Dhw Circulation Pipes - Viessmann Vitocell 300-H Technical Manual

Installation — DHW cylinders

6.2 DHW circulation pipes

For reasons of hygiene and convenience, DHW circulation pipes are
installed in DHW heating systems. Observe the applicable standards
and rules. Always fit DHW circulation pipes or DHW circulation sys-
tems with appropriate pumps, hydraulically adjust and thermally
insulate them in accordance with the applicable regulations. Take the
applicable standards and regulations into account (e.g. DVGW
Codes of Practice W551/W553 and DIN 1988/TRWI).
The flow rate of the circulation system is determined according to the
scale of the pipework, the thermal insulation and the targeted or
required maximum temperature differential between the cylinder out-
let (DHW) and the DHW circulation inlet (DHW circulation).
Depending on the type of DHW heating system, there are various
connection options for the DHW circulation pipe. Virtually all DHW
cylinders are fitted with connections for the DHW circulation pipe in
the upper third of the cylinder. The exception to this is DHW heating
systems in continuous operation such as freshwater stations or
combi cylinders with an integral DHW indirect coil (Vitocell 340-M/
Vitocell 360-M). They have a "threaded DHW circulation fitting",
which means that the DHW circulation line is routed partially into the
heat exchanger. If this is not the case, the DHW circulation pipe can
also be connected to the cold water inlet of the DHW cylinder.
DHW heating
(cont.)
Connecting to the cold water inlet is also an option for DHW cylin-
ders where, due to the ratio of the draw-off rate and/or the flow rate
of DHW circulation to the cylinder capacity, continuous mixing of the
DHW cylinder content must be expected, e.g. in case of very small
DHW cylinders. Connecting to the cold water inlet may also be
advisable in the case of extremely high DHW circulation flow rates.
In poorly insulated pipework or very widely branched systems in par-
ticular, extremely high flow rates may be necessary. It is then impor-
tant to ensure that high flow velocities cannot lead to any settling
inside the DHW cylinder. The resulting mixing in the standby part
may lead to extremely long heat-up times and fluctuating outlet tem-
peratures (DHW). In this case too, connecting the DHW circulation
pipe to the cold water inlet may be advantageous in terms of the
operating characteristics of the DHW heating system.
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VIESMANN
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