The VRF outdoor unit is scheduled to operate in heating-only, cooling-only, or heat-recovery modes, based upon the profile value of the absolute profile specified for the VRF outdoor unit control profile. The heat-recovery mode is the only mode in which simultaneous heating and cooling are permitted for a given VRF system (defined as a set of indoor units coupled to a common outdoor unit). The user must define the absolute profile according to the desired schedule or parameters that will determine the mode. A formula profile can be used to determine the operating mode based upon any variable, such as outdoor temperature, a combination of variables, such as an outdoor vs. indoor temperature differential, or mathematical operators and function available for use in formula profiles.
Profile values correspond to the following operation modes:
· Profile value of 0: Force the system off. Both the VRF outdoor unit and all connected VRF indoor units are off. Backup heating and cooling sources, if included, will also be turned off.
· Profile value of 1: Heat pump heating mode. All cooling coils will be off with no backup cooling.
· Profile value of 2: Heat pump cooling mode. All heating coils will be off with no backup heating.
· Profile value of 3: Heat recovery mode. The outdoor unit will operate in Load priority mode with simultaneous heating and cooling permitted by indoor units on the system and backup heating and cooling sources fully available.
As an example, a typical VRF outdoor unit having no capability for heat recovery might operate in cooling mode when the outside air temperature is 50°F or warmer, and in heating mode when the outside air temperature is cooler than 50°F. Assuming the outdoor unit is never completely off, the formula below is an example of this operation:
0.01*to+1
In this example formula profile, the ‘to’ variable is Temperature Outdoor. This example formula returns a value of 1.49999 or less that will round to 1 when the outside air temperature is cooler than 50°F, putting the VRF outdoor unit in heat-pump heating mode. This profile returns a value of 1.50 or greater that will round to 2 when the outside air temperature is 50°F or warmer, putting the VRF outdoor unit in heat-pump cooling mode.
Note: The back-up heating and cooling sources referred to above are those described below in sections 3.15.2.12--14. Of those, the optional ‘Electric resistance backup heat for indoor units’ is the only one intended to model actual installed equipment.
The default backup heating and cooling sources are for the purpose of allowing the VRF system model to run if it has not been correctly sized. When sized correctly (e.g., using the ApacheHVAC system-level autosizing feature), the operation of these default backup sources should be either zero or negligible.
