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

[Bruce Easterbrook]

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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