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Heating water

Water hardness

13 Heating water

13.1

Water hardness

Determine permissible water hardness for the
heating water according to ÖNORM H 5195-1

Instructions for determination:

1. Divide water content (litres) of the heat producer by

its output in kW. If the result is larger than 0.3 l/kW,
Table 1 applies. If the value is smaller or equal to
0.3 l/kW, Table 2 applies.

2. Divide the total heating water volume (in litres) by

the output (in kW) of the smallest heat producer.
The result is the specific water content and this
determines the column within the previously
calculated table.

3. Read the data for the permissible water hardness

from the respective line using the total output of the
heat producer.

Example: A heating system with a 45 kW boiler and
1500 litre total water volume.

1. The ratio of water content to output is more than

0.3 l/kW (117:45=2.6) => Table 1.

2. The specific water content is 33,3 l/kW (1500÷45 =

33.3) => middle column in Table 1.

3. The total output of the boiler is 45 kW; therefore,

only the data from the first line (

≤

50 kW) are

relevant.

The permissible water hardness in this example is 11.2
°dH.

Softening with a salt-regeneration ion exchanger

We recommend softening water with salt-regeneration
ion exchangers, just as drinking water is softened. This
method does not remove salt from the water. It
replaces the calcium in the lime with sodium from the
salt, and has considerable advantages. It is cheap and

chemically stable against contamination. In addition, it
produces a natural alkalinity that generally results in a
sufficiently non-corroding pH value of around 8.

pH value between 8 and 9 may require dosing with
trisodium phosphate

If the heating water's pH value has not itself changed
to more than 8 after a week of operation, increase it by
adding 10 g/m³ of trisodium phosphate (Na

) or

25 g/m³ of trisodium phosphate dodecahydrate (Na

.12H

O). Wait another 2-4 weeks before making

further corrections. The pH value must not exceed 9.

No hybrid installations

A disadvantage of salt-regeneration ion exchange is
the salt content with its high electrical conductivity,
which can lead to electrolytic corrosion, especially of
aluminium or galvanized steel. If only steel, brass,
gunmetal and copper are used in the heating system
and the use of stainless steel is limited to small areas,
then no corrosion problems should be expected, even
with salty water.

Galvanized and aluminium parts in a heating system
are always in danger of corrosion, especially in
combination with copper tubing. In practice, this
means no hot-galvanized fittings and no mixing of
galvanized tubing with copper tubing. There is an
illogical exception: galvanized steel tubing combined
with boilers or buffer storage tanks made of steel.
Presumably, the uniform zinc layer dissolves uniformly
and is dispersed evenly throughout the system without
localised corrosion.

Table 1

Heat producer with large (> 0.3 l/

kW) water content

Table 2

Heat producer with small (> 0.3 l/

kW) water content

Specific water content in litre/kW

< 20 l/kW

≥

20 /kW

< 50 l/kW

≥