Calculating The Required Buffer Volume - Viessmann Vitocell 300-H Technical Manual

Sizing
(cont.)
Table 13 - Excerpt from "Vitotrans 353", type PBMA/PZMA datasheet
Heating wa- Set DHW Max. draw-
ter tempera- tempera- off rate
ture in the ture from
heating wa- Vitotrans
ter buffer 353
cylinder
in °C in °C in l/min
40
45
70
50
55
60
Ó

Calculating the required buffer volume

To provide the energy required for DHW heating, a freshwater mod-
ule is normally connected to a heating water buffer cylinder. The
heating water buffer cylinder volume depends on the DHW demand
of the installation, the storage temperature in the heating water buf-
fer cylinder and the user behaviour.
The following applies:
V = ´ ̇ x t x (T /T ) x s
P
P WW
V = Required minimum volume of the heating water buf-
P
fer cylinder
= Calculated peak flow rate of the freshwater module
´
t = Length of time for which the peak flow rate is re-
quired. The value can be based on e.g. the time tak-
en to fill the bath, information provided by the user,
or the standard value from DIN 4708 (10 min).
(T /T ) = for the temperature spread between heating water
P WW
buffer cylinder and DHW:
0.5 = when temperature spread is large
(e.g. 90/45 °C)
0.7 = when temperature spread is medium
(e.g. 70/45 °C)
1.0 = when temperature spread is small
(e.g. 55/45 °C)
s = Safety factor for consideration of user behaviour:
N
1 = normal draw-off pauses
2 = short draw-off pauses
3 ... 4 = very short draw-off pauses
DHW heating
Transfer Required
output heating
water buf-
fer cylin-
der volume
per l of
DHW
in kW in l
60 125
60 146
52 144
44 137
37 127
N
At 10 °C cold water inlet temperature:
Max. draw-off rate at the mixing valve at
40 °C 45 °C
in l/min in l/min
0.4 — —
0.5 70 —
0.8 68 58
0.9 65 56
1.1 60 52
Example:
A buffer cylinder is to be selected for the detached house in the
example on page 22 (chapter "Calculating the DHW demand").
The peak flow rate is 29 l/min.
The future system user has indicated that he "enjoys long showers".
He has indicated a demand duration of 15 min.
For reasons of energy efficiency, the storage temperature in the buf-
fer cylinder should be no more than 70 °C.
The draw-off temperature is 60 °C.
This results in a low temperature spread of 70/60 °C, giving a correc-
tion factor of 1.
As the future system user has indicated that he "enjoys long show-
ers", short draw-off pauses have been assumed. Therefore, the
safety factor s is 2.
N
The minimum buffer volume V
V = ´ x t x (T /T ) x s
P
P WW
= 29 l/min x 15 min x 1 x 2
= 870 l
According to the datasheet, a Vitocell 100-E with a volume of 950 l is
selected.
Return tem-
perature to
the heating
50 °C 55 °C
water buffer
cylinder
in l/min in l/min in °C
— —
— —
— —
49 —
45 40
is therefore:
P
N
VIESMANN
14
15
17
20
23
4
23