[Equest-users] MIN-AIR-SCH and Economizer Implementation

Thu Aug 30 08:17:00 PDT 2018

Hi Daric,
Nick Caton and I had a discussion on this on 6-Sept-2017 (if you care to check the list archive). The primary takeaway was this realization:

-        I think a value of 0.001 in practice means "zero OA will be delivered, unless determined beneficial for economizer purposes.  There is no minimum OA requirement for this hour." ~Nick Caton
I haven't tested this lately but I believe it is correct. Using 0.001 in the MIN-AIR-SCH sets the hourly minimum outside air requirement to zero.
You can test this in your test model setup by using a MIN-AIR-SCH with values of -999 and looking at the custom hourly report you created with supply air flow and OA ratio to identify hours when the OA% is higher than the minimum requirement. This will confirm that the economizer is occasionally activated. I would skip the Miami model (as economizer is rarely beneficial) and potentially modify a test model to increase the likelihood that an economizer would be helpful. Perhaps use a low non-zero minimum OA requirement and an undersized air handler and see what that does to the custom hourly report numbers.
FYI your test iterations #2 and #3 below are described identically.

Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Daric Adair via Equest-users
Sent: Wednesday, August 29, 2018 4:24 PM
To: equest-users at onebuilding.org
Subject: [Equest-users] MIN-AIR-SCH and Economizer Implementation

eQuesters;

Need to learn from other's experience on proper implementation of the MIN-AIR-SCH and Air Side Economizers within eQuest. The help file language (and what I have previously followed is this):
"...Values in the MIN-AIR-SCH vary from 0.0 (no outside air flow; economizer inactive if specified) to 1.0 (100% outside air flow). A value of 0.001 actives the economizer. A value of -999 returns the hourly value to MIN-OUTSIDE-AIR or to the program's calculated minimum outside air value ..."

Based on my reading of the help file, if 0.001 'activate' the economizer, my MIN-AIR-SCH should be 0.001 for all 24 hours/day because I want the economizer to always be available. The issue is that on running with this approach on a model, and reviewing the hourly report values for the Supply Air Flow and the OA ratio - my OA was ~zero in the summer for all hours. While this shows large energy savings - it's due to lack of OA, not economization. Testing with a 0/-999 MinOA schedule yielded more realistic OA ratios, but obviously no economizer saving that could be seen.

I stepped back and started to play with eQuest Wizard to see how it configures economizer and MIN-AIR-SCH it creates. Interesting combinations/results:

All iterations are with a CHW/HW VAV System.

1.      HVAC Wizard: Screen 6 > Economizer Type = Dry Bulb and Screen 5 > Cycle Fans at Night = Cycle Fans (no OA at night). eQuest Detailed Mode yields a Dry Bulb Economizer and a MIN-AIR-SCH of 0=unoccupied and -999=occupied.

2.      HVAC Wizard: Screen 6 > Economizer Type = Dry Bulb and Screen 5 > Cycle Fans at Night = Cycle Fans (no OA at night + economizer). eQuest Detailed Mode yields a Dry Bulb Economizer and a MIN-AIR-SCH of 0.001=unoccupied and -999=occupied.

3.      HVAC Wizard: Screen 6 > Economizer Type = Dry Bulb and Screen 5 > Cycle Fans at Night = Cycle Fans (no OA at night + economizer). eQuest Detailed Mode yields a Dry Bulb Economizer and a no MIN-AIR-SCH at all(?)

4.      HVAC Wizard: Screen 6 > Economizer Type = none and Screen 5 > Cycle Fans at Night = Cycle Fans (no OA at night). eQuest Detailed Mode yields No Economizer and a MIN-AIR-SCH of 0=unoccupied and -999=occupied.

5.      HVAC Wizard: Screen 6 > Economizer Type = none and Screen 5 > Cycle Fans at Night = Cycle Fans (no OA at night + economizer). eQuest Detailed Mode yields a No Economizer and a MIN-AIR-SCH of 0.001=unoccupied and -999=occupied.

>From this testing (and other combinations) it would appear that the 'fan at night' input drives what appears in the MIN-AIR-SCH (or the existence thereof) as opposed to anything to do with Economizer. Though, iteration #5 shows a schedule with an economizer/0.001, but no economizer at the system (fixed fraction).

Quick runs of the above, the options with economizer did use less cooling than the non-economizer options (As expected) but the results were underwhelming. Tested in a 2A/Miami, 4A Kansas City, and Boise/5B. All underwhelming.

So - what is right? Is the 'right' approach for economizer implementation to have economizer enable at the unit with a MIN-AIR-SCH of 0.001=unoccupied and -999=occupied. OR does the 0.001/-999 schedule mess matter if the economizer is enabled at the unit? From my 'real' model a schedule of 0.001 appears wrong.

Also -'right' in this case is how one would expect this to perform on an actual piece of equipment; not looking to fake-bake any numbers for code/LEED/other.  Field economizer operation is relatively simple - Is OA available to help free cool my building (fully or partly/integrated) or should the system be at Minimum OA?

Note: None of the situations above involve DCV. DCV didn't appear to be something which can be implemented in the Wizard and I've always done it in Detailed mode anyways. Let me know if I've missed this in Wizard. But, more importantly, if DCV impacts your thoughts on the MIN-AIR-SCH  and economizer implementation, I'd be interested in that as well.

Thanks in advance.

Thanks,
DARIC R ADAIR PE, CEM, BEMP
Mechanical Engineer, Energy Analyst

Henderson Engineers
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Tel (913) 742-5530
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