Coil sizing problems - Loads data spreadsheet
From my hand calculations it seems the coil values calculated in the loads data spreadsheet are not as we would expect. Is this an issue with the software?
The Loads Data spreadsheet is not used for sizing coils.
The numbers written to the columns AQ, AR, AS, and AT in the VE ASHRAE Loads Results Data tab are just that: Results. They are not calculated by the spreadsheet.
The sizing of coils in ApacheHVAC is not a simple calculation comparable to a hand calc---which is essentially what's in the spreadsheet. Rather, ALL coils are sized in the SYSTEM-LEVEL SIZING run, along with water loops, boilers, chillers, and other plant equipment. The system-level sizing of coils is a function of the actual load seen by the coils during the system-level sizing run, which operates the system to condition the spaces in the model for the selected design days.
During the system-level sizing run, a VAV reheat coil, for example, will be sized according to the following:
- Heat the volume of air provided by the VAV box sufficiently to achieve the zone temperature setpoint, given the control sequence.
- The coil will be sized with sufficient capacity to heat whatever air flow rate is provided by the VAV box for that zone under the chosen design conditions. The default control sequence for heating in a VAV system is to maximize the reheat coil leaving air temperature (LAT) first while the VAV box is at its minimum flow rate. Only if/when the zone begins to drop below the room temperature setpoint will the VAV box open further, thus forcing the coil to heat a greater volume of air to the maximum LAT. Given this, the room temperature will typically be about 1 F below the setpoint when the design load is “met” (this is what’s referred to as “throttling range” as controls need some range of variation in room temperature variation over which to operate).
- The heating coil capacity will also be sized sufficient to address whatever temperature air is entering the coil at when the system is operating under the chosen design conditions. If the controls include a supply air temperature reset for the AHU cooling coil, which is included in the default control sequence for pre-defined VAV systems, then the AHU will be supplying the warmest air permitted by the user setting for the Cooling coil SAT reset (e.g., 60 to 70°F rather than 55°F).
If the user has entered a relatively high SAT reset value (typical for small or otherwise envelope-load-dominated buildings) and there is heat recovery on the AHU, which is an option on nearly all pre-defined HVAC systems, the supply air temperature may vary considerably without any active heating at the AHU. Similarly, if the user has included ductwork surface area in the duct heat gain/loss components for the HVAC system, the actual temperature entering the reheat coil may be warmer or cooler than the specified SAT leaving the air handler. Finally, for systems, such as UFAD systems with RA bypass, that add recirculated air downstream of the cooling coil, the particular ratio of outside air to recirculated air on the system may provide a mixed air from the AHU that is also warmer than the maximum cooling coil LAT.
The two ways in which the Loads Data spreadsheet contributes indirectly to the required coil capacities (but not the actual sizing process) are:
- The spreadsheet calculates the design airflow for each zone (with or without the application of optional oversizing), based upon zone loads determined by the ASHRAE Loads run. The airflows are then populated in the controllers, where they can also be edited.
- The spreadsheet is source of the default values of and a stepping stone for coil leaving temperature settings entered in the System Parameters dialog, prior to assigning these values to the system controllers. The values can also be edited either in the spreadsheet before assigning them or in the controllers themselves.
While rare, it is possible that for some system types in some applications, the default relationships and calculations for airflow sizing in the Loads Data spreadsheet may be less than ideal. It is for this reason that the spreadsheet remains fully editable. Furthermore, IES is very much open to feedback on these things.
Also, there are inherent limitations of the ASHRAE Loads approach to sizing heating systems. As required by ASHRAE, all heating equipment is sized to satisfy a static peak load. In other words, ASHRAE requires that we omit the effects transient events, such as the switch from nighttime setback to daytime temperature settings, when sizing heating equipment (both at the zone level and at the system level). This is intended to avoid excessive oversizing of equipment, and also depends upon the assumption that a typical oversizing factor (e.g., 1.25) and start-up period (e.g., 1.5 hours before morning occupancy) will be adequate to address transients. While this works well, and avoids excessive oversizing, in most applications, there are cases for which this will cause the heating airflows and/or coils for certain zones to be undersized, resulting in unmet load hours—typically in the first few hours of the day.
Example: IES recently saw this in a model that had inadequate heating for a large ground-floor corridor at its core. This corridor had exceptional small internal gains (minimal lighting, few occupants, and no other gains). As it was surrounded by other conditioned zones, it also had very low steady-state heating loads, and thus the ASHRAE Energy Balance method and sizing procedures resulted in a very small heating airflow and coil capacity for this zone. It did, however, have a large floor area of concrete slab—i.e., a large thermal mass. On cold winter nights in the northern climate the building drifted down to nighttime setback temperature fairly quickly, and by morning the slab in the corridor floor had fully cooled down to this temperature. When the thermostat was set back up in the morning, it often took on the order of four hours for the small heating capacity provided to the corridor zone to overcome the cooling effect of the chilled mass of the floor slab.
IES is considering providing optional (additional) dynamic sizing runs for heating equipment that would include transient loads, and will do so at some point in the future. Until this is implemented, it is up to users to check unmet load hours and adjust sizing for zones, such as that in the example above, that do not lend themselves to the ASHRAE approach to sizing.
It’s worth noting also that a typical hand calc also will not account for the dynamic effects of thermal mass and control transients
