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# calculated humidity ratio invalid - routine unknown

The following warnings occurred in the err-file:

* Warning * Calculated Humidity Ratio invalid (PsyWFnTdpPb) * ~~~ * Routine=Unknown, Environment=RUN PERIOD 1, at Simulation time=06/29 16:40 - 16:41 * ~~~ * Dew-Point= 104.30 Pressure= 101300.00 * ~~~ * Calculated Humidity Ratio= -4.3881 ... Humidity Ratio set to .00001

*** * Warning * Entered Humidity Ratio invalid (PsyWFnTdpPb) *** * ~~~ * This error occurred 10913 total times; *** * ~~~ * during Warmup 0 times; *** * ~~~ * during Sizing 0 times. *** * ~~~ * Max=-0.92399 [] Min=-1077.851682 []

Although no reference is given to the object that causes the problem, it is apparently due to a Coil:Cooling:WaterToAirHeatPump:EquationFit with Gross Rated Sensible Cooling Capacity=3200W and default coefficients (as predefined in OpenStudio).

Can anyone give advice on what can create negative humidity ratios and how to remedy this ?

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After some analysis I found the following explanation: overheating in a thermal zone, as shown in the image below:

How did this happen ?

In the HVAC configuration a water-air heat pump cooling coil is used to heat a domestic hot water tank. The upper limit of the tank temperature is 60°C. However, when this maximum temperature is reached, operation of the cooling coil is not shut down by EnergyPlus, and the cooling coil produces heat at the water side, which EnergyPlus simply "dumps" into the zone where the water tank is located. Although no objects in the zone can be hotter than 60°C, this leads to temperatures of up to appr. 130°C. This is presumably the reason for the illegal humidity ratios warnings above.

After inserting a EMS programmed control that ensures shutting down the cooling coil whenever the water tank reaches maximum temperature, the problem disappeared completely.

One could debate whether there is a flaw in EnergyPlus algorithms that allows such overheating to happen, including continued cooling coil operation despite water-side overheating, but with this explanation of EnergyPlus behavior this exception can be dealt with by the user.

(remark: The idf-model used is a derivative from an OpenStudio model, where the air loop and a unitary heat-pump system was modified)

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FYI, based on the description of this field you can why the heat is going to the zone. I think the tanks setpoint temp schedule only governs the coil internal to the tank, not any external plant loops feeding the tank. My guess is that if you added a SetpointManager:Scheduled to the supply outlet node of the plant loop with the water to air hp and gave it the same schedule as the tank setpoint schedule, you could avoid this issue without EMS.

( 2015-08-20 12:57:39 -0500 )edit

thank you for your comment; I will check whether this can also be solved simply by setpoint managers, but the EMS solution was very simple (only a few lines).

( 2015-08-20 14:11:58 -0500 )edit

Your suggestion did not work ! Choosing identical temperature schedules for WaterHeaterTank and the PlantLoop SetpointManager at the supply outlet node produced essentially the same simulation results, although in my example with less warnings. For me this negative outcome seems logical, because the setpoint manager(s) in the water plant loop only have control over the supply equipment, not over the equipment in the demand branches. In this application, however, we have the water side of a cooling coil situated in a demand branch of the plant loop.

( 2015-08-21 05:53:29 -0500 )edit