DX Cooling Coil Negative Bypass Factor Fatal Error

[TiejunWu]

Issue overview

I get the following error message when running a simulation with DX cooling coil.

** Warning ** Sizing: Coil:Cooling:DX:SingleSpeed "FURNACE ACDXCOIL 1": Rated air volume flow rate per watt of rated total cooling capacity is out of range.
** ~~~ ** Min Rated Vol Flow Per Watt=[4.027E-005], Rated Vol Flow Per Watt=[1.015E-005], Max Rated Vol Flow Per Watt=[6.041E-005]. See Input Output Reference Manual for valid range.
** Severe ** Coil:Cooling:DX:SingleSpeed "FURNACE ACDXCOIL 1" -- negative coil bypass factor calculated.
** ~~~ ** During Warmup, Environment=CHICAGO OHARE INTL AP ANN HTG 99% CONDNS DB, at Simulation time=01/21 00:00 - 00:10
** Fatal ** Coil:Cooling:DX:SingleSpeed "FURNACE ACDXCOIL 1" Errors found in calculating coil bypass factors

Following this UnmetHours post, looks like it is caused by the Rated Vol Flow Per Watt=[1.015E-005] very low. Following the suggestions in the post from @rraustad , I tried to reduce the cooling coil rated capacity and that got rid of the error.

However, I did not understand the physical meaning of the negative bypass factor and how that low Rated Vol Flow Per Watt could lead to a negative bypass factor. So I ran a parametric test with a simple example. In which I varied the Rated Total Capacity to see if there is a critical CFM/Ton below which the negative bypass factor error will always occur. Following is the result of that test:

From this set of results, it shows that the low CFM/Ton does not always lead to a negative bypass factor.

Then I checked the EnergyPlus source code where the coil Apparatus Dew Point temperature is solved iteratively. I think the randomness of negative bypass factor is a result of the convergence criteria of the iteration loop. The current criteria only requires the two slopes to be within 0.1% to exit the loop. This might result in a condition that the ADPEnthalpy is greater than the OutletAirEnthalpy and cause a negative bypass factor. I think there should be another criteria to require the ADPTemp to be smaller than the OuteltAirTemp.

I tested with the following convergence criteria and all the cases in the parametric study ran without that negative bypass factor fatal error
while ((Iter <= IterMax) && ((Tolerance > 0.001) || (ADPTemp >= OutletAirTemp))) {

Can you check the source code to see if it is appropriate to change that convergence criteria to avoid the negative bypass factor error?

Details

Some additional details for this issue (if relevant):

• Platform (Windows 10)
• Version of EnergyPlus (EnergyPlus 24.1, 64 bit)
• Unmethours link or helpdesk ticket number https://unmethours.com/question/47170/negative-coil-bypass-factor-calculated/

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• Defect file added (FurnaceWithDXSystem.idf)
• Ticket added to EnergyPlus Defect Complexity (Github Project)
• Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)