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
0
Q
≤
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!