Inaccuracies Resulting From Using a Large Room/Zone
In the building above, five separate zones are shown. Inside of the zones are thermally similar rooms. The loads (positive loads indicate heat gains, negative loads indicate heat losses) for the zones during a typical winter morning are as follows:
Internal LoadsEnvelope Loads:Space Load:
Office - North: 10,000 Btu/h-16,000 Btu/h-6,000 Btu/h
Office - East: 6,000 Btu/h -8,000 Btu/h-2,000 Btu/h
Office - South: 10,000 Btu/h-12,000 Btu/h-2,000 Btu/h
Office - West: 6,000 Btu/h-10,000 Btu/h-4,000 Btu/h
Conference Room: 16,000 Btu/h 0 Btu/h 16,000 Btu/h
Total: 48,000 Btu/h -46,000 Btu/h 2,000 Btu/h
If these five zones are modeled separately, then all of the offices will be in heating and the conference room will be in cooling. However, if these five zones are modeled as one large room, then 2,000 Btu/h of cooling and no heating are required. In the case of the one room model, the heating and cooling energy consumption would both be underestimated.
The above situation does not apply because the cooling and heating calculations are done separately and internal loads are typically neglected for heating design. However, large rooms/zones can still cause problems.
For constant volume systems modeled using one large room, a single time of peak is assumed. In a constant volume system a sum of the peaks methodology must be used to properly size the equipment. If one of the spaces has a different time of peak than the others, the system will be undersized if a one room/zone model is used. In the example shown above, the Office – East would peak in the morning and the Office – West would peak in the afternoon. If a single time of peak is assumed, at least one of the rooms required airflow and the size of the system serving it would be undersized. Assuming one large room can cause an underestimation of the capacity of the system by as much as 30-40%.
For variable volume systems modeled using one large room, a single time of peak is also assumed. In a variable volume system a block methodology must be used to properly size the system. Assuming one large room will allow the program to calculate a reasonable estimate for the size of the system, provided that all of the spaces are occupied and operated similarly. However, determining precise airflows to the individual rooms/zones is not possible when a single room is used.
Note: For more information on the sum of the peaks and block sizing methodologies see the document “Peak vs Block.ppt”.