Technical data
108 | Project planning manual for heat pumps for heating and domestic hot water preparation | 01.2012 www.dimplex.de
8.7 Integration of the heat pump in the heating system
8.7 Flow temperature limit of underfloor heating
Many underfloor heating pipes and screed floorings should not
be heated over 55 °C. In the case of bivalent operation or if the
buffer tank is charged externally, a limitation of the flow tempera-
ture must be effected to prevent such overheating.
NOTE
With a mixer in the underfloor heating circuit or in bivalent-renewable op-
erating mode, the mixer is closed when the temperatures are too high. A
safety temperature monitor prevents increased system temperatures due
to mixer sluggishness or failure.
8.7.1 Flow temperature limiting via a mixer limit switch
The mixer is only opened wide enough at full boiler output and
maximum boiler temperature that the maximum flow temperature
of approx. 55 °C is not exceeded. A further mixer open command
is prevented by locking the freely controllable mixer limit switch in
this position.
We recommend installing a mixer motor equipped with a limit
switch so that the drive can be switched off electrically.
8.7.2 Flow temperature limiting via a mixer bypass
At full boiler output, maximum boiler temperature and with the
mixer opened completely, the bypass is opened so wide that the
maximum flow temperature is not exceeded. This limits the flow
temperature. The regulator valve must be secured against acci-
dental adjustment.
We recommend using mixers equipped with an internal bypass.
This type of flow temperature limiting is particularly suitable for
underfloor heating.
Fig. 8.25: Bypass circuit for safeguarding the maximum
flow temperature
8.8 Mixer
The mixer is in the “closed” position (for the boiler) in heat-pump-
only operation and directs the hot flow water past the boiler. This
prevents downtime losses. The mixer is dimensioned according
to the boiler output and the flow rate volume.
The mixer drive must have a runtime of between 1 and 6 min-
utes. The heat pump manager controls the mixer and can be set
to this runtime. We recommend using mixers with a runtime of
between 2 and 4 minutes.
8.8.1 Four-way mixer
The four-way mixer is generally required for oil boilers with fixed
temperature regulation. These may not be operated below tem-
peratures of 70 °C (poss. 60 °C). The mixer mixes the boiler tem-
perature to the current required flow temperature. Using the ef-
fect of an injector, it maintains a circulation in the boiler circuit
which flows in the opposite direction of the heating system circuit.
This ensures that the heating water returning to the boiler is al-
ways hot enough to prevent the dew point in the boiler from being
undershot (which in turn would cause the return temperature to
rise).
8.8.2 Three-way mixer
The three-way mixer is used to regulate individual heating cir-
cuits and is also implemented when low-temperature boilers or
condensing boilers with burner regulation are used (e.g. “varia-
bly-regulated boilers”). These types of boilers can have cold re-
turn water circulating through them.
The three-way mixer thus serves as a kind of switching valve. It
is completely closed in heat pump only operation (prevents
downtime losses) and is completely open in boiler operation.
8.8.3 Three-way solenoid valve (switching valve)
Three-way solenoid valves are not to be recommended for this
application because they do not function reliably and can trans-
mit sound to the heating system.
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