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

Bruce Easterbrook bruce5 at bellnet.ca
Fri Aug 24 06:57:49 PDT 2012


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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