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Unable to size cooling tower

asked 2015-10-22 09:32:54 -0500

bbrannon4 gravatar image

updated 2018-01-18 03:24:05 -0500

I am using CBECC-Com for Title 24, though I think this is an E+ (v8.1) error. The actual error is:

* Severe * Bad starting values for UA

* Fatal * Autosizing of cooling tower UA failed for tower COOLING TOWER

From what I can tell all the plant sizing and cooling tower inputs are there and reasonable, but I'm not even sure where to look, as there is only one instance of this error I can find online and adjusting the loop temperatures didn't seem to affect it.

I'd appreciate any thought on where to look for the cause. I did find in the code that it is in response to a "-2" error code from the solver, but I can't find what triggers that.

For reference, here are my CT and sizing objects

Cooling Tower,           !- Name
CW Pump Outlet Node,     !- Water Inlet Node Name
Cooling Tower Outlet Node,  !- Water Outlet Node Name
,                        !- Design Water Flow Rate {m3/s}
29.454239930112,         !- Design Air Flow Rate {m3/s}
11185.5,                 !- Design Fan Power {W}
,                        !- Design U-Factor Times Area Value {W/K}
Autosize,                !- Free Convection Air Flow Rate {m3/s}
0.1,                     !- Free Convection Air Flow Rate Sizing Factor
0,                       !- Free Convection U-Factor Times Area Value {W/K}
0.1,                     !- Free Convection U-Factor Times Area Value Sizing Factor
NominalCapacity,         !- Performance Input Method
1.25,                    !- Heat Rejection Capacity and Nominal Capacity Sizing Ratio
3011305.24601958,        !- Nominal Capacity {W}
0,                       !- Free Convection Capacity {W}
0.1,                     !- Free Convection Nominal Capacity Sizing Factor
0,                       !- Basin Heater Capacity {W/K}
2,                       !- Basin Heater Setpoint Temperature {C}
,                        !- Basin Heater Operating Schedule Name
,                        !- Evaporation Loss Mode
0.2,                     !- Evaporation Loss Factor {percent/K}
0.008,                   !- Drift Loss Percent {percent}
,                        !- Blowdown Calculation Mode
3,                       !- Blowdown Concentration Ratio
,                        !- Blowdown Makeup Water Usage Schedule Name
,                        !- Supply Water Storage Tank Name
,                        !- Outdoor Air Inlet Node Name
FanCycling,              !- Capacity Control
2,                       !- Number of Cells
MinimalCell,             !- Cell Control
0.33,                    !- Cell Minimum  Water Flow Rate Fraction
2.5,                     !- Cell Maximum Water Flow Rate Fraction
1;                       !- Sizing Factor

Condensor Water Loop,    !- Plant or Condenser Loop Name
Condenser,               !- Loop Type
26.1111111111111,        !- Design Loop Exit Temperature {C}
5.55555555555556;        !- Loop Design Temperature Difference {deltaC}
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As it turns out, if I make the cooling tower variable speed, CBECC-Com has no issue, but I'm still not sure the specific issue in the constant speed one that is causing this.

bbrannon4 gravatar imagebbrannon4 ( 2015-10-23 00:18:32 -0500 )edit

Try to autosize the Free Convection Capacity, instead of 0.0.

Archmage gravatar imageArchmage ( 2016-02-10 10:39:40 -0500 )edit

@Archmage, as this is a CBECC-Com model the user isn't given the flexibility to change these inputs. That said, it's probably worth trying it natively in EnergyPlus and if that fixes it then @bbrannon4 you may want to send a message over to the CBECC-Com development team.

pflaumingo gravatar imagepflaumingo ( 2016-02-11 11:06:48 -0500 )edit

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answered 2018-01-18 00:07:01 -0500

bbrannon4 gravatar image

@rraustad was correct in terms of energyplus errors, but this turned out to be a CBECC-Com error and so I didn't have control to fix it. Like most things with CBECC-Com, bI just had to edit the inputs to be as close as possible while preventing errors. I sent a note to the development team.

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answered 2016-02-08 19:38:23 -0500

updated 2016-02-11 08:17:53 -0500

The "-2" means that the initial guess at the bounds for UA were incorrect. More likely, something else caused the result to fall outside the initial bounds. For cooling towers, the guess at UA is that the final UA will fall between the DesignLoad and 0.0001 * DesignLoad. You can see this methodology in the RegulaFalsi call.

UA0 = 0.0001 * DesTowerLoad; // Assume deltaT = 10000K (limit)
UA1 = DesTowerLoad; // Assume deltaT = 1K
SolveRegulaFalsi( Acc, MaxIte, SolFla, UA, SimpleTowerUAResidual, UA0, UA1, Par );

The model first simulates the tower using UA = 0.0001 * DesignLoad to get the tower capacity. Then UA = DesignLoad is used to get a new tower capacity. The answer for these 2 simulations should be under, then over, the actual DesignLoad. If not, a "-2" results.

As I said, it's usually something else that causes this to happen.

If you send your idf file I can look further or you can send your file to the HelpDesk or via email at

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Asked: 2015-10-22 09:32:54 -0500

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Last updated: Jan 18