Direct Digital Controller
56 Direct Digital Controller fw 4.013 – Ed. 01/2013
• Complement and substitution mode
This mode on the other hand is used for plants in which, at a high thermal load, it is foreseen that the
temperatures requested for delivery and/or return water are incompatible with the maximum permitted
temperatures of the heat pumps. In this type of plant, starting with a given thermal load we must keep the
heat pumps off and use only the auxiliary boilers; we must therefore install auxiliary boilers that are able
to provide 100% of the maximum power requested of the plant (nominal plant power). Therefore the
system assumes that the nominal plant power is equal to the total power of the auxiliary boilers and
delivers this power according to one of the following diagrams:
A - As far as is possible (low and medium thermal load) it enables the heat pumps and possibly a
fraction of the auxiliary boilers, so that the total enabled power corresponds to the nominal plant
power.
B - At high thermal load, and in particular when the requested temperatures are incompatible with
those permitted by the heat pump, it disables the heat pumps and enables all the auxiliary boilers,
so that the total enabled power is still equal to the nominal plant power.
The system passes from one power delivery layout to the other maximising the use of the heat pumps,
as far as is permitted by the temperatures requested of the plant.
If necessary it is possible to explicitly declare the nominal plant power value, using a special parameter;
this is useful in the event that the total power of the auxiliary boilers is higher than the nominal plant
power.
It is clear that this boiler operating mode requires the use of variable water set points according to the
external temperature (based on the climatic curve), otherwise the system will always work with water
temperatures that are not compatible with the heat pumps
For the description of the setting operations for these parameters, refer to the paragraph 4.4.2.6.2 “Auxiliary
boiler operation” on page 94 and 4.4.2.6.3 Rated plant power on page 95.
Warning generation for premature substitution of the heat pumps
When using the "Complement and substitute" mode it is possible to specify a delivery or return temperature
limit value; the delivery value is used if the delivery is regulated and the return value in the opposite case.
If the passage from layout A to layout B (see previous paragraph) takes place when the temperature
measured on the manifold on which the regulation takes place is less than the specified limit value, the
warning W1027 is generated and memorised in the event history. This is used to underline situations in which
the heat pumps are disabled with lower manifold water temperatures than those nominally set and which lead
to a lower use of the heat pumps; this warning acts for information only, and does not affect the system
functionality. To obtain a correct diagnostic, set an appropriate limit value according to the plant
characteristics.
These parameters are found in the hot service regulation parameters page of the base plant part (see 4.3.1
HEATING SERVICE).
Variant of the Complement and substitution: Progressive complement and substitute
Again in the case of using the "Complement and substitution" mode it is possible to select advanced settings
which substitute the heat pumps with auxiliary boilers progressively rather than in a single moment. When the
operating limit of the heat pumps is reached, corresponding to the passage from the power delivery layout A to
layout B described above, the system attempts to substitute a single heat pump with the auxiliary boilers
(rather than substituting them all as takes place in the "standard complement and substitution" mode);
subsequently it continues to monitor the temperatures; once the limit operating condition of the heat pumps is
reached again, the system substitutes another heat pump with power generated by the auxiliary boilers. This
NOTE
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