Can't get unmet hours under control for cooling

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I need some help getting these cooling unmet hours under control. I have to get this model complete and submitted by tonight.

We've tried multiple things to try to get the model to work out.

We've tried the following:

* Letting eQuest autosize the heating and cooling for the PSZ units

* Verified that the schedules are consistent with both cooling, heating, and occupancy.

* On the baseline building we have the min cfm set to 0.5 for most spaces and 1 cfm/sf for some of the larger spaces.

* We set the PSZ -fan - flow parameter - indoor fan mode to intermittent to get the unit to show partial capacity thru out the days.

One item is RTU-2 in my model. It serves only one space. We have a high amount of unmet hours on this and we are letting it auto size the capacity and airflow and it still says its not staying under control. We're assuming that we are missing one small check box that's somehow causing these unmet hours.

I've attached the pd2 files for both baseline and proposed along with weather data and inp files.

Thanks in advance for any help!

Jamie Armstrong P.E.

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Joined: 2012-05-29
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Jamie,

You have some inconsistencies in your inputs for the OA on system RTU-2 (and all the rest). See highlighted area below. You are saying that the system is 100% recirc by setting the Minimum Outside Air ratio to 0.00, but your Minimum Air Schedule (note that this is a fraction of the design supply CFM type of input) is saying otherwise and takes precedence causing it to be a 100% OA system. See my snip from the hourly reports below too. You need to develop the proper ventilation rates for each system and verify with hourly reports. Do not rely on the summary tab in the air side system tab of eQUEST to give you correct data at all times. If you just define the OA by using the Minimum Outside Air ratio, you will be supplying OA every time the fan operates. To prevent this you need to set up Minimum Air Schedule for each system which is based upon the proper fractions for that system.

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Timothy Howe, MS, LEED? AP BD+C

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In regards to OA for PSZ RTU's.

When allowing eQuest to autosize the airflow to each space if you set a ratio of OA to supply air whatever eQuest calculates the supply air needed for that space will vary the outside airflow.

If I enter the OA at the zone level to each space how do you set up a schedule for the OA? Based on Timothy's answer I would need a different schedule for each RTU with its ratio of OA to supply air. I wanted the MinOA schedule to be an on/off flag to deliver the OA that I put in at the zone level according to the occupied schedule.

Jamie Armstrong P.E.

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Joined: 2012-05-29
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Jamie,

I ran into this same issue recently. You can use the same OA for all but set the 'ON' hours to -999. This will ignore the schedule for that hour and the zone calculations and inputs will control.

FRAC/DESIGN accepts a value which is the fraction of the design quantity. Typical values range between 0.0 and 1.0. An entry of -999 causes the schedule to be ignored for that hour.

For example, in the SYSTEM:MIN-AIR-SCH, an entry of 0 or 1 will force the outdoor-air ratio to be 0% or 100% respectively. A value of -999 will cause the schedule to be ignored for the hour, and the outdoor-air ratio will be set by other calculations.
Dana

Mr. Dana Etherington, E.I.T., LEED AP BD+C

Dana Etherington, CEM, LEED AP
email: dana.etherington@crbusa.com
cell: 617-583-3009

Dana Etherington's picture
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eQuest experts,

I received some excellent support on an earlier issue from folks here
and have come across another item that I haven't found info for while
searching this forum.

Description of pertinent part of model:

* PVAVS system

* OSA requirements defined at zone level as hardcoded CFM values
(OUTSIDE-AIR-FLOW)

What I intend to have modeled is the unit providing 55degF tempered
RA/OA to the boxes, which will take the rest of the load.

Issues I've run into:

* Heating coils in central units have capacities of zero (per
SS-P) but there is a load to the system (per SS-J). It seems I need to
use HEAT-SET-T to enable a central coil, but putting that value at 55
doesn't do anything (if I make it larger the central coil sees some load
but I don't want the SAT to be above 55 nominally).

How does one go about specifying the supply air temp from a the central
unit in PVAVS system?

While trying to troubleshoot I was trying to output the outside airflows
in hourly reports. Only applicable data I saw to output was under
thermal zone "Hourly zone OA/total flow for DCV calculation" Is this
only used if you have DCV? How do you check your OA flows to zones if
you are not using DCV?

Thank you!

Erik

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Note that the supply air temperature is the mixed
air temperature which depends on the mix of OA
and return air streams. If the mixed air
temperature is at 55 then there will be no central heating.

Set up some hourly reports to monitor the OA T,
the supply air T, and the central heating coil
output. That will give some insight to how the system is working.

If the mixed air temperature is below 55 and cool
control is set to CONSTANT you will see central
heating. If cool control is set to WARMEST then
the PVAV system likely won't provide supply air at a minimum of 55.

Changing the heat control to CONSTANT or COLDEST
may also make some difference in how the central heating coil operates.

>> Christopher Jones, P.Eng.

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Jamie,

Yes based upon what you have indicated that you want to do with it you can do as Dana noted:

? Set to OA at a zone level and use the Minimum Air Schedule schedule with the -999 during on hours to have the system base it upon the zone inputs and 0.01 during off hours to allow economizer to be active when the system cycles.
If you need to simulate DCV you can do as I indicated and create a varying flow based upon occupancy levels using the fraction of design flow for each hour. This would not apply to you baseline model as DCV would not be included.

You did not have the OA defined at the zone level in the model so I did not want to confuse things in my original response with telling you to do so if you were not planning on doing this.

Hope this helps.

Tim.

Timothy Howe, MS, LEED? AP BD+C

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Jamie,
Is there a reason you have all the plenum spaces on their own separate system with indoor design temperatures similar to your conditioned zones?

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I've finally got the unmet hours under control but my heating costs are more in my design than my baseline. I'm shooting for 24% increase in efficiency and I'm at 4 now.

My design system is a PSZ with natural gas heat.

Jamie Armstrong P.E.

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Joined: 2012-05-29
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Jamie,

I would not use the compliance mode to create your baseline. I am not sure if it is accurately working yet and I am pretty sure most energy modelers on this list don't use it or have used it with limited success.

Right off I I don't see the program doing what it should (or what I expect it to do) in the baseline using the compliance mode:

1. Fan power adjustment

2. EIR for the condenser power adjustment
Also you have on a majority of your proposed systems used the keyword System Sizing at values greater than 1 (1.75, 2, etc). I would never use that keyword and always set it to 1.0. This will cause your system to be over sized and as it is constant volume your fan power consumption will be significantly increased.

If I was you, I would use the compliance mode to get started with the baseline, but make sure to validate all the inputs to make sure they are in compliance with Appendix G and the rest of 90.1.

Tim.

Timothy Howe, MS, LEED? AP BD+C

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Following up on my earlier question, it seems like you have the plenums in their respective conditioned zones in the design case, but the base case still shows
them all under their own separate system and at indoor design temps similar to the conditioned zones.

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Jamie:

Aside from ongoing advice by others, I've had a chance to give the most recent set of models a scan.

I notice you have a relatively large amount of exterior lighting. I also notice your proposed and baseline installed wattage and scheduling are identical. Think that through carefully! If you didn't do the electrical/lighting/associated controls for the project yourself, reigning that individual into the brainstorm may be productive. My LEED models rarely feature identical exterior lighting consumptions, and in your particular case this is likely low hanging fruit to improve your modeled performance towards your target.

More low fruit: I also notice your process loads are eerily close to the ubiquitous 25% mark... I hope that's coincidence! Consider your sources carefully and whether your inputs are appropriate for an energy model (vs. sizing equipment). Following recommended plug load densities in the 90.1 Users Manual (0.75W/SF for office buildings), I experimentally find your misc. equip. annual consumption drops about 7MWh - this would mirror for both models and in turn amplify the differences/improvements of all other systems. A quick visual check confirms using the prescribed receptacle usage schedule from the same source will net you further trimming in this department.

To the most recent query about gas costs: All things being equal (and perhaps they shouldn't be), I see the proposed isn't consuming a great magnitude of extra gas, and this can be normal. For this scale of project, you might well explain the additional proposed consumptions entirely by the difference in lighting LPD contributions for internals during the winter months. I would advise not getting too hung up on this fact, but if you want to feel confident I'd advise checking your heating efficiencies are correctly entered for both models and that the rates are identical.

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NICK CATON, P.E.

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