[Equest-users] Power induction unit/ reheat delta T
Bruce Easterbrook
bruce5 at bellnet.ca
Tue Sep 11 14:40:58 PDT 2012
A single system can be a lot of problem. There is just so much
variation between the zones most times and there are not enough
T-stats. I don't know the number of zones and the space size but if you
are down to the 100 hour unmet range I think you are as close as you are
going to get it. One other thing to look at is the timing of the unmet
hours. Many times I have found, more so with higher mass buildings is a
one hour warm up is not enough and you will get a lot of unmet hours
right at the start of the day. Sometimes bringing the AHU off night set
back 2 hours before is enough to get rid of these shoulder hours. There
can be a big difference in the heat and cooling capacity required from
maintaining a set point verses trying to recover the space from a 4
degree set back. Equipment these days is sized closer to the line and
it takes a little more time for the system to get the space under control.
Bruce Easterbrook P.Eng.
Abode Engineering
On 11/09/2012 12:40 PM, John Shen wrote:
> Thank you very much for the response, they were extremely helpful. I
> was able to reduce my unmet hours to ~100 hours but not without a
> whole lot of effort. It appears that my zones had a large
> load discrepancies between one another, the central zones had minimal
> load while certain outer zones had large heat loss due to window
> frames, zone area etc. This seemed to result in a system being very
> sensitive to air flow and I was previously making too large of an
> increment with my min air flow. Increasing the reheat-delta-T did make
> the system much easier to deal with as eQUEST seemed to distribute air
> flow when auto-sizing. In the end there were still zones where ~100
> unmet hours was as low as I could get it no matter how I changed the
> air flows, as the system did appear to be fighting itself to a degree.
> The only fix was to auto-size baseboards in these trouble zones. I
> suppose these issues aren't uncommon when you have a single system
> serving large a number of zones.
>
> ------------------------------------------------------------------------
> Date: Mon, 10 Sep 2012 12:42:45 -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
>
> 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.
> Bruce Easterbrook P.Eng.
> Abode Engineering
>
> On 07/09/2012 06:05 PM, John Shen 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 <mailto:bruce5 at bellnet.ca>
> To: johnshen1 at hotmail.com <mailto:johnshen1 at hotmail.com>
> CC: equest-users at lists.onebuilding.org
> <mailto: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 <mailto:bruce5 at bellnet.ca>
> To: johnshen1 at hotmail.com <mailto:johnshen1 at hotmail.com>
> CC: equest-users at lists.onebuilding.org
> <mailto: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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