[Equest-users] Is it possible to disable PSZ heat pump hot gas reheat dehumidification and enable PSZ electric coil dehumidification?
Nicholas Caton
ncaton at catonenergy.com
Fri Aug 21 14:56:10 PDT 2015
This thread/discussion has grown in a few directions (on and off-list), but
I think I may be able to trim some of the branches:
1. A visual would clearly help some to follow along with the topic
at-hand. The following illustration is lifted from an ASHRAE Journal
article discussing reheat design options
<https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCEQFjAAahUKEwjfxeH4hrvHAhUIjQ0KHTo3BPI&url=https%3A%2F%2Fwww.ashrae.org%2FFile%2520Library%2FdocLib%2FPublic%2FASHRAE-D-23127-20041201.pdf&ei=WY_XVZ-1IoiaNrrukJAP&usg=AFQjCNFhqu-mppFWzpeb46wGb5rUKD2Jcg&cad=rja>.
This picture shows one means by which a heatpump in cooling operation can
effect dehumidification by subcooling the supply airstream and then
leveraging some of the “free” hot gas reheat to bring it up to design
supply temperatures:
Summarizing the query: Is it appropriate to have dehumidification with
90.1 App.G baseline heatpump systems occur without the use of “free” hot
gas contributions, but rather with electric resistance only (and if so,
how)?
2. I believe it is mutually understood and expected that electric
resistance reheat is expected to expend more energy than a refrigerant heat
recovery cycle, all things being equal. That is why this topic is of
interest to start with (to maximize baseline energy consumption where
appropriate).
3. The entire sub-discussion regarding 6.5.2.3 exceptions seems out
of place to my thinking: This section (6.5) is solely for demonstrating
prescriptive compliance and as such has no direct bearing on Appendix G.
Baseline and Proposed case systems must follow the “X.4” mandatory
requirements (i.e. 6.4), but are not required to demonstrate prescriptive
compliance.
4. To voice my opinion regarding whether this is appropriate to
pursue at all: For a proposed design case where dehumidification
requirements cannot inherently take advantage of a heatpump reheat cycle, I
believe Kathryn’s efforts to specify the baseline system dehumidification
means is well within the realm of reasonable interpretation. The effect on
the baseline systems is more like “leveling the playing field” than “gaming
the system.”
5. Appendix G (under 2007) *directly* addresses humidity controls in
two places:
a. In Table G3.1.1.b. “…temperature and humidity control setpoints
and schedules shall be the same for proposed and baseline building designs.”
b. In G3.2.3.10, an exception (g) concerning dehumidification may
remove baseline energy recovery requirements otherwise occuring.
Appendix G does not specify how dehumidification is to occur, for any
system type. I am not aware of any further direction on this matter via
LEED C.I. All told I’d be comfortable making this distinction in a clear
fashion during preliminary submission.
6. Regarding the proposed “how” method to lock out reheat: I think
fewer steps are necessary using the eQUEST interface. Without specifying
reheat is recovered for “space heating,” you can right click the field for
the recovery fraction and assign a user default or user default expression
(effecting 0.0 or otherwise), and this produces a “space heating” enduse
consumption difference consistent with expectations. On the aside: I’m
not clear on why it defaults to ‘unused’ (i.e. “n/a” in the interface):
While “space heating” energy consumption is higher with the recovery
fraction set to zero (as might be expected if reheat is occurring via
resistance heat), I can’t say with 100% certainty a “dehumidification
electric reheat coil” is exactly what’s being simulated based on what the
DOE2 documentation omits in description of default dehumidification
operations. Perhaps a comparative model of a DX/Electric and DX/HP PSZ
(specifying a recovery fraction of 0.0 for the latter) might confirm that’s
what is happening.
Of note, I wanted to point out and request clarification on some margin
notes in Volume 3 that caught my eye and might be of concern here: *“note
that HEAT-PUMP is not a legitimate choice when humidity control is
required.”* Is this some red-herring language left over from before
heat-pump dehumidification worked with DOE2, or is there an actual
limitation requiring further explanation at play?
~Nick
*NICK CATON, P.E.*
*Owner*
*Caton Energy Consulting*
306 N Ferrel
Olathe, KS 66061
office: 785.410.3317
www.catonenergy.com
*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
Behalf Of *Daric Adair
*Sent:* Tuesday, August 18, 2015 6:36 PM
*To:* Kathryn Kerns; Sharad Kumar; equest-users at lists.onebuilding.org
*Subject:* Re: [Equest-users] Is it possible to disable PSZ heat pump hot
gas reheat dehumidification and enable PSZ electric coil dehumidification?
Kathryn;
Interesting experiment on the heating. And good investigative work.
Your results confirm the point made in that hot gas reheat is more
efficient than using electric heating for dehumidification. If the only
heat source for the PSZ HPs is the Heat Pump, which has a COP greater than
1, then it will be more efficient than electric heat which has a COP of 1.
Thus, the electric heating consuming 2 times the amount of heating energy
as compared to the heat pumps is valid and is within the range of
expectations.
The ‘why’ part is still incorrect on the basis that the reading is
incomplete. While 6.5.2.3 does allow for simultaneous heating and cooling
for dehumidification, the wording of G3.1.3.1 on electric heat control is
just as important. The Appendix G-2007 G3.1.3.1 wording:
“Electric air-source heat pumps shall be modeled with electric auxiliary
heat. The systems shall be controlled with a multistage space thermostats
and an outdoor air thermostat wired to energize auxiliary heat only on the
last thermostat stage and when outdoor air temperature is less than 40°F.”
The wording states that the electric heat should not operate when the
outside air temperature is above 40°F. It doesn’t matter than it doesn’t
explicitly say “dehumidification” and that is also covers the case of
decreasing heat output from a heat pump as outside air temperature drops.
It says electric heating cannot operate when the outside air temperature is
above 40°F. Typically when we are worried about dehumidification the
outside air temperature is greater than this. Additionally, if we are
dehumidifying we’re running a cooling coil; which means the hot gas reheat
is possible; and should be used. By forcing electric heat to run when hot
gas reheat is available will increase the energy consumed for baseline
heating, as your experimental results showed.
I am hopeful others can chime-in here and add to the discussion.
Maybe also address the fact that Dehumidification appears not be directly
address in Appendix G or a LEED CI. Can anyone confirm this?
Thanks,
DARIC R. ADAIR PE, C.E.M.
*Henderson Engineers, Inc. | *Mechanical Engineer, Energy Analyst
*dir* 913 742 5530 *tel* 913 742 5000 *fax* 913 742 5001 *tx id* #F-001236
*email* daric.adair at hei-eng.com *www.hei-eng.com* <http://www.hei-eng.com/>
Licensed in KS.
*From:* Kathryn Kerns [mailto:kathryn.kerns at bceengineers.com
<kathryn.kerns at bceengineers.com>]
*Sent:* Tuesday, August 18, 2015 4:59 PM
*To:* Daric Adair <Daric.Adair at hei-eng.com>; Sharad Kumar <
sharadcapricious at gmail.com>; equest-users at lists.onebuilding.org
*Subject:* RE: [Equest-users] Is it possible to disable PSZ heat pump hot
gas reheat dehumidification and enable PSZ electric coil dehumidification?
Daric, the “how” you have listed below is very interesting and I will try
it out. I am only guessing that the extra heating energy that appears in
the output files when the PSZ heat pump type with dehumidification active
is gas reheat. The only things I do know about using the PSZ heat pump
option is that: 1) extra heating energy occurs when dehumidification active
and 2) the amount of extra heat added for dehumidification does not use a
COP of 1. I know 1) is correct because I can see the difference in the
output files and I know 2) is correct because I performed and experiment
and can see the difference in the output files. In my experiment, I created
two duplicate energy models and substituted the PSZ DX electric coil option
for PSZ heat pump option, used heat pump cooling EIRs for DX EIRs, and
left one model with no dehumidification and one model with dehumidification
active. The output files show that the amount of heat energy providing
dehumidification for the PSZ DX electric coil models is almost 2 times
larger than the amount of heat providing dehumidification for the PSZ heat
pump models. From this, I have to conclude that the PSZ heat pump option
is using something else besides electric heat for dehumidification. I can’t
think of anything other heating source, so I am guessing hot gas reheat.
As to the “why”; it is important to get the most accurate baseline per the
LEED directions as possible in order to maximize energy savings. The
baseline directions for the auxiliary heater going off and on at 40 F etc.
has everything to do with making sure that baseline and proposed heat pumps
operate consistently at low outside air temperatures. This has nothing to
do with providing a dehumidification cycle. That is why the exceptions for
dehumidification show up in a different section of ASHRAE 90.1. The
dehumidification electric coil and the heat pump auxiliary electric heating
coil don’t have to be the same device.
This is definitely not gaming the system but an effort to make sure that
our proposed desiccant DOA/VRF HVAC system is being properly compared to a
multitude of little baseline heat pumps in a building located in a damp
climate. This is a case that has come up repeatedly for our company when
designing buildings in Hawaii and Texas.
Thanks for the input. I will try messing with ‘MAX-COND-RCVRY’. I just wish
I knew for sure that the additional dehumidification heat has something to
do with hot gas reheat.
*Kathryn Kerns*
*Systems Specialist*
*BCE** Engineers, Inc.*
*| Ph: 253.922.0446 | Fx: 253.922.0896 |*
*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org
<equest-users-bounces at lists.onebuilding.org>] *On Behalf Of *Daric Adair
*Sent:* Tuesday, August 18, 2015 2:02 PM
*To:* Sharad Kumar <sharadcapricious at gmail.com>;
equest-users at lists.onebuilding.org
*Subject:* Re: [Equest-users] Is it possible to disable PSZ heat pump hot
gas reheat dehumidification and enable PSZ electric coil dehumidification?
Kathryn;
>From your original message you are asking if there is a way to lock out
condenser heat recovery and exclusively use electric reheat as part of the
humidification sequence. Is this correct? There is a method that might
work, but I wanted to confirm the thought process first.
I reviewed the referenced 6.5.2.3 Exceptions b and c sighted, where it
allows reheat based on combinations of capacity & turndown. The fact that
is not necessarily clear is why you believe the use of electric heat
should/would take less energy than the use of the ‘free’ heat that hot-gas
reheat offers. The electric heat consumes power. The hot gas reheat is
‘free’ for use since the heat generated by the compressor & refrigeration
process must be rejected by the condenser. Directing a portion of this heat
back into the dehumidified, but overcooled, airstream would be more
efficient in most cases.
Also important to note is that you reference ‘baseline’ systems as falling
into this exception. If this is the situation, then the 6.5.2.3. exception
does not apply due to G3.1.3.1. which states that electric auxiliary heat
is only allowed below 40°F. There is also a LEED CI that states that the
compressor will continue to function in concert with the electric heat down
to 17°F. Below this, only electric heat is required. The use of electric
heat as reheat as part of a dehumidification sequence in this case is not
allowed. A review comment on the source of reheat wouldn’t be out of the
realm of possibility. At a minimum, this feels like ‘gaming the system.’
The only potential wedge issue that could be argued is if your Proposed
building was DX and not using hot gas reheat for some reason. Appendix G
does not appear to have a section dedicated to
Humidification/Dehumidification that I can tell. Please correct me if I
have overlooked/missed this. I didn’t see it in 2007, 2010, or 2013.
Now, back to the ‘how’. Under the Systems > Heating > Cap Curves/Waste Ht
tabs. In the Waste Heat section, change the ‘Waste Heat Use’ to space heat.
This should, but doesn’t, enable the ‘Max Cond Recvry for Spc Heat’ input.
The keyword is ‘MAX-COND-RCVRY’. Take this into the .inp file and paste
this keyword with ‘ =0.25’ or whatever your recovery ratio is in the System
descriptor. Reload the model. This input box is now ‘active’, and can be
modified to fit design documents. To lock it out, I would think a values of
0.0 tells eQuest that none of the heat can be recovered.
Anyone else have any input on the above two things? [The ‘Why’ of doing
this and the ‘How’ of actually doing it]. Would be curious to hear other’s
thoughts…
Thanks,
DARIC R. ADAIR PE, C.E.M.
*Henderson Engineers, Inc. | *Mechanical Engineer, Energy Analyst
*dir* 913 742 5530 *tel* 913 742 5000 *fax* 913 742 5001 *tx id* #F-001236
*email* daric.adair at hei-eng.com *www.hei-eng.com* <http://www.hei-eng.com/>
Licensed in KS.
*From:* Sharad Kumar [mailto:sharadcapricious at gmail.com
<sharadcapricious at gmail.com>]
*Sent:* Tuesday, August 18, 2015 1:24 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* Re: [Equest-users] Is it possible to disable PSZ heat pump hot
gas reheat dehumidification and enable PSZ electric coil dehumidification?
Hi Kathryn,
As per me there is no system such as heat pump hot gas reheat
de-humidification system.
The de-humidification system mainly consists of a cooling system (cooling
coil) which can form condensate to reduce the moisture content of air and
segregate it from air.
In your case the gas reheat de-humidification can be considered as
electrical coil humidifier which can bring good enough savings.
In the electric coil humidification the heat source for latent heat of
vaporization is electricity or in case of furnace, gas or fuel is the
source of heat for water to be vaporized and so on.
Thanks,
Sharad.Kumar.
Green Horizon Consulting LLP
Gurgaon
India
On Tue, Aug 18, 2015 at 1:31 AM, <equest-users-request at lists.onebuilding.org>
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Today's Topics:
1. Is it possible to disable PSZ heat pump hot gas reheat
dehumidification and enable PSZ electric coil dehumidification?
(Kathryn Kerns)
---------- Forwarded message ----------
From: Kathryn Kerns <kathryn.kerns at bceengineers.com>
To: "Marlin.Addison at asu.edu" <Marlin.Addison at asu.edu>, "
kevinm at madisonengineeringps.com" <kevinm at madisonengineeringps.com>, Jeff
Hirsch <Jeff.Hirsch at doe2.com>
Cc: "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>,
Chris Caffee <chris.caffee at bceengineers.com>
Date: Mon, 17 Aug 2015 18:49:34 +0000
Subject: [Equest-users] Is it possible to disable PSZ heat pump hot gas
reheat dehumidification and enable PSZ electric coil dehumidification?
Everyone, does anyone know of a way to disable PSZ heat pump hot gas reheat
dehumidification (at least I think that is what the extra heating energy
represents) and enable electric coil dehumidification reheat? Most of my
LEED baseline heat pump energy models with dehumidification usually have
thermal blocks HVAC units that fall into the ASHRAE 90.1-2007 section
6.5.2.3 exception b and c categories, which allows the standard
dehumidification simultaneous heating and cooling cycle. I am missing out
on energy savings.
Thanks,
*Kathryn Kerns*
*Systems Specialist*
*BCE** Engineers, Inc.*
*| Ph: 253.922.0446 | Fx: 253.922.0896 |*
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