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<font size="+1">It seems to me you have gone overboard on your zone
air flows which will generate unmet hours and also waste energy.
This is all a balancing act between several conflicting items, air
flow just being one. I don't think your delta T is the problem.
Terminal units can heat and cool and it sounds like yours are not
cooling. I would also suspect your system is fighting itself.
You are going to have to troubleshoot what is going on and figure
out a solution. Your sim reports will tell you how much heating
and cooling each zone requires. From there you can figure out how
much air you need to to accomplish this. Compare that to how much
air you are putting in. I don't do a lot of these types of
systems to give you a quick answer with the information I have.
Most of the time there is no quick answer anyway. All HVAC
systems follow a logical pattern. You need to find where and why
you have deviated. As you can see eQuest will take you way out
into left field if you let it.<br>
Bruce Easterbrook P.Eng.<br>
Abode Engineering<br>
<br>
</font>
<div class="moz-cite-prefix">On 07/09/2012 06:05 PM, John Shen
wrote:<br>
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<br>
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.
<div><br>
</div>
<div>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)</div>
<div><br>
</div>
<div>Am I allowed to change reheat_delta_T to a larger value? </div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Much of this might be brought about by the fact that my
building heating and cooling load has doubled in the baseline
model.</div>
<div><br>
</div>
<div>Any advice would be much appreciated.<br>
<div>
<hr id="stopSpelling">Date: Mon, 27 Aug 2012 09:40:41 -0400<br>
From: <a class="moz-txt-link-abbreviated" href="mailto:bruce5@bellnet.ca">bruce5@bellnet.ca</a><br>
To: <a class="moz-txt-link-abbreviated" href="mailto:johnshen1@hotmail.com">johnshen1@hotmail.com</a><br>
CC: <a class="moz-txt-link-abbreviated" href="mailto:equest-users@lists.onebuilding.org">equest-users@lists.onebuilding.org</a><br>
Subject: Re: [Equest-users] Power induction unit/ reheat
delta T<br>
<br>
<font face="Times New Roman, Times, serif" size="+1">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.<br>
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.<br>
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.<br>
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.<br>
Remember this, it is probably THE most important setting
in eQuest on the air side.<br>
Bruce Easterbrook P.Eng.<br>
Abode Engineering<br>
<br>
</font>
<div class="ecxmoz-cite-prefix">On 24/08/2012 05:16 PM, John
Shen wrote:<br>
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<div dir="ltr"> Thank you for the response, couldn't have
asked for a better explanation.
<div><br>
</div>
<div> 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.</div>
<div><br>
</div>
<div>John<br>
<br>
<div>
<hr id="ecxstopSpelling">Date: Fri, 24 Aug 2012
09:57:49 -0400<br>
From: <a moz-do-not-send="true"
class="ecxmoz-txt-link-abbreviated"
href="mailto:bruce5@bellnet.ca">bruce5@bellnet.ca</a><br>
To: <a moz-do-not-send="true"
class="ecxmoz-txt-link-abbreviated"
href="mailto:johnshen1@hotmail.com">johnshen1@hotmail.com</a><br>
CC: <a moz-do-not-send="true"
class="ecxmoz-txt-link-abbreviated"
href="mailto:equest-users@lists.onebuilding.org">equest-users@lists.onebuilding.org</a><br>
Subject: Re: [Equest-users] Power induction unit/
reheat delta T<br>
<br>
<font size="+1"><font face="Times New Roman, Times,
serif">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.<br>
Bruce Easterbrook P.Eng.<br>
Abode Engineering.<br>
<br>
</font></font>
<div class="ecxmoz-cite-prefix">On 23/08/2012 01:56
PM, John Shen wrote:<br>
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<p class="ecxMsoNormal">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.</p>
<p class="ecxMsoNormal"><br>
</p>
<p class="ecxMsoNormal">John</p>
</div>
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