Sizing Systems For Dhw Heating 1.1 Principles; General Information; Irregular Dhw Demand; Constant Dhw Demand - Viessmann Vitocell 100-H Technical Manual

Sizing systems for DHW heating
1.1 Principles

General information

1
When sizing DHW heating systems, two main principles must be taken
into consideration: For reasons of hygiene, the volume of the DHW
heating system should be sized so that it is as small as possible. How-
ever, for reasons of convenience, it should be as large as required.
This means that the system must be designed as accurately as pos-
sible.

Irregular DHW demand

For buildings with irregular demand, e.g. schools, commercial enter-
prises, hotels or sports complexes with shower facilities, sizing is often
carried out via the peak output/maximum draw-off rate over 10
minutes. It is important to ensure here that the DHW heating system
is not oversized, but also to consider the heat-up time for the DHW
cylinder until the next peak in demand occurs.

Constant DHW demand

For applications where there is constant demand for DHW, for example
businesses that prepare food or operate swimming pools, the DHW
heating system is sized according to the constant demand of the con-
sumer (continuous output). The size of the heat exchanger and the
available heating output are crucial factors.

High DHW demand

For extremely high demands, sizing the DHW heating system accord-
ing to both the peak output and the continuous output is recommen-
ded. This applies in particular to cylinder loading systems.

EDIS calculation program

For reliable sizing of DHW heating systems, Viessmann provides free
EDIS software, which can be used for calculations for both residential
buildings (in accordance with DIN 4708-2) and non-residential build-
ings such as hotels, army barracks and industrial enterprises. Various
complementary calculation processes are used.

Hydraulic connection

In addition to the sizing of the DHW cylinder, the hydraulic connection
and the operation of the entire system for DHW heating is also
extremely important for reliable and safe operation of the DHW heating
system.
Above all, the selection of the correct operating temperature and the
design of the DHW circulation pipe, as well as its connection to the
DHW cylinder are extremely important for the hygienic operation of the
DHW heating system. The applicable standards and laws must be
observed.
VIESMANN
4
In practice, various approaches are taken: For residential buildings,
systems are often configured in accordance with DIN 4708 Part 2.
Taking into account the sanitary amenities of the individual apart-
ments/residential units, the occupancy/user rate and utilisation factors,
the demand factor N can be determined.
Systems operating according to the instantaneous water heater prin-
ciple, such as e.g. freshwater modules, may also be sized according
to peak flow rate with reference to DIN 1988-300.
It is therefore also important to identify the available heating and trans-
fer output and to ensure that the DHW can be adequately heated in
the time between the peaks in demand.
Sizing according to the continuous output is also practical when
special consideration must be given to the return temperatures of the
heating system (e.g. district heating systems).
Particular reference is made to DVGW Code of Practice W 551, TRWI
(DIN 1988) and the valid Drinking Water Ordinance [Germany]
(TrinkwV) or Directive 98/83/EC of the Council of the European
Union.
DHW heating