The default generic backup cooling and heating sources simply allow for a simulation model that always addresses the full load at the coil. Therefore, when the VRF equipment is properly sized to meet cooling and heating loads, these generic backup cooling and heating sources should have zero or insignificant operating time during an annual simulation.
This virtual backup heating source is grayed out and does not apply when the ‘Electric resistance backup heat for indoor units’ (above) is selected, as this actual back up will be modeled as addressing any heating loads that the indoor unit VRF coil is incapable of meeting.
The efficiency setting is provided so that users may set a value here that will reasonably approximate VRF operation if the backup does come into play (i.e., if the autosized or manually sized system does not quite cover all loads all the time).
A meter selection is provided to facilitate separate tracking of energy used for this virtual backup. Again, if the VRF system is properly sized, the energy used for this backup heating source should be either zero or insignificant.
Figure 3-138: Design parameters tab of the VRF system editing dialog, shown for a water-source VRF system with Design condition – outdoor unit sub-tab selected.
