Setting DOAS system type airflow rates

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Caution note when using DOAS systems - you cannot simply define the OA volume in the flow parameters tab of the DOAS fan.

In DOE-2.3, there is now a system type 'DOAS' that will properly provide outdoor air either directly to zones or to the mixing section of the system it serves. The volume of outdoor air delivered by this system is determined by the outdoor air flow rates of the zones that take air from this system. The screenshot below suggests that the unit will be delivering 6,545 cfm of OA but in fact this system was only delivering 500 cfm as the served zones did not have their OA volumes set properly.

[cid:image003.jpg at 01D509B3.D8F50570]

When the OA was defined in the zones taking air from this system, the volume of OA in the DOAS was as expected.

Today's lesson - define outdoor air rates properly, and hourly reports are your friend.

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Brian Fountain P.Eng., LEED AP BD+C, BEMP
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Brian.Fountain at sa-footprint.com

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Thanks, Brian! You make an important point on design sizing of DOAS systems - the hourly ventilation rate is determined by the systems/zones that the DOAS serves, and the corresponding ventilation controls for those systems.
Some additional observations:

* If you enter the DOAS SUPPLY-FLOW (Design cfm):
* The design fan power will be based on the SUPPLY-FLOW multiplied by SUPPLY-KW/FLOW (if entered).
* Any central airhandler coils will be autosized based on the SUPPLY-FLOW. (See the SV-C report in the SIM file.)
* If instead, you let the DOAS SUPPLY-FLOW (Design cfm) default by leaving it blank:
* The design fan power will be based on the design ventilation rate determined by the served systems multiplied by SUPPLY-KW/FLOW (if entered).
* Any central airhandler coils will be autosized based on the design ventilation rate.
* If you enter zone ventilation as OA/FLOW-PER (cfm per person), to simulate DCV or to size the ventilation per ASHRAE 62.1 guidelines, the design ventilation rate will be based on the highest hourly value in the occupancy schedule. So you have to have at least one hour with 1.0 occupancy either in the weekday, weekend and/or CDD/HDD schedules. Otherwise, the design ventilation rate will be reduced.

~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
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Thanks Bill,
Re. DOAS coils - is there an option to specifically size the coils?

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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Hi Chris.

Yes - you can explicitly set the heating and/or cooling capacities of the DOAS system type.

[cid:image002.jpg at 01D50B03.D24CE5C0]

Cheers,

Brian

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Brian Fountain
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For my most recent project for which I have a DOAS system, I've noticed a
constant (8760 hrs) heating energy consumption. I've got the DOAS set up as
a gas-fired furnace and DX cooling coil. I've set my auxiliary fuel
consumption set to 0 and no preheat. During the summer where there is
active cooling my furnace is running at about 19% of it's design capacity
(simultaneous heating and cooling). Has anyone noticed their DOAS system
doing this? Any suggestions?

Thanks,
Chris

On Wed, May 15, 2019 at 9:52 AM Brian Fountain via Equest-users <

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A fast shotgun guess: Is it possible the boiler(s) serving that hot water loop feature a 5:1 turndown and the loop operation is set to standby, meaning the boiler(s) can?t turn off completely?

~Nick

[cid:image001.png at 01D50BD6.7DD4C2A0]
Nick Caton, P.E., BEMP
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Energy and Sustainability Services
Energy Performance Contracting
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Ah sorry, I retract my guess ? I read too fast and my eyes locked to Brian?s demonstrative screenshot, which had a hot water loop indicated for heating.

Taking a step back, have you tried some of the custom hourly outputs specific for DOAS systems? It may be helpful to figure out if this gas is being consumed 24/7, in sync with system operations, day vs. night, etc.

One new thought is that this might be a piece of subcooling and reheat to accomplish dehumidification ? something to look out for in those hourlies.

~Nick

[cid:image001.png at 01D50BDF.71E42F90]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Regional Energy Engineering Manager
Energy and Sustainability Services
Energy Performance Contracting
D
M
F
E
913 . 564 . 6361
785 . 410 . 3317
913 . 564 . 6380
nicholas.caton at se.com
15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States
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Hi Nick,

It was with the custom hourly outputs specific for DOAS systems that I
noticed this behaviour. I have no dehumidification controls specified (and
no dehumidification reheat energy indicated from my hourly reports). My
DOAS fan is set to run 24/7. The hourly reports show a heating load (and
corresponding fuel consumption) all hours of the year, even during cooling
hours. Using the hourly outputs available, I am unable to find out the
source of this heating load...really not sure what to make of it.

Chris

On Thu, May 16, 2019 at 1:04 PM Nicholas Caton
wrote:

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It's been suggested I share my pd2 and inp (and weather) file to allow
others to perhaps help with the troubleshooting. Those 3 files have been
attached. I am using build 7175, DOE 2.3.

Thank you to all that take a look and provide suggestions.

Chris

On Thu, May 16, 2019 at 12:00 PM Nicholas Caton
wrote:

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Your COOL-SET-T and MIN-SUPPLY-T are both defaulting to 65 F, which results in cooling and reheating. The solution is to drop your heating setpoint slightly and enter cooling setpoints that are slightly higher than the heating setpoint, as in the attached file.

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That does fix the issue - thank you Doug!

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