.31 (.08)
10.5 (31.4)
0.61
0.7
4-row
1.34 (.08)
2.8 (8.4)
1.22
1.5
2-row
1.40 (.09)
8.8 (26.3)
0.72
0.7
03
3-row
1.70 (.11)
3.8 (11.3)
1.33
1.5
4-row
1.81 (.11)
5.3 (15.7)
1.20
1.5
2-row
1.71 (.11)
3.2 (9.5)
1.45
1.5
04
3-row
2.12 (.13)
6.4 (19.0)
1.27
1.5
4-row
2.28 (.14)
9.1 (27.3)
1.15
1.5
2-row
2.70 (.17)
8.9 (26.5)
1.38
1.5
06
3-row
3.31 (.21)
4.2 (12.6)
2.46
2.5
4-row
3.59 (.23)
5.7 (17.0)
2.29
2.5
2-row
3.39 (.21)
4.0 (11.9)
2.58
2.5
08
3-row
4.11 (.26)
6.9 (20.7)
2.38
2.5
4-row
4.45 (.28)
9.4 (28.1)
2.21
2.5
2-row
4.32 (.27)
6.8 (20.4)
2.52
2.5
10
3-row
5.55 (.35)
13.4 (40.2)
2.30
2.5
4-row
6.00 (.38)
18.3 (54.8)
2.13
2.5
2-row
4.99 (.32)
9.6 (28.6)
2.45
2.5
12
3-row
6.10 (.38)
10.5 (31.4)
2.86
2.5
4-row
6.48 (.42)
7.3 (21.8)
3.65
4.0
