Carrier 30RY/RYH Demand limit, Night mode Reset example in cooling mode, Capacity control

5.6.2 - Reset

Reset means that the active setpoint is modified so that less machine capacity is required (in cooling mode, the setpoint is increased, in heating mode it is decreased). This modification is in general a reaction to a drop in the load. For the Pro-Dialog control system, the source of the reset can be configured in the User 1 configuration: it can be provided either by the outdoor temperature (that gives a measure of the load trends for the building) or by the return water temperature (delta T that gives an average building load). In response to a drop in the outdoor temperature or to a drop in delta T, the cooling setpoint is normally reset upwards in order to optimise unit performance:

In both cases the reset parameters, i.e. slope, source and maximum value, are configurable in the Setpoints menu (see section 4.3.8). Reset is a linear function based on three parameters.

•A reference at which reset is zero (outdoor temperature or delta T - no reset value).

•A reference at which reset is maximum (outdoor temperature or delta T - full reset value).

•The maximum reset value.

5.7 - Demand limit

Generally, demand limit is used by an energy management system to restrict the unit electricity consumption. The PRO- DIALOG control system enables the capacity of the unit to be limited by means of user-controlled volt-free contacts. Single- circuit units only have one contact (control contact 3), available in the User Configuration function for demand limiting or setpoint selection. Dual-circuit units have two volt-free contacts that permit several limit levels. The capacity of the unit cannot exceed the demand limit setpoint activated by the position of the contacts (see section 3.6.4 and 3.6.5 for the contact description). The demand limit setpoints are adjustable via the setpoint menu.

The demand limit is active in all operating types: Local, Remote or CCN. However in CCN operating type, demand limit can be controlled directly with the aid of CCN commands.

A limitation value of 100% means that the unit may call upon the full array of its capacity stages.

5.8 - Night mode

Reset example in cooling mode

Reset value	% Building load
	Outdoor temperature (OAT)
	Evaporator Delta T

Legend

AMaximum reset value

BOAT or delta T for no reset

COAT or delta T for full reset

DBuilding Load

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 (but a minimum capacity value can be configured).

5.9 - Capacity control

This function adjusts the number of active compressors to keep the heat exchanger water temperature at its setpoint. The precision with which this is achieved depends on the capacity of the water loop, the flow rate, the load, and the number of stages available on the unit. The control system continuously takes account of the temperature error with respect to the setpoint, as well as the rate of change in this error and the difference between entering and leaving water temperatures, in order to determine the optimum moment at which to add or withdraw a capacity stage. If the same compressor undergoes too many starts (per hour) or runs below one minute each time it is started this automatically brings about reduction of compressor starts, which makes leaving water temperature control less precise. In addition, the high pressure, low pressure or defrost unloading functions can also affect the temperature control accuracy. Compressors are started and stopped in a sequence designed to equalise the number of start-ups (value weighted by their operating time).

NOTE: Circuits with 3 compressors (reversible heat pump units) only provide 2 capacity stages (2 compressors are started/stopped together).

5.10 - Head pressure control

Condensing pressure control is automatically ensured by a two- speed fan and can also be ensured by an additional fan on each circuit (no adjustment).

Fanstart-up:

ATTENTION: In accordance with the operating conditions the fans can be cleaned periodically. A fan can start at any time, even if the unit has been shut down.

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