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5.5
By-pass function
The by-pass function, sometimes called summer by-pass function or
keep warm function, implements a feature to keep the supply side of the
HIU hot. It is necessary to heat up the supply side pipe work after a long
period of inactivity, this will minimise the time taken to produce hot
water at the tap or radiator.
This function is provided by a thermo-mechanical by-pass [3]
( Table 2, Model matrix for standard component or accessory), this
allows circulation of the district water at the appliance.
An electronic option is also available when connecting a Sense II. The
domestic hot water control valve [2] is used to allow water to run
through the HIU, this heats the supply side piping. This function is
provided via the HIU control module.
Electronic keep hot function
▶ This function will allow a small amount of district
water to flow through the heat meter.
The trigger temperature for this function is adjustable between 0-60 °C,
this function is activated by a Sense II [1].
Thermo-mechanical by-pass valve set temperature
▶ The thermo-mechanical by-pass valve is fixed to
number 4 (approximately 45 °C).
Fig. 37 By-pass functions
6 720 808 928 (2017/06)
5.6
The limit return function helps to ensure high system efficiency. Since a
high return temperature results in a lower efficiency for the heating
supply company, there may be contractual financial penalties for
customers when the return temperature is over a certain threshold. This
situation can be avoided by limiting the hot water flow through the HIU,
the return temperature is measured and the CH control valve closed if
the return flow temperature is too high.
Although the limit return flow function is offered as a special HIU feature,
it should not be necessary if the system is designed correctly. If high
return temperatures are experienced it may be, for example, that the
radiator surface areas are too small and not enough heat is transmitted
into the room. If the limit return temperature function reduces the flow
of hot water it may be that the required room temperature is not met.
The limit return temperature function is always active and ensures a
highly efficient use of the energy. The default value for the return
temperature is 50 °C.
If a lower limit for the return temperature is required, e.g. 40 °C, a Sense
II must be used to set this temperature. The CH function then compares
the difference between set-point and control value for flow and controls
the minimum deviation.
If a Sense II is not used for controlling the system, it is also possible to
connect a Sense II to the HIU control unit during commissioning only, in
order to a set the maximum allowable return temperature.
1
2
3
6720808928-34.1Wo
Limit return temperature
Installation
29
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Troubleshooting
