DOAS and Fan Coils

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Howdy,

I'm trying to simulate a four pipe fan coil system with a DOAS ERV providing outdoor air in DOE 2.3 and getting some errors I haven't been able to figure out, I'm hoping someone here knows the answer!

The fan coil system runs by itself, connected to a campus CHW and steam loop, though I get the following error in my sim file:

***ERROR***********************************************************************
Heat-Exchanger: L1West Perim Zn (G.W1) UA calculation
failed to converge.

This seems to be a random zone and switches every time I run it. Besides defining OA flow at the zone, there are no other custom inputs at this time on the zone. When I add the DOAS (without the ERV) to the "OA From" field in the fan coil system, I get an aborted simulation and the following errors:

***NOTE************************************************************************
Convergence issues in Design-Day mode 14
Hours Slow: 3209; Hours Failed: 0

***ERROR***********************************************************************
Heat-Exchanger: HENE Perim Zn (M.ENE16) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HSouth Perim Zn (Mb.S22) UA calculation
failed to converge.

***ERROR***********************************************************************
Fatal HVAC simulation errors.

Adding the ERV to the DOAS gets me a new set of errors:

***NOTE************************************************************************
Convergence issues in Design-Day mode 14
Hours Slow: 3384; Hours Failed: 1834

***ERROR***********************************************************************
Heat-Exchanger: HNorth Perim Zn (M.N17) UA calculation
failed to converge.

**WARNING**********************************************************************
PredictX parabolic solution failed. Please
send this input and weather file to program developers 3015
***ERROR***********************************************************************
Heat-Exchanger: HEast Perim Zn (T.E39) UA calculation
failed to converge.

***ERROR***********************************************************************
Fatal HVAC simulation errors.

Same notes and warnings as before but it adds the PredictX warning around 50 times (I edited the above).

I tried stepping back to DOE 2.2 but of course you can't define a DOAS in that engine and in 2.2 and 2.3 I'm unable to put an ERV on a fan coil system type.

So then, can anyone shine a light on what's going on?

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Building Efficiency Engineer | TBCx

E-mail: msg at tbcxinc.com
Mobile: 801-907-5654
Direct: 801-401-9492

324 S. State Street, Suite 400
Salt Lake City, UT 84111
www.tbcxinc.com

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Matt,

Well-documented!

It's been somewhat established through a couple threads over the past few months that "convergence errors" can sometimes, if not always, be related to difficulties in the system sizing portion of the simulation, and in those scenarios may be resolved by providing DESIGN-DAY inputs and/or manually specifying coil capacities you are relying on eQuest to auto-size.

You might check this possibility off the list by navigating to your first tab (Project & Site), deleting any design days you see there in the component tree, then importing the attached to see if that has any effect.

I believe references to convergence in the errors can also sometimes be a red herring, but in this case I'm not seeing some related telltale language in those screen snips. If you do encounter something to the effect of "MsgBomb call argument: " followed by some number of digits that is something you'll want to document/share as well.

~Nick

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Thanks for the responses! Unfortunately adding design days didn't help. I still get the following errors:

With just four pipe fan coil system:
**WARNING**********************************************************************
DOAS System: ERV-1 does not serve any
any systems (or zones), and will not be simulated.

**WARNING********************************************************************** (This x50)
PredictX parabolic solution failed. Please
send this input and weather file to program developers 3015
***ERROR***********************************************************************
Heat-Exchanger: HSouth Perim Zn (T.S40) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HWSW Perim Zn (G.WSW5) UA calculation
failed to converge.

***ERROR***********************************************************************
Fatal HVAC simulation errors.

With the DOAS serving the fan coil system (No heat recovery) and delivering OA to conditioned zones:
***NOTE************************************************************************
Convergence issues in Design-Day mode 14
Hours Slow: 8; Hours Failed: 0

***ERROR***********************************************************************
Heat-Exchanger: HWest Perim Zn (M.W10) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HEast Perim Zn (M.E12) UA calculation
failed to converge.

***ERROR***********************************************************************
Fatal HVAC simulation errors.

With the DOAS and heat recovery serving the fan coil system:
***NOTE************************************************************************
Convergence issues in Design-Day mode 14
Hours Slow: 7; Hours Failed: 138

**WARNING********************************************************************** (this x50)
PredictX parabolic solution failed. Please
send this input and weather file to program developers 3015

***ERROR***********************************************************************
Heat-Exchanger: HSouth Perim Zn (M.S13) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HWSW Perim Zn (T.WSW41) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HNorth Perim Zn (G.N8) UA calculation
failed to converge.

***ERROR***********************************************************************
Heat-Exchanger: HSouth Perim Zn (Mb.S22) UA calculation
failed to converge.

***ERROR***********************************************************************
Fatal HVAC simulation errors.

I tried switching the weather file (This was a custom built file) and got the same errors so I don't think the error lies there. I've attached the INP, PD2 (3.65 build 7175) and weather file for anyone with the time and desire to look into it.

Thanks again for the help, you guys rock!

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Mobile: 801-907-5654

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Noticed in your .inp file that you have a CHW design temperature of 14F.
Maybe that's the problem? Try something like 44F.

-Erik

*Erik Kolderup, PE, LEED AP*
erik at kolderupconsulting.com | 415.531.5198 | www.kolderupconsulting.com

On Fri, Jan 17, 2020 at 8:21 AM Matthew S. Garlick via Equest-users <

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Great catch! Same aborted simulation though?

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Mobile: 801-907-5654

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I got it to simulate.

* Changed the CHW design temperature to 44F per Erik?s suggestion.
* Set the DHW heater capacity to -1.0 (1 MMBtu/h). The tank volume of 3239.75 gallons is very large ? maybe it was interfering with the design sizing.
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

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