System coil capacities in the baseline building are automatically oversized by the program (25% for heating and 15% for cooling). Equipment is sized using design day data and weather files for the building location. These are discussed in Section 3.2.3 of this manual. The coil capacities shall be based on sizing runs for each orientation in accordance to Section 2.6 of this manual.
Oversizing would be carried out at the zone level, where the sizing parameters would be applied to the zone design coil loads, but not to the supply airflow. The system sizing calculations would sum the zone design airflow rates to obtain a system level airflow rate. The design conditions and the outdoor airflow rate would be used by the simulation program to calculate a design mixed air temperature. The thermostat setpoint plus the design supply air temperatures (SATs) would allow for the calculation of system design heating and cooling capacities. The sizing option would be specified as “Coincident,” which specifies that the central system airflow rate will be sized on the sum of the coincident zone airflow rates. There would be no oversizing factor specified at the system level or the central plant level. Plant capacities shall be based on coincident loads.
For cooling sizing runs, schedules for internal loads including those used for infiltration, occupants, lighting, gas and electricity using equipment shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day. Schedules for infiltration for the cooling sizing runs shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day
For heating sizing runs, schedules for internal loads including those used for occupants, lighting, gas and electricity using equipment shall be equal to the lowest value used in the annual simulation runs. Schedules for infiltration for the heating sizing runs shall be equal to the highest hourly value used in the annual simulation runs and applied to the entire design day.
For the sizing run, thermostat schedules for heating and cooling should be set to the most typical 24 hour profile from the annual simulation run.
Exception: For cooling sizing runs in residential dwelling units, the infiltration, occupants, lighting, gas and electricity using equipment hourly schedule shall be the same as the most used hourly weekday schedule from the annual simulation.
If the automatic oversizing percentage is not sufficient to meet demands, then UMLH are evaluated at the building level by looking at the UMLH for each thermal zone being modeled. The first step would be to determine if the UMLH are high (>300) for the proposed building design as well as the baseline building. If that is the case, the issue is usually related to fan operation, HVAC availability, and occupancy schedules where the HVAC system has an incorrectly specified schedule that makes it unavailable during occupied hours. Optimal start controls, if a part of the building design, can also help eliminate UMLH during startup times. Since the same schedules are used for the baseline design, UMLH are seen in the baseline building as well. Other user inputs that could cause UMLH include incorrectly specified zone minimum airflows, which could result in unmet heating load hours. In this case, the software should notify the user and ask the user to verify schedules of operation. If a space is being conditioned via transfer air, it might be that the temperature of the transfer air is not sufficient to meet space conditioning requirements.
- If this is not the case and UMLH are seen only with the baseline design, the user or software tool is required to incrementally increase system airflows and equipment capacities, following the steps outlined below.
a. In the case where UMLH for cooling are a bigger problem, the equipment in the baseline building model is resized by first increasing the design airflow of all zones with significant UMLH (greater than 150 for an individual zone) by 10%, increasing the design airflow of all zones with some UMLH (between 50 and 150) by 5%. Then, the equipment capacity for the system(s) serving the affected zones is increased to handle the increased zone loads. For the central plant, the chiller(s) and towers are resized proportionally to handle the increased system loads.
b. In the case where UMLH for heating are a bigger problem, the same procedure is followed, with zone airflows resized first, then heating secondary equipment capacity and then boiler capacity as necessary. The capacity of the boiler or furnace shall be increased in proportion to capacities of coils required to meet the increased airflows at the baseline supply air. For heat pumps, the capacity of the coil is increased so that the additional load is not met by auxiliary heat.