Power induction unit/ reheat delta T

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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

John Shen's picture
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Joined: 2012-11-12
Reputation: 0

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.

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0

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.

On 23/08/2012 01:56 PM, John Shen

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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John Shen's picture
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Joined: 2012-11-12
Reputation: 0

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.

Bruce Easterbrook's picture
Joined: 2011-09-30
Reputation: 0