How to model a DOAS serving corridors with a Chilled Water Coil

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Hi Julien,

I think there are multiple options that could fit the criteria you?ve
provided, more than one could work depending on circumstances of your
project?

*General* advice for ?what system should I use? queries follows.

1. Ask a few questions of the system equipment/design:

- How the actual system is controlled ? is it constant volume with
heating/cooling triggered via one space/duct thermostat only, or something
more complex?

- Do you have just one central fan or additional fans in each zone
for independent circulation?

- One set of central heating/cooling coils or are there separate
reheat / cooling coils occurring at individual zones?

2. QC your work:

- Answer for yourself how the system?s fans/cooling/heating should
operate, and when/to what degree ventilation OA is delivered

- After initially setting up your systems/zones (or perhaps the
first set of many), perform some test runs to QC your efforts

- Verify each intended behavior/output via hourly reporting

Considering you can generally force 100% OA and constant volume with
system and/or zonal minimum OA/flow ratio inputs, as appropriate, peruse
the help file diagrams for systems of interest that include features of
interest. You probably want to consider/review the articles for FC, VAVS,
and PIU.

to leverage a 2-pipe loop set to an outdoor snap temperature (or perhaps a
seasonal schedule) ? see the operation tab under the loop in detailed edits.

Hope that helps get you started!

~Nick

*NICK CATON, P.E.*
*Owner*

*Caton Energy Consulting*
1150 N. 192nd St., #4-202

Shoreline, WA 98133
office: 785.410.3317

www.catonenergy.com

*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
Behalf Of *Julien Marrec
*Sent:* Monday, December 08, 2014 3:29 AM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] How to model a DOAS serving corridors with a
Chilled Water Coil

Hi,

I've got a building with a DOAS (AHU with 100% outside air) serving
corridors (no reheat, no fan in there, just ventilation grilles). The DOAS
has one coil, served by hot water in winter and cold water in summer.

There is no economizer, and the supply temperature is simply set by a
thermostat set as 70?F year round.

I have modeled DOAS before but I was using a PSZ or a PVVT. Those have DX
cooling coils, so it's not an option.

Seems like I could use MSZ or VAV for example, but I'm not sure which would
be more appropriate.

What type of system do you recommend using in my case?

How would you control it in order to get the proper supply temperature
(70?F) and avoid simultaneous heating and cooling (Is using an availability
schedule the only option?)?

Also, if you have an .inp snippet I could use to set it up correctly, I'll
greatly appreciate it!

Thanks,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

Nicholas Caton's picture
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Hi,

I've got a building with a DOAS (AHU with 100% outside air) serving
corridors (no reheat, no fan in there, just ventilation grilles). The DOAS
has one coil, served by hot water in winter and cold water in summer.
There is no economizer, and the supply temperature is simply set by a
thermostat set as 70?F year round.

I have modeled DOAS before but I was using a PSZ or a PVVT. Those have DX
cooling coils, so it's not an option.
Seems like I could use MSZ or VAV for example, but I'm not sure which would
be more appropriate.

What type of system do you recommend using in my case?

How would you control it in order to get the proper supply temperature
(70?F) and avoid simultaneous heating and cooling (Is using an availability
schedule the only option?)?

Also, if you have an .inp snippet I could use to set it up correctly, I'll
greatly appreciate it!

Thanks,
Julien
--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

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Bill,

There is nothing fancy in my AHU. Just a fan and a coil. Always 100%
outside air, the same amount of CFM (no economizer), and no return airflow.
Discharge temperature is fixed (Tstat in discharge air stream is
controlling a three way valve on the coil). The system doesn't know and
doesn't care about what's happening in the space it serves.

As Nathan was saying, I was thinking about using a Dummy zone to control
the discharge temp indeed, like I usually do with PSZ systems: you have to
give it a control zone, and the dummy serves this purpose.

But as I'm trying to switch from VAVS to 2pipe FC right now, I realize this
isn't the way it works with FC. I think I'll keep my VAVS solution for now.
It seems too cumbersome to change it now and I'd have to attribute an
outdoor air CFM numbers to all my zones served by the system.

Thanks all,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2014-12-08 16:38 GMT+01:00 Nathan Miller :

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I honestly don?t know, which is why I asked. I have never needed to use a dummy zone, but am always open to the possibility that it is a useful workaround for (something). I?ll check out the video.
Thanks,
Bill

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Thanks for your message Nick.

I now have a 2-pipe loop with the SNAP-T, thanks for pointing it out.

David Elridge also wrote that I could just subsitute PSZ for a 2pipe FC.

I've got something working right now that is based on VAVS.

I have read the help file before posting.I understand there are many ways
to skin a cat in eQuest, but I'm still confused what fundamental difference
it would make between using a VAVS or using a 2pipe FC with a dummy zone.
Aside from the fact that the dummy zone setup is easier to implement I
guess... Is there a wrong way to do this? Or just a better way?

Thanks,
Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2014-12-08 13:20 GMT+01:00 Nicholas Caton :

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Julien,
Why would you model this with a dummy zone? If your DOAS is only supplying the corridor, just model the corridors as separate zones and assign the DOAS system to them.
You didn?t say whether or not the DOAS has heat recovery. That will further dictate which system type you select. FC does not allow heat recovery, but VAVS and Single Zone Reheat do.
Regards,
Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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I personally use dummy zones on corridor units when I?m trying to model a system that uses discharge air temperature control rather than one that responds to a thermostat in the corridor. On single zone systems with control given to a dummy zone (with no internal or envelope loads), you can force the system to discharge air with-in a specific temperature band year-round. Of course you then have to give the actual corridor spaces a t-stat schedule that is rather accommodating or else you might end up with lots of unmet load hours.

Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com

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How does using a dummy zone give you extra capability and/or accuracy modeling discharge air temperature control, and how is the simulated energy consumption more accurate?
Thanks,
Bill

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I'm in no way an expert here, otherwise I wouldn't be asking for help.

But I don't think it does anything you can't do otherwise. Last time I
switched between a PVAVS and PSZ and was able to recreate the same results
(PSZ with dummy zone, and PVAVS with some more complex steps to set
discharge temp correctly. The PVAVS solution is more flexible though).

It's just easy to use a dummy zone, it's what comes up mostly when
searching for "equest" and "doas" keywords, and there's even a tutorial
video .

Do you think differently Bill?

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr

2014-12-08 17:16 GMT+01:00 Bishop, Bill :

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Many of the projects I model have 100% outside air corridor units that only deliver tempered air (60-70 degrees, year round. It is easy to check the total annual heating energy (or cooling I suppose, but I primarily do work in heating driven climates) of the system against a bin-calc.

I?ve found that when I give system t-stat control to one of the corridors, the amount of annual heating energy comes up way short. Seems to be some combination of heat gain from the adjacent spaces (fully conditioned) and lighting gains, etc. Additionally when I check the hourly reports, I can see that the supply air temperature is bouncing around, even if I?ve specified max and min supply temps, etc. When I use the dummy-zone approach, my total annual heating use pretty much lines right up with my bin calcs, and I can verify in the hourly reports that SAT stays right where I want it.

Nathan Miller, PE, LEED AP BD+C ? Mechanical Engineer/Senior Energy Analyst
RUSHING | D 206-788-4577 | O 206-285-7100
www.rushingco.com

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