Table 9 – Draw-off demand per point of use at a DHW temperature
of 45 ºC
Point of use Volume of hot
water drawn off
per use in l
Draw-off demand Q
h max.
per single room
in kWh
per double
room in kWh
Bath 170 7.0 10.5
Shower cubi-
cle
70 3.0 4.5
Washbasin 20 0.8 1.2
Calculating the required cylinder capacity
Q
h max.
=
Draw-off demand per draw-off point in kWh
n = Number of rooms with identical draw-off demand
ϕ
n
= Utilisation factor (simultaneity); can be applied conditionally:
Number of rooms 1 to 15 16 to 36 35 to 75 76 to 300
ϕ
n
*14
1 0.9 to 0.7 0.7 to 0.6 0.6 to 0.5
ϕ
2
= Hotel grading factor
The following factors can be applied to reflect the category of
hotel:
Hotel category Standard Good High
ϕ
2
1.0 1.1 1.2
Z
A
= Heat-up time in h
The heat-up time is subject to the rated heating output available
for DHW heating. Subject to the rated boiler heating output, you
can select a smaller Z
A
value than 2 hours.
Z
B
= Duration of the peak DHW demand in h.
Assumption: 1 to 1.5 h
V = Volume of the DHW cylinder in l
T
a
= Cylinder storage temperature in ºC
T
e
= Cold water inlet temperature in ºC
a = 0.8; this takes into account the heat-up condition of the DHW
cylinder
Example:
Hotel with 50 rooms (30 double rooms and 20 single rooms)
■ Amenities of the single rooms:
5 single rooms with bath, shower cubicle and washbasin
10 single rooms with shower cubicle and washbasin
5 single rooms with washbasin
■ Amenities of double rooms:
5 double rooms with bath and washbasin
20 double rooms with shower cubicle and washbasin
5 double rooms with washbasin
■ Heating water flow temperature = 80 ºC
■ Required heat-up time of the DHW cylinder 1.5 hours
■ Duration of peak demand 1.5 hours
Heat demand for DHW heating
Type of room Equipment level
(draw-off point)
n Q
h max.
in kWh
n × Q
h max.
in kWh
Single room: Bath 5 7.0 35.00
Shower cubicle 10 3.0 30.00
Washbasin 5 0.8 4.00
Double rooms: Bath 5 10.5 52.50
Shower cubicle 20 4.5 90.00
Washbasin 5 1.2 6.00
Σ (n · Q
h max.
) = 217.50
860 · Σ(n · Q
h max.
) · φ
n
· φ
2
· Z
A
V =
(Z
A
+ Z
B
) · (T
a
– T
e
) · a
860 · 217.5 · 0.65 · 1 · 1.5
=
(1.5 + 1.5) · (60 – 10) · 0.8
=1520 l
Selected DHW cylinders:
3 × Vitocell 300-H, each with 500 l capacity
or
3 × Vitocell 300-V, each with 500 l capacity
Calculating the required heat-up output
1500 · (60 – 10)
Z
A
² = Φ =
860 · 1.5
= = 58 kW
V · c · (T
a
– T
e
)
² or Φ = Heat-up output in kW
V = Selected capacity in l
c = Spec. thermal capacity
T
a
= Cylinder storage temperature in ºC
T
e
= Cold water inlet temperature in ºC
Z
A
= Heat-up time in h
The boiler and circulation pump for cylinder heating must be sized
accordingly for the required heat up output.
To guarantee adequate heating of the building during winter too, this
heat volume must be added to the heat load.
Calculating the heat demand for DHW heating in commercial saunas
Assuming:
The sauna is used by 15 people/h.
5 showers with 12 l/min are available, i.e. the showers are utilised 3
times in a row. A showering time of 5 min results in a DHW demand of
60 l per use.
The heat load of the building is
²
N
= Φ
HL build.
= 25 kW.
Two points must be observed to safeguard adequate DHW heating:
a) Adequate cylinder capacity (sized according to peak output).
b) The boiler must be large enough to cover the DHW heating and
²
N
.
Regarding a)
Calculating the cylinder capacity:
15 persons @ 60 l = 900 l at 40 ºC at the DHW outlet.
The cylinder storage temperature is 60 ºC.
As a low temperature boiler is to be installed, the peak output at a
heating water flow temperature of 70 ºC must be calculated; see tables
in the datasheets for the relevant DHW cylinders.
Conversion to an outlet temperature of 45 ºC results in:
V
(45°C)
= V
(40°C)
·
ΔT
(45 °C – 10 °C)
30
= 900 ·
35
=771 l
ΔT
(40 °C – 10 °C)
Suggestion: 2 Vitocell 300-V, each with 300 l capacity with a peak out-
put of 375 l per cylinder and 698 l as a cylinder bank (DHW temperature
45 ºC).
Regarding b)
Required boiler size
*14
For spa hotels, trade fair hotels or similar installations, select a utilisation factor of ϕ
n
= 1.
Sizing
(cont.)
18
VIESMANN
DHW heating
4
5414 646 GB
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