Calculating The Dhw Demand; Calculating The Required Buffer Volume - Viessmann Vitocell 100-H Technical Manual

Sizing
(cont.)

Calculating the DHW demand

This is based on determining the peak flow rate ´
b
= a (Σ ´ ) - c
´
S R
(Valid for ´ max. = 500 l/s)
R
= Peak flow rate
´
S
= Total flow rate (sum of calculation flow rate of all consumers)
´
R
a, b, c = Constants subject to building type of use (see table)
Table 11
Building type
Residential buildings
Hospital ward
Hotel
School
Administration building
Facility for supported living, retirement
home
Care home
describes the total flow rate of all consumers. The values of the
´
R
DHW calculation flow rate of individual consumers is added to this.
Information on the calculation flow rate are available from the manu-
facturers of the consumers (e.g. tap manufacturers). If they are not
available, the values in DIN 1988-300 can be used:
Table 12 - Calculation flow rate for the connections on the cold
and warm water sides
Mixer taps for type of draw-off point
Shower tray
Bath
Kitchen sink
Washbasin
Bidet
Example:
Detached house with 2 bathrooms, 1 kitchen with kitchen sink, 1 guest
toilet with washbasin.
Equipment, bathroom 1: Shower, washbasin
Equipment, bathroom 2: Bath, shower with body showers, 2 washba-
sins
Table 13 - Excerpt from "Vitotrans 353" 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 module
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 buffer cylin-
der and the user behaviour.
DHW heating
to DIN 1988-300.
S
Constants
a b c
1.48 0.19 0.94
0.75 0.44 0.18
0.70 0.48 0.13
0.91 0.31 0.38
0.91 0.31 0.38
1.48 0.19 0.94
1.40 0.14 0.92
DN Calculation
flow rate ´
R
15 0.15 l/s
15 0.15 l/s
15 0.07 l/s
15 0.07 l/s
15 0.07 l/s
Transfer Required
output heating
water buf-
fer cylinder
volume per
litre of
DHW
in kW in l
65 135 0.5
64 155 0.7
54 149 0.8
45 141 0.9
37 129 1.1
Assuming:
A manufacturer datasheet is available for the shower with body
shower.
The calculation DHW flow rate is: 20 l/min = 0.33 l/s.
Standard values from Table 12 are used for the remaining consum-
ers.
The total flow rate of the detached house is:
= shower 0.15 l/s + washbasin 0.07 l/s + bath 0.15 l/s + shower
´
R
with body shower 0.33 l/s + 2 washbasins 0.07 l/s + kitchen sink
0.07 l/s + washbasin 0.07 l/s
= 0.98 l/s
To calculate the peak flow rate, factors a, b, c for a residential building
are selected from Table 11:
a = 1.48
b = 0.19
c = 0.94
Peak flow rate:
b
= a (Σ ´ ) - c
´
S R
= 1.48 x 0.98 0.19 – 0.94
= 0.53 l/s
The calculated peak flow rate of 0.53 l/s is greater than the sum of the
two simultaneously operating consumers (shower in bathroom 1 =
0.15 l/s and shower with body shower in bathroom 2 = 0.33 l/s) =
0.48 l/s. Therefore, the value of 0.48 l/s is taken as the peak flow
rate.
The DHW heating system must heat 0.48 l/s = approx. 29 l/min of DHW
from 10 to 60 °C. This results in a transfer rate of approx. 101 kW.
Subject to the heating water temperature or heating water storage
temperature in the heating water buffer cylinder (assumption: 70 °C),
a Vitotrans 353 freshwater module can now be selected from the data-
sheet.
Example: Vitotrans 353, type PZM for installation on a Vitocell 100-E
buffer cylinder (see Table 13).
The values for Vitotrans 353, type PBM (for wall mounting) are the
same as those for the Vitotrans 353, type PZM (for installation on a
cylinder).
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
— —
74 —
71 61
67 57
62 53
The following applies:
V = ´ ̇ x t x (T /T ) x s
P P WW N
Return tem-
perature to
the heating
50 °C 55 °C
water buffer
cylinder
in l/min in l/min in °C
— —
— —
— —
50 —
46 41
VIESMANN
4
19
21
23
26
31
21