FAQ

Given a VTCV system, if CFM is sized for heating, then the cooling load in the plant form is oversized. At maximum cooling load, the cooling coil valve will be closed some, due to high CFM. The program is sizing the chiller assuming the cooling valve will be full open and the fan full tilt, when this will never happen.

(You can tell whether a system is sized based on heating or cooling loads by looking in the Zone | Sply Air data forms. There you will see the cooling supply air requirement, the heating supply air requirement and the ventilation requirement. The largest of these will set the zone supply air requirement. The zonal supply air requirements for all zones attached to a system are then added together to determine the default supply fan sizing. If the zonal supply air requirements are sized based on heating requirements, then the system is sized for heating.)

The same is true for DD/MZ systems. If CFM is sized for heating, then the cooling load in the plant form is oversized as well. Not all the air flow is going to pass through the cold duct at design conditions.

To size equipment for cooling when the system is sized for heating, do this:

1. In the Plant form, unclick autosizing for zone air, click on autosizing for fans and cooling equipment
2. In the Zone | Sply Air form, zero out the heating CFM requirement, so that the zonal supply air requirement equals the zonal cooling CFM requirement.
3. Now your fans and cooling equipment are autosized using the cooling CFM requirement.
4. In the Plant form, unclick autosizing for fans and cooling equipment. Click on autosizing for zone air again.

If a system is sized for cooling, you can go through the same procedure to size heating equipment.

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