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OA below specified minimum for zone served by VAV

asked 2015-08-27 17:01:29 -0500

jkjenner gravatar image

I'm attempting to model a pretty simple multiple zone VAV system in eQuest. This system serves a large gymnasium and a few small offices and storage rooms in a cold climate (Calgary, Canada). The problem I'm having is that the outdoor air rate being supplied to the gymnasium is below the minimum I've specified.

I've selected the "Pkgd Var Vol" system type, and specified minimum outdoor air flow rates for each zone (6313 cfm in the case of the gym). At the system level, I've selected the "fraction of design flow" OA control method, and the "set by critical zone" OA sizing method. The gymnasium zone terminal has a reheat coil, and the zone also has baseboard heaters.

I've set the minimum supply air flow for the gymnasium zone to 0.4 cfm/ft2, which works out to 2652 cfm. The problem is that this number seems to be taking precedence over the required O/A of 6313 when DOE-2 determines the zone's minimum flow. (Because there are baseboards in the zone, the S/A continually rides the minimum).

It seems to me that the "minimum" ought to be the larger of the specified minimum S/A and the minimum O/A... is this just not how it works? If so, is there any way to force eQuest/DOE-2 to work like this?

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answered 2015-08-31 09:04:36 -0500

updated 2015-08-31 12:42:26 -0500

eQUEST defaults to calculating the system design outdoor air flow fraction by summing the zone outdoor air requirements and dividing by the design supply air flow rate. With this method, the outdoor air fraction for a given zone may fall below the desired minimum outdoor air flow rate. For example, if a given zone requires 80% outdoor air, but all other zones only require 30% outdoor air, the system will supply outdoor air at a fractional rate that is between 30 and 80%. If you want to ensure that each zone is getting the required outdoor air you need to select the outdoor air sizing method "SET-BY-CRITICAL-ZONE". Using this method, the system design outdoor air fraction is based on the zone with the highest outdoor air fraction requirement. In this case, using our example above, the system will supply 80% outdoor air to ensure that all zones receive their minimum. All of this behaviour is reasonably well described in the DOE2 Dictionary, which can be found with other valuable documentation on the download page for eQUEST.

To change this setting in eQUEST to the Outdoor Air tab in the system editing window and look for the "Minimum OA Sizing Method". Change the default from "Sum of Zone OA" to "Set by Critical Zone".

Updated in response to the comment below: I see what you are asking now. Apologies for misreading your question. As you have noted, it appears that the input for minimum supply air flow rate for the gymnasium is taking precedence over the input for the outdoor air flow rate. Under the entry for SUPPLY-FLOW, the DOE-2 dictionary states that when you allow the program to calculate the MIN-FLOW-RATIO rather than input, the values are corrected taking into account the specified outdoor/exhaust air or the peak heating load. It sounds like the entry for MIN-FLOW-RATIO or MIN-FLOW/AREA is overriding the supply air flow calculation and that eQUEST is not using the outdoor air flow rate to set the minimum. I tried working this out in a simple wizard model and when I blanked out the inputs for the minimum flow ratio, eQUEST sized the minimum flow to equal the outdoor air flow rate. However, the outdoor air ratio did not go to 1.0 as I would have expected in minimum flow conditions. The only way I was able to force the system to maintain the minimum was to set the "Minimum OA Control Method" to "DCV Zone Sensors". I'm not exactly sure what's happening here, but it appears that when using "SET-BY-CRITICAL-ZONE" the system calculates the design outdoor air flow fraction based upon the zone with the highest OA requirement, but the calculation doesn't appear to be updated correctly on an hourly basis. Note that at supply air flow rates higher than the minimum outdoor air rate, the outdoor air may dip below the specified minimum. This is to be expected as a given percentage of the return air is assumed to ... (more)

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Thanks for the response. As I mentioned in my question, I'm already using the "set by critical zone" sizing method, which is part of what makes this behaviour so puzzling...

jkjenner gravatar imagejkjenner ( 2015-08-31 10:25:02 -0500 )edit

Thanks for the response Daniel--the behaviour you describe in the the wizard model you created is exactly what I'm experiencing. I will try that workaround. For what it's worth, I ended up just setting up a spreadsheet to calculate for each zone MAX(0.4, OA / AREA), and then just pasted the results into eQuest. (which works because in eQuest I'm dealing with a 100% OA system).

jkjenner gravatar imagejkjenner ( 2015-08-31 13:11:21 -0500 )edit
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answered 2015-08-28 08:22:44 -0500

Chris Jones gravatar image

updated 2015-08-28 20:01:08 -0500

The egroup for equest is Equest-users equest-users-bounces@lists.onebuilding.org. The signup page is: Equest-users Info Page

I suggest posting your question to the eQuest egroup along with the .pd2 and .inp files. The eQuest group responds quickly to this sort of question.

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@jkjenner It would be great if you could also summarise the response here so that it can helps others in the future.

Jamie Bull gravatar imageJamie Bull ( 2015-08-28 08:46:54 -0500 )edit

Hey Chris, where can I find the group you refer to?

jkjenner gravatar imagejkjenner ( 2015-08-28 17:36:20 -0500 )edit
1

The eQUEST user's list is a fantastic resource with a vibrant and helpful community. Unfortunately it suffers from the problems common to many email lists that this website is trying to solve, namely that the archives are hard to find and search and that many common questions are often repeated. Hopefully by addressing eQUEST questions on this page we can encourage a slow build up of a community of eQUEST users here.

Daniel gravatar imageDaniel ( 2015-08-31 09:07:31 -0500 )edit
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answered 2015-10-26 16:25:44 -0500

updated 2015-10-26 16:31:31 -0500

This is my understanding about the OA setup in eQuest:

1, OA setup can be defined in either zone level (ex: Flow per Area, Flow per Person) or system level (Minimum Outside Air ratio). Usually zone level definition will overide system, but not always. We should be very careful when we had inputs in both levels.

2, If we input 1.00 into "Minimum Outside Air" of "Outside Air and Economizer" tab in system level, at the same time we input OA in the zone level "Flow per Area", actual OA ratio varies in different situations. If the OA defined in zone level is more than the system design supply air flow rate (could be defined by system or autosized based on load), result would be 100% OA. If the OA required by zone is less than the system design supply air flow rate, OA ratio would be "OA required by zone/supply air flow rate" which is less than 1. An interesting example is DOAS in eQuest. If we define a DOAS on a small zone, many other zones use OA from this system, the OA required by zones would be a very large number and definitely larger than the supply air flow rate of the small zone. In this case, OA ratio would be 1.

3, If we only defind OA in system level, input system air flow rate and minimum outside air ratio as 1.00 and NO INPUTS in ZONE LEVELS! The results would be absolutely 100% OA.

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Asked: 2015-08-27 17:01:29 -0500

Seen: 569 times

Last updated: Oct 26 '15