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# DOAS input Problem

I have started my simulation and It shows the following warning and I am not sure what is it and what to do about it. I have trying to set up a DOAS System

First of all, I am not very sure what are these errors come from and how can I solve it?

** Warning ** SimHVAC: Maximum iterations (20) exceeded for all HVAC loops, at HONG KONG SAR CHN CityUHK-45007 WMO#=450070, 01/01 03:15 - 03:30
**   ~~~   ** The solution for one or more of the Air Loop HVAC systems did not appear to converge
**   ~~~   ** Air System Named = DOAS did not converge for mass flow rate
**   ~~~   ** Check values should be zero. Most Recent values listed first.
**   ~~~   ** Demand-to-Supply interface mass flow rate check value iteration history trace:
0.421904,0.571668,0.774595,1.049554,1.422117,4.239839,0.421904,0.571668,0.774595,1.049554,
**   ~~~   ** Supply-to-demand interface deck 1 mass flow rate check value iteration history trace:
0.421904,0.571668,0.774595,1.049554,1.422117,0.311375,0.421904,0.571668,0.774595,1.049554,
**   ~~~   ** Air System Named = DOAS did not converge for temperature
** ~~~   ** Check values should be zero. Most Recent values listed first.
** ~~~   ** Demand-to-Supply interface temperature check value iteration history trace:
3.462632E-002,2.555499E-002,1.886016E-002,1.391922E-002,1.027269E-002,0.103233,3.462632E-002,2.555499E-002,1.886016E-002,1.391922E-002,
**   ~~~   ** Supply-to-demand interface deck 1 temperature check value iteration history trace:
2.555499E-002,1.886016E-002,1.391922E-002,1.027269E-002,0.103233,3.462632E-002,2.555499E-002,1.886016E-002,1.391922E-002,1.027269E-002,
**   ~~~   ** Air System Named = DOAS did not converge for energy
**   ~~~   ** Check values should be zero. Most Recent values listed first.
** ~~~   ** Demand-to-Supply interface energy check value iteration history trace:  9612.881052,13025.189533,17648.773708,23913.603146,32402.274792,96602.722233,9612.881052,13025.189533,17648.773708,23913.603146,
**   ~~~   ** Supply-to-demand interface deck 1 energy check value iteration history trace: -9612.881052,-13025.189533,-17648.773708,-23913.603146,-33006.735202,-7094.521112,-9612.881052,-13025.189533,-17648.773708,-23913.603146,


My next warnings are the following. As This is a DOAS System I am not sure how to adjust the input to Energyplus

Yes, !- Use Outdoor Air DX Cooling Coil
7.2; !- Outdoor Air DX Cooling Coil Leaving Minimum Air Temperature {C}


The maximum Outdoor Air DX Cooling Coil Leaving Minimum Air Temperature is 7.2C But What If my leaving air temperature is larger than 7.2? Can it still be a DOAS Coil?

   ** Warning ** CalcDoe2DXCoil: Coil:Cooling:DX:SingleSpeed "DOAS COOLING COIL 1" - Full load outlet air dry-bulb temperature < 2C. This indicates the possibility of coil frost/freeze. Outlet temperature = -1.12 C.
**   ~~~   **  ...Occurrence info = HONG KONG SAR CHN CityUHK-45007 WMO#=450070, 01/02 08:00 - 08:04
**   ~~~   ** ... Possible reasons for low outlet air dry-bulb temperatures are: This DX coil
**   ~~~   **    1) may have a low inlet air dry-bulb temperature. Inlet air temperature = 16.701 C.
**   ~~~   **    2) may have a low air flow rate per watt of cooling capacity. Check inputs.
**   ~~~   **    3) is used as part of ...
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I don't have an answer, but just wanted to point out that this is occurring during your design day sizing, rather than during the actual RUN PERIOD. You may be able to safely ignore this warning.

( 2014-12-22 13:19:20 -0500 )edit

Is it just me or the two drawings you inserted don't match? (on the second one, the area around the heat pipe doesn't make much sense: missing supply air, outdoor air not labeled right and not in correct location and exhaust air arrow is in the wrong direction)

(Also, I've never seen such a system with -4°C outlet conditions after DX coils. I'm curious, what application is that?)

Anyway, can you post the full idf or at least the sections where you defined your coils and heat pipe?

( 2014-12-23 03:30:09 -0500 )edit

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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.

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