[Equest-users] Confirmation for finding the Baseline Energy Model Supply Airflow

[Hwakong Cheng]

Wait a minute! I think we are mixing very different keywords and functions here, both in the modeling world and in reality. This is fundamental to understanding VAV systems.
 
Kathryn and Carol were discussing FLOW/AREA or the minimum design airflow. This is the zone design airflow (or VAV max airflow) that eQUEST starts with as a minimum when autosizing. If you are letting eQUEST autosize the zone design airflow, then the program will increase the airflow from that starting point to meet the load. Most zone loads will require more airflow than this so this value is not really that important, until you start setting it too high. Typical office interior zones are usually less than 1 cfm/sf so FLOW/AREA=1 will probably oversize interior zones. To the original question, in my experience, insufficient zone airflow is seldom the cause for unmet hours. Neither are system airflow or primary coil capacity, especially if you are letting eQUEST autosize these. Appendix G (G3.1.2.1) gives bad advice on this situation. You have to dig a little deeper in the reports (like SS-R) to understand the cause of the unmet hour. If it's unmet heating hours, I would try looking at the REHEAT-DELTA-T or MAX-SUPPLY-T or HEAT-SET-T depending on your system type. If you are VAV, you need HEAT-SET-T defined in order to have a heating coil in your AHU. Also, I think a lot of people confuse the COOL-TEMP-SCH and DESIGN-COOL-T (and same for heating side). The schedule describes the zone temperature setpoint at each hour, not DESIGN-COOL-T. There are tons of discussions in the archives about how to deal with unmet hours. Although it can be very effective at reducing unmet hours, I think increasing the zone THROTTLING-RANGE should be among the very last options to fix unmet hours, when nothing else fixes the problem. 
 
Otto, you are describing MIN-FLOW/AREA or the VAV zone minimum airflow. This is the minimum airflow that is required when the zone is either in deadband, or deadband and heating, depending on when you have a reverse-acting or proportional thermostat. This keyword should have no impact on whether the zone meets cooling loads. If you have a proportional t-stat, then a low MIN-FLOW/AREA could impact whether you have enough reheat capacity. If you have a reverse-acting thermostat, this shouldn't really have any impact on unmet hours, unless you have a zone with very low internal loads and insufficient reheat capacity. In that case, you should probably be fixing the reheat capacity, not the zone minimum. 
 
Although the keywords and names are very similar and easy to confuse, the eQUEST dictionary help files describe all of these variables very clearly.
 



Date: Wed, 8 Sep 2010 09:47:17 -0400
From: oschwieterman at fhai.com
To: cmg750 at gmail.com; kathryn.kerns at bceengineers.com
CC: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Confirmation for finding the Baseline Energy Model Supply Airflow








Appendix ASHRAE 90.1 2007 Appendix G3.1.3.13 VAV Minimum Flow Setpoints (System 5 and 7) states:
 
“Minimum volume setpoints for VAV reheat boxes shall be 0.4 cfm/sf of floor area served or the minimum ventilation rate, whichever is larger.”
 
For systems 5 & 7, I will set the minimum flow rate to 0.4 CFM/SF. If I have high unmet hours due to the small amount of CFM, I will increase the value on a room by room basis. The baseline’s flow rates are typically 30% less than my proposed model. The fans typically run more often in the baseline model. 
 
Does anyone else have a different way to model the flow rates for the LEED Baseline model?
 

From: Carol Gardner [mailto:cmg750 at gmail.com] 
Sent: Tuesday, September 07, 2010 9:02 PM
To: Kathryn Kerns
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Confirmation for finding the Baseline Energy Model Supply Airfllow
 
You must use the 1.15 and 1.25 multipliers to meet LEED requirements. I would look at the throttling range that eQ is defaulting to. if it's .5 F, which is I think what it is for PSZ, I would raise that to1 or 2 F, which is reasonable. Then I would look at the space loads, equipment especially and make sure all that looks good, then I'd look at the schedules, make sure they are good, and then I'd look at the cfm/sf. It's not unreasonable to match it to your proposed building and I don't know of any issue LEED has with doing that.

Carol

On Tue, Sep 7, 2010 at 4:23 PM, Kathryn Kerns <kathryn.kerns at bceengineers.com> wrote:


 
Yes, Carol I know the default airflow value is very low. Usually the eQuest airflow auto sizing routine will select larger values than the default. The problem is: what if the eQuest baseline energy model selects airflows larger than the default cfm/sqft airflow values but the baseline energy model still ends up with greater than 300 unmet hours or the baseline energy model has more than 50 unmet hours greater than the proposed energy model? 
 
Do you start increasing the baseline energy model airflows above the auto sized airflow value in the thermal blocks that are having unmet hours issues, or do you retain the auto sized airflow value and start manually increasing the baseline heating and cooling capacities that were also auto sized by eQuest using the 1.15 and 1.25 oversized coefficients?


 
 
Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 | 
 
 




From: Carol Gardner [mailto:cmg750 at gmail.com] 
Sent: Tuesday, September 07, 2010 2:22 PM
To: Kathryn Kerns


Cc: equest-users at lists.onebuilding.org

Subject: Re: [Equest-users] Confirmation for finding the Baseline Energy Model Supply Airfllow
 
Kathryn,




I think that .5 cfm/sf is too low. I never saw it used when I was designing HVAC systems except for an occasionally used interior space. I immediately change it to1.0 cfm/sf which I feel is a good place to start. For highly loaded rooms with lots of internal gain I sometimes up that amount after I have checked the other possible inputs.

Carol



On Tue, Sep 7, 2010 at 1:55 PM, Kathryn Kerns <kathryn.kerns at bceengineers.com> wrote:


 
Everyone, 
 
We are having a discussion at the office about how to define the supply airflow and energy model baseline. I pick the baseline system type per Appendix G (packaged single zone, packaged VAV, etc., set the heating sizing ratio to 1.25 and the cooling sizing ratio to 1.15, leave the heating and cooling capacities blank so the model will auto-size, and then set the default airflow to 0.5 cfm/sqft and let eQuest run. I then check to see if the baseline energy model unmet hours are less than 300 and are within 50 hours of the proposed model. If the baseline energy model is above 300 unmet hours or isn’t within 50 hours of the proposed model, I will then start tweaking the airflow in the baseline energy model thermal blocks that are causing the unmet hour issues to solve the problem. 
 
Is this what everyone else is doing or do leave the airflow with the default values it calculated with the 0,5 cfm/sqft limit and enter capacity values in the previously blank HVAC cooling and heating capacities slots so the baseline energy model stops auto-sizing? 
 
Thanks.
 
 
Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 | 
 
 

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