[Bldg-sim] EPlus Dehumidification Subcooling Temperature Too Low?
Michael J Witte
mjwitte at gard.com
Wed Mar 18 13:56:31 PDT 2015
The overall logic is:
Determine the supply air temp to meet the cooling setpoint in the
control zone
Check to see if the humidity setpoint is met, if not go colder until the
humidity setpoint is met
if this results in cooling the zone below the heating setpoint, reheat
to the heating setpoint
It's the humidity setpoint that is driving what the cooling coil is
asked to do. This is a unitary heatcool system, so it does not operate
on setpoints per se. It's responding to the load request in the control
zone. It will run the DX coil as much as it needs to meet the humidity
control requirement. So, if you raise the heating setpoint, you'll use
the same amount of cooling plus more reheat. But on the other hand, the
absolute humidity ratio can be higher for the same RH if the zone is
warmer, so it will shift from cooling energy to heating energy - hard to
know which will use more.
Regarding the fan. The furnace object fan control is currently set to
cycle for part of the day and run continuously for the rest of the day:
outside_air_schedule_compact, !- Fan
Operating Mode Schedule Name
That's why it needs Fan:OnOff.
Mike
On 3/18/2015 3:48 PM, Jacob Dunn wrote:
>
> Ivan –
>
>
> Thanks for the info! I did get your previous message, my apologies
> for not responding sooner.
>
> Your answers were very helpful, but as you mentioned they are
> workarounds. The single setpoint t-stats are a nice idea, but forcing
> the building to have a single setpoint thermostat designated for a
> seasonal period is not ideal.
>
> Additionally, I’ve swapped out the Fan:onoff to Fan:constantvolume as
> you mentioned, but am getting some strange results. Can you send me
> the .idf that you modified? I essentially changed the fan object’s
> name and modified the “Supply fan object type” in the
> “Airloophvac:unitary:furnace:heatcool” object. However, when I ran the
> simulation I got a severe error. I had to modify the “Fan Operating
> Mode Schedule Name” to a schedule that had “1s” constantly. I also
> changed the fan’s availability schedule to the same thing. Thus, the
> fan runs all the time.
>
> The EUI jumped up considerably (to 111 kBtu/sf-yr) because of the fan
> and cooling energy, but both the zone RH looks good and the supply air
> temperature never goes below 15 deg C.
>
> Here’s an image -
> https://drive.google.com/file/d/0B6aA8ZsOIiibc0ZhSTdDVmtQSVE/view?usp=sharing
>
> Am I missing something?
>
>
> Cheers and thanks 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/>___
>
> *From:*Ivan Korolija [mailto:ivankorolija at yahoo.com]
> *Sent:* Wednesday, March 18, 2015 2:53 PM
> *To:* Jacob Dunn; Jim Dirkes
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] EPlus Dehumidification Subcooling
> Temperature Too Low?
>
> Hi Jacob,
>
> It seems my respond to your question hasn't gone through so I'll
> repeat it again.
>
> I think I identified a problem but not the full solution for it. If
> you check the zone temperature during summer period you'll see that
> during unoccupied period the zone is cooled down to 15C which is
> actually your heating setback temperature. To achieve such a low
> temperature the supply air has to be sub-cooled to 5C which, as you
> mentioned, is unrealistic. I cannot source the root of this issue.
>
> The partial solution is to control the zone conditions by single
> heating and single cooling thermostats. As soon as I changed to single
> cooling thermostat during summer period, the system behaves as it
> should (supply air temperature around 15C, zone air temperature
> according to cooling occupied/unoccupied setpoints).
>
> Another possible way of overriding the issue is to replace Fan:OnOff
> with Fan:ConstantVolume and keep DualSetpoint control. This doesn't
> completely remove periods when the supply air temperature is around
> 5C, although it significantly reduces it.
>
> I hope you'll find this answer useful.
>
> Best regards,
>
> Ivan
>
> p.s. I'll post this reply to UnmetHours and Yahoo e+ group as well.
>
> _____
>
> Dr Ivan Korolija
>
> IESD, De Montfort University, Leicester, UK
>
> It seems my respond to your question hasn't gone through so I'll
> repeat it again.
>
> On Wednesday, March 18, 2015 7:38 PM, Jacob Dunn
> <jdunn at eskewdumezripple.com <mailto:jdunn at eskewdumezripple.com>> wrote:
>
> 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*
>
> *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/>_
>
> *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
> <mailto:bldg-sim at lists.onebuilding.org>
> *Subject:* Re: [Bldg-sim] EPlus Dehumidification Subcooling
> Temperature Too Low?
>
> 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 <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*
>
> *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 <mailto: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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