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SetpointManager:WarmestTemperatureFlow interferes with proper autosizing in WinterDesignDay

asked 2023-02-18 14:32:55 -0500

Keigo's avatar

updated 2023-02-20 11:04:17 -0500

A bug report. I found that zone air temperature does not meet heating temperature setpoint in WinterDesignDay if AirLoop is controlled by SetpointManager:WarmestTemperatureFlow. You may think SetpointManager:WarmestTemperatureFlow is irrelevant to heating, but actually it affects sizing run in WinterDesignDay. You can see the bug by running the EnergyPlusV22-2-0 ExampleFile: 5ZoneCAVtoVAVWarmestTempFlow.idf for sizing periods.

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You will see a larger temperature gap if you set higher thermostat heating setpoint.

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Only about one-third of the predicted load is actually heated.

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Here are my findings from some tests for now:

  • The bug occurs in models with AirTerminal:SingleDuct:ConstantVolume:Reheat or AirTerminal:SingleDuct:VAV:Reheat , but does not occur in models with AirTerminalSingleDuct:SeriesPIU:Reheat or AirTerminalSingleDuct:ParallelPIU:Reheat. I have not tested all the AirTerminals.
  • It's not that the reheat coil does not work at all. The reheat coil heats the zone to some extent, but it's insufficient.
  • The gap between Zone Thermostat Heating Setpoint Temperature and actual Zone Air Temperature is noticeable when the reheat coil type is Coil:Heating:Water. The gap is relatively small (0.1 to 0.3ºC) for Coil:Heating:Electric.
  • The bug occurs regardless of control strategy (FlowFirst or TemperatureFirst)
  • The weird thing is that unmet hour (Zone Heating Setpoint Not Met While Occupied Time) is not detected (i.e. 0) during the sizing run even though the zone heating setpoint is not met.
  • No warnings/errors are reported in the err file.

Originally, I found the bug in my ASHRAE90.1 Baseline case: System 7(VAV with Reheat). You know, system 5 through 8 need to have Supply Air Temperature Reset Control as per ASHRAE90.1 G3.1.3.12, and SetpointManager:WarmestTemperatureFlow needs to be used for the control (Note that SetpointManager:Warmest is inappropriate because the control strategy should be FlowFirst rather than TemperatureFirst according to ANSI/ASHRAE/IES Standard90.1 Performance Rating Method Reference Manual 3.6.2.2 Cooling Control, Cooling Supply Air Temperature Control). I got so many unmet hours in my annual simulation as the reheat coil was not autosized properly.

I would like to know if there is a way around the bug. If not, the reheat coil capacity has to be hardsized with reference to Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate.

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Keigo's avatar Keigo  ( 2023-05-08 02:07:22 -0500 )edit

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answered 2023-07-16 13:04:59 -0500

nigusse's avatar

Two issues drive the problem: (1) differences in heating coil design capacity and available capacity and (2) air terminals maximum available supply airflow limit.

Heating Coils Available Capacity: The heating coils design and available capacity for hot water heating coils differ from Electric and gas heating coils. Why did the heating coils fail to deliver to meet the heating load?

Coil:Heating:Electric and Coiil:Heating:Fuel: The available capacity for these heating coil types is not impacted by the operating supply air flow rate unless the user caps the maximum supply air temperature. These heating coil types can deliver as high as the design heating capacity regardless of the operating supply air flow rate. This explains why the system with these heating coil type met the space thermostat heating set-point. The slight deviation you observed (0.1K to 0.3K) from the thermostat heating set-point for the air terminal with these reheat coil types must have been caused by something else. It could be driven by the user input inconsistency or the air distribution unit leakage, which we are investigating.

Coil:Heating:Water The available capacity for this heating coil type depends on the supply air flow rate, other inputs being the same as the design condition. The hot water heating coil’s available heating capacity tends to be lower if the operating air flow rate is reduced from the design flow. The physics of the water coils dictates this. So, if the supply air flow rate is reduced from the design flow by other objects specified in the system, the hot water reheat coil cannot meet a heating load as high as the design capacity. A combination of system configuration or user input inconsistencies can cause this.

Air Terminal Maximum Operating Supply Air Flow Rate: If the maximum available supply air flow rate at the air terminal unit is kept well below the design flow rate, then the hot water reheat coil in the air terminal can’t meet a heating load as high as the design load. In your test case, the maximum supply air flow rate at the air terminal was capped by the SetpointManager:WarmestTemperatureFlow object Minimum Turndown Ratio input field (you specified 0.2). Hence, the hot water reheat coil in an AirTerminal:SingleDuct:ConstantVolume:Reheat could not meet the heating load since it is not operating at the full design supply air flow rate. You can test your example file by increasing the “Minimum Turndown Ratio” to as high as 0.5 or higher to prove it to yourself. So, the conclusion here is that a system configuration with a combination of SetpointManager:WarmestTemperatureFlow and AirTerminal:SingleDuct:ConstantVolume:Reheat object with a hot water reheat coil may limit the system from meeting the heating thermostat set-point. So, it is not a bug but not a good system configuration. If you choose such a configuration, you must design the system to operate at a higher Minimum Turndown Ratio. However, with consistent user inputs, the ... (more)

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In your test case, I see that the “Minimum Turndown Ratio” was 0.2 while the ‘Central Heating Maximum System Air Flow Ratio’ was 0.3.

I have not uploaded my test case. I just ran the ExampleFile 5ZoneCAVtoVAVWarmestTempFlow.idf. Minimum Turndown Ratio in the ExampleFile is 0.3, not 0.2. I'm afraid you are mistaken about something.

Keigo's avatar Keigo  ( 2023-07-16 21:32:21 -0500 )edit

In your test case, the maximum supply air flow rate at the air terminal was capped by the SetpointManager:WarmestTemperatureFlow object Minimum Turndown Ratio input field (you specified 0.2).

No, Minimum Turndown Ratio is not what caps the maximum supply air flow rate. It limits the "minimum" flow fraction.

Keigo's avatar Keigo  ( 2023-07-16 21:37:47 -0500 )edit

The SetpointManager:WarmestTemperatureFlow sets the minimum flow and modulates (increases) until it meets the cooling load. However, this set-point manager does not modulate the supply airflow when the space needs heating. So, when there is no cooling load (heating mode operation) the supply air flow rate remains capped at the minimum flow fraction. In other words, the supply airflow rate in heating mode operation will remain capped at the minimum flow fraction set by this set-point manager. Note that my response explains why the system does not meet the thermostat heating set-point.

nigusse's avatar nigusse  ( 2023-07-17 07:49:38 -0500 )edit

the supply airflow rate in heating mode operation will remain capped at the minimum flow fraction set by this set-point manager.

You seem to be talking about VAV systems. No, the supply airflow rate in heating mode increases above Minimum Turndown Ratio as the heating load increases with Reverse action. One example with AirTerminal:SingleDuct:VAV:Reheat (V23-1-0).

Keigo's avatar Keigo  ( 2023-07-17 12:39:52 -0500 )edit

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Asked: 2023-02-18 14:32:55 -0500

Seen: 135 times

Last updated: Jul 16 '23