[Equest-users] Excessive PVAV cooling in proposed

[Chris Baker]

How I resolved this was I followed direction of a couple really helpful folks on the boards that contacted me via email.

I first set all my cooling controls to “warmest” with 70 F max reset.

This cut space cooling but the main cause of excessive cooling in winter was my kitchen zone.
The kitchen range hood was exhausting 2500 CFM on a schedule (not operating on weekdays and only 4-5 hours on weekends and holidays).

But the OSA was set to 900 CFM – Plus, Supply air was set to 2600 CFM with no min flow schedule.
So it was basically a 2600 cfm design flow for 330 square foot kitchen.

To address this I just set a min flow schedule (frac/design sched)  that corresponded with the kitchen hood schedule.

Anytime the hood is on, the makeup air is 2600 CFM (or 1.0 frac/design)
Anytime the hood is off, the supply air is about 154 CFM or so. (or .059 frac/design)

I updated both models to show the same exhaust flow, schedule, and I modified the OSA in the space to correspond with this schedule (ensuring OSA was the same in both models).

So the cause wasn’t really excessive cooling it was excessive heating in winter with this insane 2600 cfm design flow – causing the space to intermittently call for cooling, then heating, etc…

These updates cut my heating energy by 400 MBTU and my electric usage for space cooling went down by 4,000 KWH or so!

Awesome!

-ChriS

From: Jeff Hirsch [mailto:James.J.Hirsch at gmail.com]
Sent: Sunday, June 28, 2015 2:07 PM
To: Chris Baker
Subject: Re: [Equest-users] Excessive PVAV cooling in proposed

PSZ is constant volume variable temperature (song;le unit per zone) whereas PVAV is constant temperature variable volume (central AHU with zone boxes or dampers) .  PSZ is intended as single zone with the supply set by that zone demand for either heat or cool. So it can minimize heat/cool but the constant volume uses more fan energy. PVAVS has the central AHU with a discharge supply temperature then a vav box in each zone controlled by the zone thermostat. But the box has a min flow (default is set by the OA requirement for the zone, thus a min supply from the AHU which is controlled by cool-control which defaults to constant and uses cool-set-t as the setpoint.

You can optimize the VAV by allowing the AHU to reset the supply temperature to match the cooling demand, but if there is varying demand in the zones (some requiring heat while other require cool) you either get some unsatisfied cooling or some overcooling. Using a warmest control on the AHU sets the supply to satisfy the zone with the highest cooling demand, but that may be over cooling for other. You can use a reset but then there is the risk of both under and over cooling. You can use a combined single zone variable volume and variable temperature system (PVVT) and that will try to sequence both supply temperature and volume and putting one in each zone (like a PSZ) gets the best result.

You need to read about the control modes and ask the designer what system type (how many, sizes and placements in zoning) and control they will spec ... then model that.

________________________________
Jeff Hirsch
James J. Hirsch & Associates
12185 Presilla Road -  Camarillo, CA 93012-9243 USA
phone: (805) 553-9000 mobile: (805) 532-1045 fax: (805) 532-2401
email: James.J.Hirsch at gmail.com<mailto:James.J.Hirsch at gmail.com>

From: Chris Baker<mailto:CBaker at cci-alliance.com>
Sent: Sunday, June 28, 2015 2:44 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Excessive PVAV cooling in proposed

My baseline system is PSZ-AC (type 3) and my proposed system is PVAV with economizer cooling.

There is no actual cooling system installed in the building. Because of this, I modeled the proposed the same as baseline (with DX coils) – per Appendix G.

I’ve been told that the reason my proposed model is showing four times as much cooling as the baseline is likely because of the setpoints for cooling.

My setpoints for cooling are 76 (occupied) 82 (unoccupied).
That’s how the setpoints were designed for the economizer cooling, basically – which is why I set these for cooling.

But that doesn’t account for a DX coil system (which is how the proposed has to be modeled).

I’ve been told that this is a typical issue when modeling cooling in a PVAV system, specifically.  Especially when modeling preheat/reheat coils with a cooling setpoint that is too low.

As you can see in the report, the cooling loads in the baseline seemed to work out OK - No cooling in winter!  That’s what we want.

The proposed is not only showing excessive cooling use in summer but cooling in winter months also.

I will obviously experiment with slightly higher setpoints and look at the results.  I’ve been told sometimes people set these as high as 95 to avoid showing excessive cooling (even cooling showing up in winter months).

Has anyone else experienced this in equest?

I can post my models if it would help.  But I think the reports summarize this issue.

Chris Baker
CCI CAD Drafter


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