Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
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 `AirTerminal:SingleDuct:VAV:Reheat object with hot water reheat coil and the same set-point manager should not have a problem meeting the heating thermostat set-point. 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. If you match these two inputs, you allow the VAV reheat air terminal to operate at the design supply air flow rate. Hence your hot water reheat coil should have delivered a reheat load as high as the design. If you observe a slight deviation of not meeting the space heating thermostat set-point, it must have been caused by leakage fractions specified at the Air Distribution Unit. We are investigating the impact of the leakage problem we found while working on this issue, but it’s not the main driver for not meeting the thermostat heating set-point.
Let us know if these suggested changes resolve the problem.
