1.2.1 Unmet Load Hours tests
Unmet load hours are any hours of operation when conditioned spaces are outside the throttling range for heating or cooling controls. While this test is performed automatically in the 90.1 Performance Rating Method (PRM) Navigator for the associated reports, this can readily be done as a manual check within Vista Results. To do so, complete all of the following steps (an example is provided below):
1. In Vista Results, select the conditioned spaces in the model from the Room Browser tree at left.
2. Select the Air temperature results variable.
3. Open the Range Test tool.
4. Date/Time: check the tick box below the list of week days and select When conditioned (incl. setback) from the drop-down menu next to this tick box.
5. Test: Between room setpoints (+/- differential tolerances)
6. Under Test temperatures in controlled band, tolerances in °F (or, for metric users, … in °C ), enter 2 for both heating and cooling if working in IP units and 1.11 for both if working in metric units (these settings will apply for most user most of the time; see further notes below).
7. Check the tick box for Averaged, shared hours (for ‘unmet hours’ test).
8. Click Apply.
The range test below shows results for a system autosized perfectly to meet all loads for the simulation period of eight days in January (this test was aimed at confirming heating performance). This outcome may actually be less than ideal if meeting all loads under the most extreme conditions causes the system to be significantly oversized relative to more typical conditions. And, over a full year, even autosized systems will normally have some unmet heating or cooling hours as a result of varying conditions and related system dynamics or differences between the design sizing conditions vs. the simulation weather file. The results of an unmet loads hours test should, however, generally appear as in this example.
Figure 9 - 10 : Unmet Load Hours test performed using the Range Test tool in Vista Results
Figure 9 - 11 : Heating setpoint profile (purple), Cooling setpoint profile (blue), zone air temperature (green), and plant profile (red) for a selected space in the model.
As can be seen in Figure 9-11: Heating setpoint profile (purple), Cooling setpoint profile (blue), zone air temperature (green), and plant profile (red) for a selected space in the model. above, the profiles set in the Space Data dialog for a particular space (either via a Thermal Template, via System Schedules dialog, or manually) for heating and cooling setpoints are recorded at the time of simulation and can readily be placed on graph along with the zone/room air temperature. The profiles show the setpoints for occupied hours and setback for unoccupied hours.
The right-hand graph shows the heating setpoint and room temperature once again with the Plant profile (red). The plant profile toggles between 0 and 1 to indicate the times during which the normal daytime setpoint should be fully met (future versions of the VE may use this profile to provide more detailed information regarding system status relative to setpoints, night-cycle operation, and so forth).
It is important to keep in mind that the heating and cooling profiles show the setpoint for occupied hours as a target for the morning start-up and after-hours operating periods. Thus you may see the room temperature lagging behind the setpoint profile, particularly in the early morning hours. The definitions below describe how unmet load hour tests use nighttime setback values while the modeled spaces are transitioning between nighttime setback and daytime setpoint. This avoids over-counting unmet hours.
Note that in the illustrative example on the preceding page there are some spaces in the model for which the hours in all three columns are zero. These spaces are plenums and an unconditioned vestibule. While they still have profiles assigned to them in Space Data (via System Schedules or manually) for timed heating and cooling setpoints and setback, they have their heating and cooling on/off profiles (on/off schedules) in Space Data set either individually or via thermal templates to “off continuously.” This is the essential means of indicating that a space in unconditioned with respect to unmet load hours tests.
When the VE detects heating and cooling on/off profiles set to “off continuously” and thus determines that a particular room is fully unconditioned, a nominal unconditioned values range of 20 ° C +/-80 ° C (68 ° F +/-144 ° F) is applied. This equates to an unconditioned heating value of -76 ° F (-60 ° C) just shy of the -80 ° F lowest external temperature ever recorded in the US, and an unconditioned cooling value of 212 ° F (100 ° C)—the boiling point of water. These values are recorded at the time of simulation as continuous setpoints for any fully unconditioned space.
