[Equest-users] Power induction unit/ reheat delta T

[Robby Oylear]

ASHRAE does not mandate anything for reheat delta T in the baseline.  You
can make this whatever it needs to be to meet the load, within reason of
course.  Maximum recommended discharge air temperature to the zone is 15
deg F above the room temperature setpoint.  Reheat delta T's of 30-40 deg F
are not unreasonable for the baseline building.



Robby Oylear, PE, LEED AP

*Mechanical Engineer*

*Senior Energy Analyst*

* *

*D* 206-788-4571

*www.rushingco.com* <http://www.rushingco.com/>

On Fri, Sep 7, 2012 at 3:05 PM, John Shen <johnshen1 at hotmail.com> wrote:

>
> I am working on a building with air handling units on each floor. Interior
> zones are served by the AHU while exterior zones are served by the AHU as
> well as FC unit which provides furthur heating and cooling. I modeled the
> system as a Power Induction Unit with series PIU to represent the FC units.
> I used a reheat delta T determined from AHU and FC's spec'd heating
> capacity and air flow rates and allowed eQUEST to autosize and distribute
> airflow. Thanks to the advice given from members on this list I was able to
> reduce my unmet hours to a reasonable range.
>
> I am working on the baseline model and am again having issues with unmet
> hours. As mentioned the heat capacity of VAV systems are determined by the
> design flow rate and the reheat delta T. In order to reduce unmet hours I
> would increase flow rates to particular zones by manually increasing Min
> design flow rates with my reheat delta T set at 20F which is what is
> required by ASHRAE. I've been at this for a while and my building is
> basically a wind tunnel, I have floors with supply cfm upwards of 100,000
> CFM and I still have thousands of unmet hours. (my throttling range are at
> 6)
>
> Am I allowed to change reheat_delta_T to a larger value?
>
> For a power induction unit with series PIU boxes, is there any way for the
> boxes to provide zone cooling? Looking at hourly reports for "zone coil
> cooling" I have 0's.
>
> Much of this might be brought about by the fact that my building heating
> and cooling load has doubled in the baseline model.
>
> Any advice would be much appreciated.
> ------------------------------
> Date: Mon, 27 Aug 2012 09:40:41 -0400
>
> From: bruce5 at bellnet.ca
> To: johnshen1 at hotmail.com
> CC: equest-users at lists.onebuilding.org
> Subject: Re: [Equest-users] Power induction unit/ reheat delta T
>
> For reference on the sim reports go to help/Tutorials and
> Reference/Detailed Simulation Reports Summary.  It will explain all the
> reports.  I don't use the SS-G report much.  I also don't do much chilled
> water cooling.  I looked at a DX design I had done and the SS-G shows no
> cooling.
> Cooling is the most difficult to get right because the delta-T is almost
> always lower than the heating side and on the heating side you can always
> cheat a little with baseboards to do a final trim even if you don't have
> them in your system.  You can get a quick idea of the magnitude you are
> short.  Cooling has no final trim and the eQuest focus is on cooling.  You
> have also bumped into another limit on cooling capacity.  You are correct
> that the cooling capacity is dependent on the delta-T and the air flow.
> There are 2 delta-T's, the one on your water and the one on your air.  You
> can play with the water one all day but if you don't have enough air you
> won't get anywhere.  Equest has a default on the airflow to a zone that is
> 0.6 cfm/SF.  Your main AHU maximum flow will be based on this value for the
> sum of all the zones it is supplying.  Most of the time this value is too
> small for a sun side perimeter zone.
> I would start with the zone with the worst unmet hours.  You want to use
> zone reports, start with LS-A which will give you your peak heating and
> cooling loads for all your zones.  LS-B will give you the components of
> that load except OA.  SS-R is handy.  Keep an eye on SV-A, it will list the
> zones and their airflow's as sub-components of the main AHU.
> You want to go to the air-side HVAC tab and select the zone you want to
> work on.  Right click and select "Properties".  In the "Basic
> Specifications" tab you will see on the right side half way down "Zone
> Design Flow Rates", the first value is "Min Design Flow" and is probably
> set at 0.6 cfm/SF, start increasing this value.  You may need to be in the
> 1.2 cfm/SF range, it depends on the zone and the cooling load.    You can
> calculate what you need too.  As you bring the number up you will see the
> AHU airflow increase and also the cooling/heating capacity of the AHU
> increase in SV-A.  Remember that you just want enough air.  Excess air
> moving wastes energy.  Your unmet hours should be decreasing for that
> zone.  Some of the other zones will begin to decrease as well but your main
> problem zones should still have unmet hours.  Deal with each one the same
> way.
> Remember this, it is probably THE most important setting in eQuest on the
> air side.
> Bruce Easterbrook P.Eng.
> Abode Engineering
>
>  On 24/08/2012 05:16 PM, John Shen wrote:
>
>  Thank you for the response, couldn't have asked for a better explanation.
>
>   I was able to get the heat unmet hours down to a reasonable range.
> However, I am having difficulties with the Cool unmet hours. It appears I
> am unable to acquire any cooling capacity which is resulting in a few unmet
> hours (~150). As mentioned the heating capacity is dependent on the reheat
> delta t and airflow. I can't seem to find a place to enter delta T for
> cooling, I have coil delta T set along with an appropriate CHW loop. Yet I
> don't get any cooling in any of my zones in the SS-G section of the report.
> Any further help would be appreciated.
>
>  John
>
>  ------------------------------
> Date: Fri, 24 Aug 2012 09:57:49 -0400
> From: bruce5 at bellnet.ca
> To: johnshen1 at hotmail.com
> CC: equest-users at lists.onebuilding.org
> Subject: Re: [Equest-users] Power induction unit/ reheat delta T
>
> You can try changing your interior control zone as this will effect the
> base capacity that the AHU will supply to the floor.  As for the exterior
> zones your heat/cool capacity is based on the delta-T and the airflow.  You
> said you specified the air flow.  Let eQuest auto-size the flow.  By
> specifying the air flow you have removed most of eQuest's ability to adjust
> capacity.  One other thing to remember is eQuest is working on the maximum
> flow as well as all the intermediate conditions as well.  It is these
> intermediate loads which will trigger most of your unmet hours.  As the sun
> moves across your exterior zones through the day the peak load will be
> shifting through the exterior zones as well.  So a zone which was the peak
> at 10 am won't be the peak at 3 pm.  eQuest will count an unmet hour if you
> are high or low on a zones temperature.  By specifying the cfm to the zone
> I would guess you may be over cooling some of the zones and triggering
> unmet hours.  eQuest, if allowed, would reduce the airflow to the 10 am
> zone later in the day and shift the CFM to the 3 pm zone.  This is mostly
> under the hood stuff and you have to drill pretty deep into your reports to
> have an idea of what is going on and you won't find the direct answer as to
> the lower cfm to the zone.  You can get an idea of the heat/cool capacity
> eQuest is dealing with in each and follow it across the building.
> Bruce Easterbrook P.Eng.
> Abode Engineering.
>
>  On 23/08/2012 01:56 PM, John Shen wrote:
>
>  Working on a building with an air handling unit which serves the entire
> floor. In the exterior rooms there are Fan coils to supply further heating
> and cooling to the floor. I have split the floor up into interior zones and
> exterior zones (each containing a fan coil unit). The AHU I have modeled as
> a power induction unit which serves all the zones on the floor, the
> interior zones have terminal type std VAV and the exterior zones have
> series PIU. With in the exterior zones I have specified flow rates,
> heat/cool capacity reflecting the FC. However I am getting lots of unmet
> hours from these exterior zones (~500 hrs each). I inputted a
> REHEAT-DELTA-T in of 57F (based on fan flow rates and heat capacity) into
> each of the exterior zones. This greatly reduced the unmet hours; however I
> find it strange because the unmet hours are completely dependent on
> REHEAT-DELTA-T and completely independent of the zone heat capacity. I
> appear to have a poor understanding of how the PIU system works, if anyone
> could provide further insight it would be much appreciated.
>
>
>  John
>
>
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