[Bldg-sim] EPlus Dehumidification Subcooling Temperature Too Low?

[Jim Dirkes]

Jacob,
I'm  big fan of Output:Variables in E+ and use them liberally to see
whether setpoints are met and to get clues about why setpoints are NOT met.
I see that you have a number of them in your IDF; have you evaluated those
values against the HVAC controls?

   - Is the RH setpoint consistently met?
   - You expect cooling, heating and overall energy to increase with
   controlled dehumidification (and they did), so are you concerned only with
   the magnitude of the jump?

I think some review of the energy for depressing discharge air temp to get
lower dewpoint, along with confirmation that the heating energy is
appropriate, will help you a lot.  You may also benefit from reporting the
variables at "timestep" instead of "hourly".

(I missed your post on the E+ forum)

On Tue, Mar 17, 2015 at 4:56 PM, Jacob Dunn <jdunn at eskewdumezripple.com>
wrote:

>  I posted this question to the EnergyPlus yahoo support group and the
> Unmet Hours online forum and have received no replies after a week!  So I
> thought I’d pass it along to this group:
>
>
>
> Hello all!
>
> I'm attempting to model dehumidifcation in EnergyPlus and I'm getting some
> strange results. When using a constant 62.5% Relative Humidity setpoint
> versus no dehumidification in New Orleans LA, the EUI jumps from 70
> kBtu/sf-yr to 91 kBtu/sf-yr. Cooling energy increased by about 30%, and
> heating increases by a factor of 5.6!
>
> Image -
> https://drive.google.com/file/d/0B6aA8ZsOIiibQUJSa2g5Z2pnMkE/view?usp=sharing
>
>
>
> The project is a 400 square foot classroom "shoebox" that I'm running some
> infiltration sensitivity analysis on. Thus, I want to make sure I'm
> capturing the energy impact of dehumidification accurately. I'm using the
> HVACTemplate:System:Unitary template (everything is autosized), and I've
> input "CoolReheat" in the "! - Dehumidification Control Type" field. I've
> added a "ZoneControl:Humidistat" object that uses a compact schedule with a
> "percent" schedule type limits that uses 62.5 all year. After inspecting
> the expanded file, the AirLoopHVAC:Unitary:Furnace:HeatCool object has a
> "CoolReheat" designation in the "! - Dehumidification Control Type" and an
> additional reheat coil appeared. Everything looks good.
>
> So, why is the reheat and cooling energy increase so high? The scheme
> seems to be working, only 6% of all hours are above 62.5% relative
> humidity. I think the main problem is that with dehumidification, the
> cooling coil drops the temperature to 5 deg C (42 deg F) in some cases (see
> below graph). This seems unreasonable, and may be why there is so much
> reheat and coling energy, as actual systems would never really drop it
> below 55 deg F. I don't know how to control this, and I can't find a
> setpont manager that controls the subcooling routine for the
> dehumidification.
>
> Image -
> https://drive.google.com/file/d/0B6aA8ZsOIiibQm9rdDNUZFYwS0E/view?usp=sharing
>
>
>
> here appears to be a "!-minimum supply air temperature" field in the
> "SetpointMnager:SingleZone:Reheat" object set at 13, but it doesn't seem to
> have an effect.
>
> Is there a way to control this? Am I missing anything?
>
> Here is a link to the .idf in case it helps -
> https://drive.google.com/file/d/0B6aA8ZsOIiibLWpqR1JCbVhyOWs/view
>
> Thanks ahead of time for your help!
>
>
>
>
>
> *Jacob Dunn LEED AP BD+C*
>
>  *ESKEW+DUMEZ+RIPPLE, APC*
>
> 2014 AIA National Architecture Firm Award
>
>
>
> 365 Canal Street Suite 3150
>
> New Orleans LA 70130
>
> 504.561.8686
>
> *eskewdumezripple.com <http://www.eskewdumezripple.com/>*
>
>
>
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-- 

James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
1631 Acacia Dr, GR, Mi 49504

Direct: 616.450.8653
jim at buildingperformanceteam.com

Website <http://buildingperformanceteamcom>l  LinkedIn
<https://www.linkedin.com/pub/jim-dirkes/7/444/413>

 Anger is the wind that blows out the candle of the mind.
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