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Why Return fan CFM ≠ Supply fan CFM - Exhaust fan CFM?

asked 2020-05-21 07:58:53 -0500

TheEnergyTutor gravatar image

updated 2020-05-21 12:07:24 -0500

When Open Studio auto-sizes my air handling unit (which has a supply and return fan), it selects the same CFM for each, even though i have a coupled Exhaust fan in the zone. For example, if my supply fan was 6000 CFM and i have a 1000 CFM bathroom exhaust fan, the return fan should only be sized for 5000 CFM, but it thinks it should be 6000 CFM. Why is this? In addition, the economizer should not be balancing the intake and exhaust air from the AHU, it should also reflect this +1000 intake air to keep the building neutral. (i.e. 1000 CFM minimum OA while the exhaust is 0 CFM at the AHU mixing box) Does anyone have advice?

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answered 2020-05-21 11:58:58 -0500

EnergyPlus requires a single loop (obj AirLoopHVAC / PlantLoop / CondenserLoop) to be balanced: if you supply 6000 CFM you also need to return 6000 CFM. There is no other possibility.

For Exhaust fan look at "System Availability Manager Coupling Mode" and "Balanced Exhaust Fraction Schedule Name" but be aware that this obj takes only into account the Power/Energy of the fan, not its thermal effect on the zone air thermal balance

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If i could insert a duct branch, i could include both the exhaust and the return fans in the Air Loop. It's unfortunate because the exhaust requirements often override the ventilation requirements and this needs to be accounted for. Most HVAC projects historically ( i work with retrofits) have exhaust fans that make the return to the AHU less than the supply, except for DOAS systems which are more common in new construction. .

TheEnergyTutor gravatar imageTheEnergyTutor ( 2020-05-21 12:33:09 -0500 )edit

Yes, it is very common that HVAC systems has supply air flow> return air, especially to avoid unwanted infiltration. But as far I worked with E +, I always had to compromise on this aspect.

Ag gravatar imageAg ( 2020-05-22 15:55:40 -0500 )edit

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Asked: 2020-05-21 07:58:53 -0500

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Last updated: May 21