[Equest-users] Is it possible to disable PSZ heat pump hot gas reheat dehumidification and enable PSZ electric coil dehumidification?

Kathryn Kerns kathryn.kerns at bceengineers.com
Tue Aug 18 14:59:27 PDT 2015

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] 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?


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 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 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…

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]
Sent: Tuesday, August 18, 2015 1:24 PM
To: equest-users at lists.onebuilding.org<mailto: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.

Green Horizon Consulting LLP

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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<mailto:kathryn.kerns at bceengineers.com>>
To: "Marlin.Addison at asu.edu<mailto:Marlin.Addison at asu.edu>" <Marlin.Addison at asu.edu<mailto:Marlin.Addison at asu.edu>>, "kevinm at madisonengineeringps.com<mailto:kevinm at madisonengineeringps.com>" <kevinm at madisonengineeringps.com<mailto:kevinm at madisonengineeringps.com>>, Jeff Hirsch <Jeff.Hirsch at doe2.com<mailto:Jeff.Hirsch at doe2.com>>
Cc: "equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>" <equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>>, Chris Caffee <chris.caffee at bceengineers.com<mailto: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 exception b and c categories, which allows the standard dehumidification simultaneous heating and cooling cycle. I am missing out on energy savings.


Kathryn Kerns
Systems Specialist
BCE Engineers, Inc.
| Ph: 253.922.0446 | Fx: 253.922.0896 |

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