4.2 Sizing according to peak flow rate with reference to DIN 1988-300
Application
For DHW heating systems operating according to the instantaneous
water heater principle, e.g. freshwater modules, the DHW demand
can be determined according to the peak flow rate principle.
For this, the assumption is made that the peak flow rate to DIN
1988-300 determined for calculating the pipe dimensions for the
DHW pipework will also have to be heated by the DHW heating sys-
tem.
The peak flow rate is the sum of all connected individual consumers
(total flow rate), reduced by a simultaneity factor. This is subject to
the type of building.
However, to avoid oversizing, the calculated peak flow rate must not
be higher than the sum of the two largest individual consumers that
may be operating simultaneously. For systems with several inde-
pendent consumers, e.g. in apartment buildings, also carry out this
check with the total flow rate of the respective largest consumer, e.g.
of all apartments.
Calculating the DHW demand
This is based on determining the peak flow rate ´
S
to DIN 1988-300.
´
S
= a (Σ ´
R
)
b
- c
(Valid for
´
R
max. = 500 l/s)
´
S
= Peak flow rate
´
R
= Total flow rate (sum of calculation flow rate of all con-
sumers)
a, b, c = Constants subject to building and its type of use (see ta-
ble 11)
Table 11
Building type Constants
a b c
Residential buildings 1.48 0.19 0.94
Hospital ward 0.75 0.44 0.18
Hotel 0.70 0.48 0.13
School 0.91 0.31 0.38
Administration building 0.91 0.31 0.38
Facility for supported living, retirement
home
1.48 0.19 0.94
Care home 1.40 0.14 0.92
´
R
describes the total flow rate of all consumers. The values of the
DHW calculation flow rate of individual consumers is added to this.
For details of the DHW calculation flow rate, see consumer manufac-
turers, e.g. tap manufacturer. If no details are available, use values
from DIN 1988-300:
Table 12 - Calculation flow rate for the connections on the cold
and warm water sides
Mixer taps for type of draw-off point DN Calculation
flow rate ´
R
Shower tray 15 0.15 l/s
Bath 15 0.15 l/s
Kitchen sink 15 0.07 l/s
Washbasin 15 0.07 l/s
Bidet 15 0.07 l/s
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
Assumptions:
A manufacturer datasheet is available for the shower with body
shower.
The calculated 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:
´
R
= Shower 0.15 l/s + washbasin 0.07 l/s + bath 0.15 l/s + show-
er with body shower 0.33 l/s + 2 washbasins 0.07 l/s + kitch-
en 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 build-
ing are selected from Table 11:
a = 1.48
b = 0.19
c = 0.94
Peak flow rate:
´
S
=
a (Σ ´
R
)
b
- c
=
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 (assump-
tion: 70 °C) select a Vitotrans 353 freshwater module from the data-
sheet.
Example: Vitotrans 353, type PZMA for installation on a
Vitocell 100-E buffer cylinder (see Table 13).
The values for Vitotrans 353, type PBMA (for wall mounting) are the
same as those for the Vitotrans 353, type PZMA (for installation on a
cylinder).
Sizing
(cont.)
22
VIESMANN
DHW heating
4
5414646
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