Negative humidity in DOE reference model

asked 2018-06-13 12:23:47 -0600

mldichter
2565 ●2 ●11

updated 2018-06-13 13:39:25 -0600

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

A Calculated Humidity Ratio= -410.2300 warning occurs in the err file for the DOE reference model StandAloneRetail. This is one of the DOE reference models WITHOUT modification, so any idea what's wrong?

Strangely, the warning only appears for the last design day in the model, but if I delete the offending design day, then a different design day is referenced. I think the error is occurring for all design days, but Energyplus is only reporting the very last design day in the IDF file.

Here is a google drive link for the zip of the idf and generated files.
DOE_StandAloneRetail

Here is a google drive link for the idf that has been significantly cleaned up with different design days.
cleaned up DOE_StandAloneRetail

Both the err files have the same warning.

answered 2018-06-19 15:00:51 -0600

MJWitte

9740 ●6 ●10 http://www.gard.com/

In a nutshell, for this model, the thermostat cooling setpoint for Front_Entry ZN is always 100C, an attempt to communicate no cooling for this zone. This results in the psychrometric routines attempting to find the humidity ratio at a dewpoint of 100C which stretches the boundaries of the function, resulting in the negative humidity ratio. Change those 100s to 99 and the error goes away.

If you want more details . . .

After all of the sizing periods are simulated for sizing, there are some final zone accounting steps to set the zone conditions at the heating and cooling peak. If the zone has no cooling load, the zone temp at cooling peak is set to the minimum value in the thermostat profile . . . blah blah blah . . . and because it's happening after the sizing periods and before the next simulations, that's why the timestamp says <last design="" day="" name=""> at Simulation time=07/21 24:00 - 24:10 (which even in the EnergyPlus world is not a real timestep). A new issue #6787 has been posted to revisit the error check for function PsyWFnTdpPb (return humidity ratio as a function of dry-bulb and pressure) when the saturation pressure comes back greater than the barometric pressure (which is the case here).