Air side HVAC

3 posts / 0 new
Last post

Sambhav:

If you want Exhaust CFM Fan energy, then?I believe you need to specify Exhaust CFM.

OK, eQuesters,?I think I have this straight. If you specify an Exhaust CFM LESS THAN OSA CFM, nothing airwise changes, and Exhaust CFM translates to an Exhaust?fan HP. If Exaust CFM EXCEEDS OSA CFM, the OSA CFM is increased to account for no negative airflow. And Exhaust CFM STILL creates and Exhaust HP.

Comments from our studio audience, please..

John Aulbach's picture
Offline
Joined: 2011-09-30
Reputation: 1

John and Sambhav,
You are absolutely correct. If you specify exhaust CFM that exceeds OSA cfm, the OSA cfm is indeed increased to prevent negative pressurization of the zone. This is true if the Exhaust Air Source is specified as "Air Handler" (see screenshot). The other options are "Infiltration" and "Balanced Infiltration". If either of these latter two options are selected, the OSA cfm is not increased, but the exhaust air is made up through outside air infiltration into the zone (note: this is different than zonal infiltration, or space-to-space air transfer). I personally have been unsuccessful using either the "Infiltration", or "Balanced Infiltration" options because my unmet load hours tend to skyrocket when either option is selected. Again, my interpretation is that eQuest is making up the exhaust air with unconditioned outside air, which then dramatically impacts the temperature of the space. If anyone has had any success using these other methods, or some work-around to simulate space-to-space air transfer, I would be very interested in hearing about it.
As John has already stated, with any of the three options, eQuest will account for fan power based on your input KW/cfm or a combination of Static Pressure and Efficiency.

Seun Odukomaiya, LEED AP

Seun Odukomaiya2's picture
Joined: 2011-09-30
Reputation: 0

The other thing to consider is that DOE-2 forces an air balance in every
zone. If the zonal (local) exhaust is less that the OA delivered then the
balance is exhausted at the central system exhaust.

In terms of heat recovery, only air exhausted at the central unit is
available for heat recovery. By specifying zonal exhaust, that exhaust
flow rate is not passed through the heat recovery device (if specified).

bfountain's picture
Offline
Joined: 2011-09-30
Reputation: 201