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

Jacob Dunn jdunn at eskewdumezripple.com
Wed Mar 18 12:38:11 PDT 2015

Jim –

Thanks for the reply!  According to the hourly outputs (currently the simulation is at a “1” timestep) the relative humidity setpoint is indeed being met 94% of the time, so the system seems to be working well, almost too well for a subcool reheat system if you ask me.

I do expect an increase, and I’m indeed questioning the magnitude of the jump.  I don’t really have much of a frame of reference as research/simulation study material on this is scarce.  The ASHRAE 1254-RP from 2006 is the only thing I’ve seen quantifying the energy effect and effectiveness of commercial HVAC systems and dehumidification.  They modeled 17 different system configurations in E-plus for a variety of building types/climates, and reported the hours above 65% RH and the increase in annual HVAC cost. For a DX system w/subcool Reheat for Miami and a 9 month school ------ 2222 hours (25%) of hours are above the RH threshold and this creates an 11% increase in HVAC energy.  So my 6% of hours above the RH setpoint and 30% energy increase seems too big.

However, the main reason why I think it’s unreasonable is that I don’t believe that the supply air temperature should ever drop to 5 deg C (41 deg F) to dehumidify.  I think most dx systems won’t go past 55 deg F, but I can’t seem to control this setpoint in Eplus.

Cheers and thanks for your help!

Jacob Dunn LEED AP BD+C

2014 AIA National Architecture Firm Award

365 Canal Street Suite 3150
New Orleans LA 70130

From: Jim Dirkes [mailto:jim at buildingperformanceteam.com]
Sent: Tuesday, March 17, 2015 4:20 PM
To: Jacob Dunn
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] EPlus Dehumidification Subcooling Temperature Too Low?

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<mailto: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
2014 AIA National Architecture Firm Award

365 Canal Street Suite 3150
New Orleans LA 70130

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James V Dirkes II, PE, BEMP, LEED AP
The Building Performance Team Inc.
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