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Minimum return temperatures
System
Radiator central heating
less than 20l/kW
Design 80/60
Low-temp. central heating
with less than 20l/kW;
Design temperature
55/45 and above
Systems with more than
20l/kW underfloor/
low-temp. central heating
Design temperature
under 55/45
t
= min. boiler temperature
Kmin
t
= min. return temperature
Rmin
Please note: A weather-compensated control unit is equipped with soft boiler start, which prevents condensate being formed
during the start-up phase.
Sizing shunt pump
System example
Minimum return temperatures /
return temperature raising facility
To prevent falling below the dewpoint, the following measures for raising the return
temperature are required in order to maintain the minimum temperatures t
in the table.
Fuel
Control unit
t
°C
Kmin
t
°C
Rmin
Return temp. raising
acc. to sys. example
t
°C
Kmin
t
°C
Rmin
Return temp. raising
acc. to sys. example
t
°C
Kmin
t
°C
Rmin
Return temp. raising
acc. to sys. example
The flow rate of shunt pump V is calculated according to the following formula:
V
= Shunt pump volume flow in m³/h
Q
= Rated boiler output in kW
N
c
= Specific energy 4.2 kWs/kgK
p
ρ
= Specific weight of water 1000 kg/m³
W
∆t
= Temperature differential between the flow temperature and bypass
temperature (recommended design temperature differential 30k)
The shunt pump head results from the boiler pressure loss at the selected volume
flow, the pipework pressure loss as well as all individual boiler circuit pressure
losses.
A return temperature limit stat T switches the shunt pump ON, when the permissible
return temperature is no longer maintained.
A return temperature limit stat T1 switches the shunt pump ON, when the minimum
return temperature is no longer maintained.
If the return temperature falls further, the return temperature limit stat T2 signals to
the control unit R, which throttles the heat transfer via one or several mixers M in the
heating circuit.
A return temperature limit stat T regulates a distribution valve M, so that if the
permissible return temperature is no longer maintained, the heating circuit flow will
be partially or fully closed.
After the boiler has been heated up, the distribution valve M will open as wide as the
permissible return temperature requires.
The distribution valve M, can also be installed as mixer into the return.
Oil
R31, R32
R33, R33/4
38
30
1-3
38
30
1-3
38
30
2/3
Q
x 3600
N
V =
in m
3
/h
x ∆t x ρ
c
p
W
listed
Rmin
Gas
R31, R32
R33, R33/4
50
40
2/3
50
40
2/3
50
40
2/3
9
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