Solar System Design Guide
27
6020 457 - 01
Verifying Solar Control Energy Calculation (SCU 124 / 224 / 345)
Suppression of DHW Tank Reheating by the Boiler with SCU 124, 224 and 345
Legend
A
Solar control unit wiring terminals
B
External Relay (field supplied)
C
Resistor 20
Ω
, 0.25 W (field supplied)
D
Tank temperature sensor of the boiler control unit
E
To the boiler control unit; connection for tank
temperature
sensor
F
Resistor 10 k
Ω
, 0.25 W (field supplied)
Systems with Viessmann Solar Control Units SCU 124,
SCU 224 & SCU 345
Reheating of the DHW tank by the boiler will be
suppressed by the solar control unit if the DHW tank
(thermal load #1) is being heated. A resistor simulates
an DHW temperature that is 5ºK or 9ºRa higher than the
actual temperature. The DHW tank is only heated by the
boiler if the set DHW temperature cannot be maintained
by the solar thermal system.
PTC
−
DHW Tank Sensors
NTC
−
DHW Tank Sensors
Long Hand Calculation Example:
Imperial Units
Metric Units
Solar Flow Rate (m) = 1 gpm
Solar Fluid Specific Heat (cp) = 1 Btu/lb°F (water)
0.83 Btu/lb°F (50% glycol)
Solar Fluid Density (P) = 8.33 lbs/gal (water)
8.73 lbs/gal (50% glycol)
Incoming fluid temperature (t
1
) = 120°F
Leaving fluid temperature (t
2
) = 100°F
Calculate energy gained by solar system (50% glycol)
Q= m(cp)(P)
∆
T
Q = 1 gpm x (60 mins/hr. x 8.73 lbs/gal x 0.83 Btu/lb°F) x 20°F
Q= 8,695 Btuh
Solar Flow Rate (m) = 3.8 lpm
Solar Fluid Specific Heat (cp) = 1.163 Wh/kg°C (water)
0.96 Wh/kg°C (50% glycol)
Solar Fluid Density (P) = 1.000 kg/L (water)
1.047 kg/L (50% glycol)
Incoming fluid temperature (t
1
) = 49°C
Leaving fluid temperature (t
2
) = 38°C
Calculate energy gained by solar system (50% glycol)
Q= m(cp)(P)
∆
T
Q= 3.8 lpm x (60 min x 1.047 kg/L x 0.96 Wh/kg°C) x 11°C
Q= 2521 Wh or (2.5 kWh)
Concentration of
Propylene Glycol
Freeze Protection
Specific Heat
Density
°C
°F
J/kgºK
Wh/kgºK
Btu/lbºF
kg/L
lb/gal
0% (water only)
0
32
4190
1.1638
1.000
1.000
8.33
25%
-10.7
12.7
3930
1.0917
0.939
1.024
8.53
30%
-14.0
6.8
3850
1.0694
0.920
1.029
8.58
35%
-17.6
0.3
3770
1.0472
0.900
1.034
8.62
40%
-21.5
-6.7
3680
1.0222
0.879
1.039
8.66
45%
-26.0
-14.8
3580
0.9944
0.855
1.043
8.69
50%
-32.4
-26.3
3480
0.9667
0.831
1.047
8.73
55%
-40.4
-40.7
3380
0.9389
0.807
1.050
8.75
60%
-48.4
-55.1
3280
0.9111
0.783
1.053
8.78
Fluid Properties for Standard Propylene Glycol Based Heat Transfer Medium
Solar Control
