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6.13 - Controlling space temperature
The controller uses two loops in tandem to control space
temperature. The purpose of the first loop (called the external
loop) is to calculate the blower temperature required to
maintain space temperature at space setpoint. This blower
temperature, which is called the blower setpoint or forced
air setpoint, is transmitted to a second loop (called the
internal loop). This second loop determines the cooling or
heating capacity needed to maintain blower temperature
at the setpoint transmitted to it by the external loop. This
enables it to actuate the compressors and if necessary the
additional heating stages or a valve on the hot water system.
The precision of the controller depends on the load and the
number of stages available on the unit.
On high inertia systems such as these, tandem control has
the advantage of allowing space temperature to be finely
controlled whilst guaranteeing a good response to disturbing
elements associated with non-linear systems.
NOTE: In some cases Carrier Service may adjust the loop
gains to suit the installed configurations.
6.14
– Capacity limitation
Capacity limitation is generally used by an energy management
system to restrict the amount of electricity consumed by the
unit. The SmartVu control system enables the capacity
of the unit to be limited by means of a user-controlled voltfree
contact (active when closed).
The action of this contact depends on the type of unit and
the configuration (see GENCONF configuration):
•
Heat Pump unit with gas heater: All compressors are
stopped and only gas is used.
•
Heat Pump unit with hot water system: The compressor
heater authorised to operate are limited to the value
configured and the hot water system is used.
•
Electric stages: The electric stages authorised to operate
are limited to the value configured.
•
Compressor stages: The compressor stages authorized to
operate are limited to the value configured.
•
Unit in defrosting mode: The electric stages are authorised
to operate if authorised by the configuration.
6.15
– Capacity control
In heating or cooling mode the capacity control regulates
the compressor start-up and shut-down sequence. The
control determines the compressor start-up in a way that
equalises the start-up number for each compressor (value
weighted by the operating time). This means, the compressor
with the least number of start-ups is always the first to start
up and the first to shut down.
For VSPD unit with dual circuits, when capacity need to be
loaded, one fixed speed compressor shall be firstly turned
ON. Then VSPD compressor shall be added to control
more precisely the capacity. Its capacity can vary between
25% with step of 5%. If capacity is not enough, then another
fixed speed compressor shall start.
Moreover, the VSPD speed will be equal to the circuit A
capacity. It means maximum SIOB output will be vspd_max/10
Volts.
6.16
– Head pressure control
Condensing pressure control is automatically ensured by a
two-speed fan (no adjustment).
6.17
– Defrost function
Defrost is activated, when the unit is in heating mode, in
order to reduce frost build-up on the air heat exchanger.
The defrost cycle can only be applied to one circuit at a
time. During the defrost cycle the fans of that circuit are
stopped, and the four-way refrigerant valve is reversed,
forcing the circuit to cooling mode. The fan can
temporarily be restarted during the defrost cycle. The
defrost cycle is fully automatic and does not require any
setting. A condensate heater prevents ice formation at the
bottom of the heat exchangers, if the defrost cycles are
taking place at low outdoor temperature.
NOTE: The indoor fans run during the defrost cycle except
in single-circuit units without additional heat option
(electric heat stage or hot-water coil).
6.18
– Additional heater
Additional heater can be used on both cooling only and
heat pump unit. There are three types of additional
heaters: electrical heater, hot water coil and gas heater. On
heat pump unit, electrical and hot water coil can only be
used as backup heater, but modulated type gas heater can
be used both as primary or backup heater.
6.19
– Electrical heater
Electrical heater is staged type. Up to 4 staged electrical
heaters can be controlled independently by the unit.
On cooling only unit, electrical heaters may be switched on
when unit need to do heating. On heat pump unit, electrical
heaters may only be switched on when the unit is at 100%
of its available compressor capacity and when outside
temperature is below OAT threshold (default value is 5 °C
with 1°C hysteresis to be switched off). When a circuit is in
defrost mode, electric heaters are authorized to start up
regardless of the compressor capacity.
6.19.1
– Gas heater
Gas heater is modulated type. On cooling only unit, the
modulated type gas heaters will be switched on when unit
need to do heating.
On heatpump unit, modulated type gas heater can be used
as primary heater(default) or backup heater. If used as
primary heater, gas will be the first heating source to
satisfy the load. The compressors can be added after the
gas capacity is 100%. In reverse, compressor will be the
first to satisfy the load and modulated type gas heater may
only be switched on when the unit is at 100% of available
compressor capacity and when outside temperature is
below OAT threshold (default value is 1.6 °C with 1.6°C
hysteresis to be switched off).
For only 190-220 heat pump sizes, gas heater cannot be
used at the same time with the compressors. It will be used
when the outside temperature is below OAT threshold
(default value is 1.6 °C with 1.6°C hysteresis to be switched
off). Above that temperature, the compressors are used
again and the gas heater is halte
6.19.2
– Hot water coil
Hot water coil is modulated type. Units may control the
heating capacity by adjusting a valve on a hot water
system. It is always used as backup heater.
When the unit is operating in heating mode and
