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5.8 - Controlling room temperature
The controller uses two loops in tandem to control room
temperature. The purpose of the first loop (called the external
loop) is to calculate the blower temperature required to maintain
room temperature at room setpoint ± half the active dead band
(see section 5.5). 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 tempera-
ture at the setpoint transmitted to it by the external loop. This
enables it to actuate the compressors and if necessary the addi-
tional 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 room temperature to be finely con-
trolled 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.
5.9 - Capacity limitation
Capacity limitation is generally used by an energy management
system to restrict the amount of electricity consumed by the
unit. The PRO-DIALOG Plus control system enables the
capacity of the unit to be limited by means of a user-controlled
volt-free contact (active when closed).
The action of this contact depends on the type of unit and the
configuration (see user configuration, section 4.3.11.3):
•
Reversible unit with gas stages: all compressors are stopped
and only gas is used.
•
Reversible unit with hot water system: the compressor
stages authorised to operate are limited to the value con-
figured 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 authorised to
operate are limited to the value configured.
•
Unit in defrosting mode: the electric stages are authorised
to operate if authorised by the configuration.
5.10 - Night mode
The night period is defined (see User configuration) by a start
time and an end time that are the same for each day of the week.
During the night period, the fan runs at low speed, if permitted by
the current operating conditions. In addition, the user can
reduce the unit capacity.
5.11 - 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.
26
5.12 - Head pressure control
Condensing pressure control is automatically ensured by a two-
speed fan (no adjustment).
5.13 - 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).
5.14 - Electric or gas stages
In heating or dehumidification mode, the unit may control up to
three electric or gas stages.
5.14.1 - Electric stages
On reversible units, electric stages may only be switched on
when the unit is at 100% of available compressor capacity.
When defrosting a circuit is in progress, electric stages are
authorised to start up regardless of the compressor capacity at
the time.
5.14.2 - Gas stages
On reversible units in heating mode, gas stages cannot be used
at the same time as the compressors. Gas will be used when the
outdoor temperature is below 1.6°C (this gas utilisation thres-
hold can be changed by Carrier Service). Above that temperature,
the compressors are used again and the gas is halted (hysteresis
1.6°C).
5.15 - Controlling a hot water system valve
Units may control a valve on a hot water system. When the unit
is operating in heating mode and compressor capacity is 100%,
the valve is continually adjusted to maintain the blower tempe-
rature calculated by the external loop (see section 5.8). In cooling
mode this valve is kept fully open (completely bypassing the
hot water system). If the unit is reversible, the valve is kept
closed during defrosting sequences (hot water system feed).
5.16 - Economiser
The unit may control an economiser which is activated to
enable fresh air to be fed in from outside when conditions
permit (this is called free cooling). As in the case of controlling
gas or electric stages, the controller uses two loops in tandem to
control room temperature with the aid of the economiser. The
external loop calculates the supply air temperature required to
maintain room temperature at room control point. The internal
loop determines the economiser position needed to maintain
this supply air temperature.
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