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| MarketManager » Diagnostics |
A:
You might be expecting that the average zone temperature would drop in cases where the heating equipment or fans are not large enough to meet heating load. In real life, of course the average zone temperature would drop. In MarketManager, and other simulation programs, this is not the case.
Most simulation programs have a non-iterative algorithm (I will detail MarketManager's below, but most other programs are very similar):
When you press the simulate button, for each hour of simulation:
- MarketManager determines Zone Loads.
- MarketManager applies Zonal Equipment to Zone Loads.
- If Zone Loads are met by Zonal Equipment, then the simulation is done for this time step. Otherwise, any remaining unmet Zone Load is passed to the System level.
- The System will combine unmet Zonal Loads from all Zone children, and will introduce outside air, economizers, and fan heat, adding to the Zone Load. The unmet Zone Loads combined with loads added by the System are called System Loads. These are the loads experienced by the AHU. The System will turn on supply and return Fans.
- The System Loads are transferred to the Plant level. The Plant will combine System Loads from all System children.
- The Plant has Heating and/or Air-Cooling and/or Chillers that then meet the load at the Plant level.
- Finally, all equipment usage in kWh, therms, etc., is translated into dollars using the rates.
The drawback of this simulation method (which, I repeat, other simulation programs share) is that the algorithm is not iterative. The program assumes that zone loads are met before it even knows if there is any heating or cooling equipment to meet the loads.
If your heating and cooling equipment is unable to meet loads at the Plant level (step 6), then you will get an "unmet hours" message upon simulation.
If your fans are sized too small to meet the zone loads, (say 1 cfm), you will not get any post-simulation messages. As well, the program will assume that zone average temperature is at set point. (There is a warning in QC, however.)
An alternative approach would be to have the program iterate at each time step. The program could go through the same steps, but once it gets to step 6 (step 7 could be done at the very last), it could then revisit step 1, and adjust the zone average temperatures.
To ensure that your HVAC configuration is meeting set point, you need to ensure that your fans are at least autosized, and then ensure that your heating and cooling equipment is at least autosized. If you encounter no unmet hours messages, then MarketManager is telling you that the unit is meeting load.
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