6
Corrosion protection
Oxygen always plays a role if metal materials in a heating system corrode. pH values and the salt content
also play a major role. The challenge: A licenced plumber who would like to be able to guarantee his cus-
tomers a hot water heating system not at risk of corrosion from oxygen - without the use of chemicals - must
pay attention to the following:
n
Correct system design by the heating contractor/planner and
n
subject to the installed materials: filling the heating system with demineralised softwater or fully deionised
water, checking the pH value after 8 to 12 weeks.
See the following table for the requirements in accordance with VDI 2035 Part 1 with regard to total hard-
ness.
Total hardness [°dH] subject to the specific system volume
Overall output in kW
<20 l/kW
³
20 l/kW and <50 l/kW
³
50 l/kW
to 50 kW
£
16,8 °dH
£
11,2 °dH
£
0,11 °dH
The following table provides the allowed oxygen content in connection with the salt content.
Reference values for the hot water in accordance with VDI 2035 Part 2
low-salt
salline
Electrical conductivity
at 25°C
μS/cm
< 100
100-1500
Oxygen content
mg/l
< 0,1
< 0,02
pH value at 25°C
8,2 - 10,0 *)
*) For aluminium and aluminium alloys the pH value range is limited: the pH value at 25°C is 8.2-8.5 (max.
9.0 for aluminium alloys)
In low-salt water and the correct pH for a short time even to oxygen concentrations up 0.5 mg / l are
tolerated.
Water treatment with chemicals
Adding chemicals to treat water should only be
done as an exception. VDI 2035 Part 2 requires
explicitly under Point 8.4.1 that all water treatment
be explained and documented in the system log
book. This has reasons:
n
The improper use of chemicals often leads to
the failure of elastomer materials
n
To blockages and deposits because of sludge
formation
n
To defective floating seals in pumps
n
To the formation of biofilm which can cause
microbial influenced corrosion or significantly
impair heat transfer.
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