[Equest-users] Exhaust Air Tracking

[Chris Baker via Equest-users]

In your example the EF modulates to exhaust a consistent ratio of the Supply Air (with the OSA ratio remaining constant).  This is pretty straightforward to model in equest - However, exhaust fans can sometimes be tricky.

It doesn’t sound like it would be much of an issue in your model but I’ve seen cases where exhaust fans can cause some drama.

Ideally, your supply flow should correspond to the exhaust flow.  In other words, if your SA fans are on a specific schedule, your exhaust fan should be on a corresponding schedule if the intended use is only to activate the exhaust fans when the system is operating.

That way the system can reach the intended equilibrium and reflect the info in the design documents.

If your exhaust fan isn’t on a schedule (or if the Exhaust CFM exceeds the supply air CFM for some weird reason) you can get excessive OSA coming into the zone as the system tries to make up for the exhaust flow going out.  This can throw the fan power usage WAY out of whack as the zone struggles to maintain setpoint.

In winter, if you have heat and reheat coils they can become overwhelmed too – throwing cold into the zone.  The system then calls for heat (and/or Baseboard heat) to meet the setpoint because of all the cold air feeding the zone.

The zone might then overheat, then call for cooling, call for heat, call for cooling, etc.. All in the dead of Winter.

But the excessive fan power usage and unmet hours would be obvious in the final reports after running your simulation and you’d be able to address it.

Some of it is just trial and error.  And I think it’s normal to see problems/issues in the final reports that need addressing.  Those are problems that you might not be able to pin down without running a successful simulation.

Seems like when you think you’re close to being done is when the real fun always begins in eQuest.

Chris Baker
CCI Alliance of Companies
Fort Wainwright, AK

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Darryl Kasun via Equest-users
Sent: Wednesday, August 24, 2016 12:50 PM
To: equest
Subject: [Equest-users] Exhaust Air Tracking

Hi eQUESTers,

I am looking for confirmation that I understand how eQUEST is going to handle zonal exhaust air.

I have several zones that are supplied by an AHU and exhausted by a separate common EF.  The AHU also supplies other zones and air is returned from those zones to the AHU from the zones.

In the zones where air is exhausted directly, the E/A tracks the S/A.

For the zones where there is direct E/A, will eQUEST match the O/A requirements caused by the E/A at the zone level or system level?  From my understanding, I believe it is at the system level but just want to confirm.

For example, let's say an AHU has a min O/A ratio of 0.2 and a zone has a design S/A flow = 100 CFM with direct E/A flow matching S/A flow (E/A = 100 CFM).  For this example, the design O/A from the AHU to Zone 1 will be 20 CFM.
Will eQUEST boost the O/A requirements to Zone 1 to 100 CFM to match the E/A flow or will it bring that 100 CFM O/A requirement back the system level.

Thanks
Darryl

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