A chemical water treatment can be ordered from Viessmann for fill-
ing.
Prevention of damage through corrosion on the water side
The corrosion resistance of ferrous materials on the heating water side
of heating systems and boilers depends on the absence of oxygen in
the heating water.
The oxygen introduced into the heating system with the first fill and the
top-up water reacts with the system materials without causing dam-
age.
The characteristic blackening of the water after some time in use indi-
cates that free oxygen is no longer present.
The technical rules and in particular VDI Directive 2035-2 therefore
recommend that heating systems are designed and operated so that
a constant ingress of oxygen into the heating water is prevented.
Opportunities for oxygen ingress during operation:
■ Through open expansion vessels overflowing
■ Through negative pressure in the system
■ Through gas-permeable components
Sealed unvented systems, e.g. with a diaphragm expansion vessel,
offer good protection against the ingress of airborne oxygen into the
system, if correctly sized and operating at the correct pressure.
At every part of the heating system, even at the suction side of the
pump, and under all operating conditions, the system pressure should
be above ambient atmospheric pressure.
The pre-charge pressure of the diaphragm expansion vessel should
be checked at least during the annual service.
The use of permeable components, e.g. plastic pipes that are perme-
able to gas in underfloor heating systems, should be avoided. Provide
system separation if such components are nevertheless used. This
must separate the water flowing through the plastic pipes from other
heating circuits, e.g. from the heat source, by the provision of a heat
exchanger made of corrosion-resistant material.
No further anti-corrosion measures are required for sealed hot water
heating systems subject to the above points being observed.
However, take additional precautions where there is a risk of oxygen
ingress, for example by adding oxygen binder sodium sulphite (5 - 10
mg/litre into the excess). The heating water should have a pH value
between 9.0 and 10.5.
Different conditions apply to systems that contain aluminium compo-
nents.
Where chemicals are used as part of the corrosion protection, we rec-
ommend that the manufacturer of the chemicals issues a certificate of
suitability of the additives with regard to the boiler materials and the
materials of the other heating system components.
We recommend you refer questions of water treatment to Viessmann
industrial services or an appropriate specialist.
Further details can be found in VDI Directive 2035-2 and EN 14868.
a
: In Austria, detailed information can be found in the
ÖNORM H 5195-1: Heat exchanger for domestic systems, part 1
Appendix
8.1 Sizing the expansion vessel
In accordance with EN 12828, water heating systems must be equip-
ped with a diaphragm expansion vessel. The size of the expansion
vessel is subject to the heating system specification and should be
checked in each case.
Quick selection table for calculating the vessel size V
n
Safety valve
p
sv
bar
3.0
V
n
Pre-charge
pressure
bar
1.0
1.5
1.8
litres
System vol-
ume V
A
litres
220
—
—
25
340
200
—
35
510
320
200
50
840
440
260
80
1050
540
330
100
1470
760
460
140
2100
1090
660
200
2630
1360
820
250
3150
1630
990
300
4200
2180
1320
400
5250
2720
1650
500
Selection example
Datum:
p
sv
= 3 bar (response pressure of the safety valve)
H
= 13 m (static ceiling of the system)
Q
= 18 kW (rated output of the heat source)
v
= 8.5 l/kW (specific water capacity)
panel radiators 90/70 °C
V
PH
= 1000 l (volume of buffer cylinder)
The specific water capacity v was determined as follows:
■ Radiators: 13.5 l/kW
■ Panel radiators: 8.5 l/kW
■ Underfloor heating: 20 l/kW
Calculation:
V
A
= Q x v + 1000
V
A
= 18 kW x 8.5 l/kW + 1000 l
= 1153 l
If possible, when calculating the gas pre-charge pressure, select a
supplement of 0.2 bar:
p
0
≥ H/10 + 0.2 bar
p
0
≥ (13/10 + 0.2 bar) = 1.5 bar
Design information
(cont.)
VITOLIGNO 300-H
VIESMANN
57
5672 507 GB
8
