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.
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
8345 Lenexa Drive, Suite 300 | Lenexa, KS 66214
Tel (913) 742-5530
daric.adair at hendersonengineers.com
hendersonengineers.com
Licensed in KS
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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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Bill;
Thank you. I knew this had to have been discussed before, but searching "MIN-AIR-SCH" on the archives doesn't yield the thread from Sept 2017. May be a case of "-" meaning don't include results with xxx in it. Meh.
Sorry on the #2 and #3 descriptions below. Have added a strikethrough on #3 as it is the case with no economizer and it had no MIN-AIR-SCH coming out of wizard.
So marrying the previous thread (which was on 100% OA systems) along with this thread (not 100% OA VAV / 'typical' VAV units) - for systems with economizer. There are 4 situations just tested and confirmed with hourly reports:
1. System has economizer (OA-CONTROL set to anything but 'fixed fraction'); a MIN-AIR-SCH which is -999 for occupied hours and 0.001 for unoccupied hours.
a. Allow the unit to bring in extra OA during unoccupied and occupied hours to provide air-side economization. Typical 'real' economizer.
2. System does not have economizer (OA-CONTROL set to 'fixed fraction'); a MIN-AIR-SCH which is -999 for occupied hours and 0 for unoccupied hours.
a. Does not allow the unit to bring in extra OA during unoccupied or occupied hours. Truly no air-side economization.
3. System has economizer (OA-CONTROL set to anything but 'fixed fraction'); a MIN-AIR-SCH which is -999 for occupied hours and 0 for unoccupied hours.
a. Allow the unit to bring in extra OA during occupied hours only to provide air-side economization.
4. System does not have economizer (OA-CONTROL set to 'fixed fraction'); a MIN-AIR-SCH which is -999 for occupied hours and 0.001 for unoccupied hours.
a. Allow the unit to bring in extra OA during unoccupied hours only.
5. System does not have economizer (OA-CONTROL set to 'fixed fraction'); a MIN-AIR-SCH which is 0.001 for all hours.
a. Unit hardly brings in any OA all hours.
6. System has economizer (OA-CONTROL set to anything but 'fixed fraction'); a MIN-AIR-SCH which is 0.001 for all hours.
a. Unit hardly brings in any OA all hours, but will 'jump' to economize levels, both during occupied and unoccupied hours when economizing would help. This is thinking the MinOA is 0.001 (overriding what is input at the zone level?) but allowing economizer when it helps. This is too low of OA and is probably wrong.
Case 1 and 2 are probably the most common, from review of real controls sequences.
Case 3 is possible, but maybe odd/rare/special in my experience.
Case 4 is really odd, but possible I guess? Would need some convincing before a real application, but that's a separate topic.
Case 5 isn't bringing in OA and is (most likely) wrong.
Case 6 the Min OA is the 0.001, and while it can economize is not providing the right amount of OA during occupied hours. But hey, it's economizing right? (sarcasm)
Probably the most concerning thing (to me) is with the way I understood the help file was that Case 6 was the only way to get to economizer ('enables economizer'). I now believe thi is wrong approach. Really Case 1 is the 'right' way to air-economize.
Any diagreements on the above finiding or inputs?
Again; haven't tested with DCV implmented - but that's a next step in the process!
Thanks,
DARIC R ADAIR PE, CEM, BEMP
Mechanical Engineer, Energy Analyst
Henderson Engineers
Tel (913) 742-5530
daric.adair at hendersonengineers.com
Licensed in KS