14
Filling Water
Minimum Return Temperatures
Guide values for water in heating equipment according to VDI 2035 for operating
temperatures up to 100°C:
*) See VDI 2035 for exceptions
(+) Necessary in some circumstances, see VDI 2035
Guide values for water in heating equipment according to VdTÜV guidelines for
operating temperatures over 100°C:
Filling and supplementary
water
Circulating water
Residual hardness
mval/kg
pH value
Hydrazine Na
2
SO
3
p value
mval/kg
<0,03 0,5-1,5 8,5-10,0 0,5-25 10-40
Group
Total of rated
boiler powers Q
Filling and
supplementary
water
Heating water
Total of alkaline
earths in Mol/m³
pH value Hydrazine Na
2
SO
3
Excess in mg/kg
0Q ≤ 100 -*) - - -
1 100 < Q ≤ 350 1 to 3 8 to 9,5 - -
2 350 < Q ≤1000 1 to 2 8 to 9,5 2 to 5(+) 5 to20(+)
kW
Excess in mg/kg
Filling Water
Equipment Fuel Oil Gas
Controller R31, R32 R31, R32
R33, R33/4 R33, R33/4
Radiator heating t
Kmin
°C 38 50
less than 20ltr./kW t
Rmin
°C 30 40
Design 80/60 Return boost acc. 1-3 2/3
to example equip.
Low temperature heating t
Kmin
°C 38 50
with less than 20 ltr./kW; t
Rmin
°C 30 40
Design temperature Return boost acc. 1-3 2/3
55/45 and above to example equip.
Equipment with more than t
Kmin
°C 38 50
20ltr./kW floor/ low t
Rmin
°C 30 40
temperature heating
Design temperature Return boost acc. 2/3 2/3
below 55/45 to example equip.
Minimum Return Temperatures
The following measures to maintain a high enough return temperature are needed
in order not to fall below the minimum return temperatures, t
Rmin
, listed in the table and
thus to avoid falling below the dew point!
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