Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The first warning (SimHVAC: Maximum iterations exceeded) is likely due to the HX in the outdoor air system. Not that a HX is not allowed, rather that the HX will provide a different mixed air temperature when the main air loop air flow rate changes. This can lead to an oscillating condition within the simulation. The air loop's zone terminal units determine a mass flow rate to meet the zone temperature set point based on the supply air temperature. This mass flow rate is fed back to the OA system would could calculate a different mixed air temperature. This change in temperature can lead to a change in air loop mass flow rate and the simulation may exceed the iteration count.
Look at the Demand-to-Supply and Supply-to-Demand interface checks for mass flow rate. Notice how these historical values are the same for the 1st through the 5th values (previous 5 iterations). These values do change from 1 to 2, and 2 to 3, etc, indicating the solution is iterating on a solution. However, the 6th number in each history array is different from one another. This reflects a change in mass flow rate that the simulation did not correctly resolve. Also note that iteration 7 through 10 are a repeat of 1 through 4. This is an indication that the simulation will continue to iterate, and continue to find the same values over and over again until the max iteration count is reached.
This problem "might" be avoided by closely controlling the supply air temperature in the main air loop. If these iteration warnings are not prevalent, you can disregard these warnings.
The second warnings (DX coil outlet < 2C) are troublesome since you have specified a 100% outdoor air coil. This input is used to tell the simulation to switch models to a DX coil that can, and usually do, have lower outlet air temperatures. This warning is troublesome since you have specified a 7.2C outlet temperature yet the simulation is reporting "full-load" outlet temperatures below 2C. The catch is that the simulation cannot limit the coil outlet temperature to 7.2C until the simulation has determined what that outlet temperature is without controlling the outlet temperature. By the time the simulation has calculated the full-load outlet temperature, the warning has been issued. This issue could be fixed with proper programming so this warning can be ignored IF the outlet temperature is correctly controlled (check the variable output results to be sure of the 7.2C minimum temperature control).
Your last question regarding the use of 7.2C or higher leaving air temperatures, yes you can use a DX cooling coil in a DOAS system even with higher leaving air temperatures.
