Excessive PVAV cooling in proposed

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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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Chris Baker2's picture
Joined: 2014-03-25
Reputation: 0

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!



Chris Baker2's picture
Joined: 2014-03-25
Reputation: 0