Using inlet fan option and outlet fan option

I am working on a simulation in which I have both inlets modeling fans and outlets modeling fans.
The simulation runs fine and converges well when I have both the inlets and outlets set to fixed velocities that are surface normal. However, I do not get pressure readings that are realistic at all (well beyond what the fans are actually capable of generating).
I have tried even using the the default fan-curve points that Caedium loads automatically when the fan option is chosen - to make sure that I am not loading the data erroneously - but the solver won't run. It appears to fail on the first iteration; invariably I get an error message such as: "Warning:
volume_157 [www.1793.5] RANS Flow: Failed
Solver singularity: smoothSolver: Solving for Ux, Initial residual = nan, Final residual = nan, No Iterations 20."
I even created a very simple simulation involving a box with one side being a fan inlet and the opposite side being a fan outlet, and I still got this kind of error message.
Do any other settings need to be changed when the fan options are utilized?
Thank you in advance for any help.

Paired inlet and outlet fan conditions will not work

A fan inlet matched to a fan outlet is ill posed, that is why you are seeing unrealistic pressures and failures. You need to either add a normal outlet (vent) in addition to the fan pair, or reassign the fan outlet (or inlet) to be a default outlet (or inlet).

Thank you. I tried both

Thank you. I tried both options, but still get the same error message. I will troubleshoot further and see what I can come up with.

No matter what, I continue to

No matter what, I continue to have problems using the fan mode of either the inlet (or outlet) boundary conditions.
I simply cannot get either to work.
I keep getting an error message as noted above.
Is there a tutorial example involving an inlet working in fan mode (or an outlet working in fan mode)? I looked but didn't see it if there is one.
Thank you very much for any help.

Try default inlet and outlet conditions

Have you tried a default inlet with a default outlet? I ask because the problem you are seeing may not be related to the fan condition.

Also is your simulation incompressible? I ask because the fan condition does not support compressible flow.

There are no tutorials for fans. The input parameters for fan conditions are SI units only, there is no unit conversion support for fans.

Is there anything else that is special/unusual about your simulation?

The issue turned out to be

The issue turned out to be the substance reference velocity.
Ideally, the simulation was to run in still air, with a reference air U of 0, 0, 0.
However, this didn't work.
I tried having the reference velocity be 0, 0, and 0.0001 ft/sec. Then the simulation ran. That was it!
I went back and forth repeatedly; it wouldn't run if U was 0, 0 ,0, and it always ran when just one of the values of the reference velocity of U was > 0.
I adjusted some of the relaxation factors and it has run about 800 iterations so far.
Thank you.