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MarketManager » Loads and Sizing
Q: When I reduce lighting loads, the cooling system ventilation load increases, and the total cooling loads remain unchanged. Why?


For each zone, in the Zone | Sply Air sub form, MarketManager determines the cooling CFM required to bring the zone to setpoint. MarketManager also determines the heating CFM required to bring the zone to setpoint. The maximum of the cooling CFM requirements, the heating CFM requirements, and the Minimum Ventilation is used as the default zone supply air CFM. This means that if the heating CFM requirement is larger than the cooling CFM requirement and the Minimum Ventilation, the heating CFM requirement will be used as the default zone supply air CFM.

Assuming fans were sized to meet total zone CFM requirements (i.e. autosized), during cooling season, the fans will be oversized, while during heating season, they will be correctly sized.

If the cooling system ventilation loads increase when you reduce lighting W/sq-ft, then most likely you have a project in which the heating loads determine the supply air CFM to the zone.

When you decrease lighting W/sq-ft, the zone requirement of cooling supply air decreases due to smaller cooling load (see Zone|Sply Air Sub-Data Form) This decreased amount is still below the heating CFM requirement, therefore the heating CFM requirement is still used to size the zone CFM which does not change. (Remember, lights do not affect design heating load calculations.)

As a result, during cooling season, the fans are even more oversized than they were with the less efficient lighting scheme. And as a result, the cooling ventilation load will increase. You can see this on the System|Loads form under either "System Ventilation" or "Oversizing", depending upon the version of MarketManager you have.

If the design cooling load is greater than the heating load, then you will see a reduction in total system load when lowering the lighting density.

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