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# Differences between VAV:Reheat and VAV:HeatAndCool:Reheat

I gave it the exact same title as this old post. It seems that AirTerminal:SingleDuct:VAV:Reheat and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat have not changed that much since 8 years ago, but I'm not satisfied with the answer in the old post. To be more precise, I want to clarify the difference between AirTerminal:SingleDuct:VAV:Reheat with Reverse damper heating action and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat.

For AirTerminal:SingleDuct:VAV:Reheat, the Reverse damper heating action is also called the dual maximum control logic as illustrated in the following figure.

For AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat, Adam Hilton, the questioner of the old post interpreted Input/Output Reference as follows:

VAV:HeatAndCool:Reheat increases the air flow to maximum first, then modulates the reheat valve to 100%.

This is incorrect, at least according to simulation results. When the heating load is small enough to handle with the Mnimum Air Flow Fraction, the airflow starts from the Mnimum Air Flow Fraction, and the supply air temperature is gradually increased. The aiflow increases only after the heating load increases and the supply air temperature reaches the Maximum Reheat Air Temperature, which is the same as the figure above! If this simulation result is not as intended for AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat, this should be a bug.

I don't know the difference between AirTerminal:SingleDuct:VAV:Reheat with Reverse damper heating action and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat. Both have Mnimum Air Flow Fraction and Maximum Reheat Air Temperature. When the heating load increases, both start with Mnimum Air Flow Fraction and both increase airflow after the supply air temperature reaches Maximum Reheat Air Temperature.

Actually, the simulation results of AirTerminal:SingleDuct:VAV:Reheat with Reverse damper heating action and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheatare not exactly the same, but their control logic looks very similar.

Could someone clarify the difference between AirTerminal:SingleDuct:VAV:Reheat with Reverse damper heating action and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat? I would appreciate it even more if you could illustrate it. The problem is that Input/Output Reference does not illustrate the control logic of AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat.

UPDATE

I plotted simulation results as shown below according to @rraustad 's advice. The models are baesd on the ExampleFile HVACTemplate-5ZoneVAVWaterCooled.idf. The figures appear to be similar, but the model with AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat has less outliners, 3% less annual heating energy (51.9GJ vs 53.7GJ) and 50% less unmet hours (66h vs 127h) compared to the model with AirTerminal:SingleDuct:VAV:Reheat with Reverse damper heating action. I'm going to use AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat to model variable heating airflow.

I overlooked an important difference between VAV:Reheat and VAV:HeatAndCool:Reheat. AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat does not have an input field for Design Specification Outdoor Air Object Name. It cannot supply stable OA flow. It turned out to be useless.

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( 2024-06-11 01:38:12 -0500 )edit

So according to the above plots the HeatAndCool terminal unit is not responding as originally intended. Given the 2 independent variables, air flow rate and coil capacity, the air side damper fraction and the coil capacity fraction would have to be linked together. I think the first thing needed is to look for CBVAV sequence of operations documents. If not found then some consensus on how CBVAV terminal units should respond to heating loads. If I recall correctly, CBVAV supply temperature is set to keep the maximum damper position in one of zones at 1.

( 2024-06-11 12:31:35 -0500 )edit

The CBVAV model calculates a target supply air temperature required to operate a single terminal unit at its maximum air flow rate. The remaining terminal units will modulate as required to maintain the dry-bulb temperature in the zone they are serving according to the thermostat schedule for their respective zone. The system air flow rate remains constant. The excess system air flow is “bypassed”.

( 2024-06-11 20:36:58 -0500 )edit

@rraustad You are correct. VAV:Reheat with reverse damper heating action and VAV:HeatAndCool:Reheat have different control logic. And in detail, both controls may differ slightly from the common VAV control logic for heating in my region:

1. The heating supply air temp setpoint is fixed.
2. Each VAV box modulates airflow according to each zone temp.
3. The system airflow is modulated according to the airflow requests from VAV boxes.
4. If the system airflow drops to the minimum airflow and is still excessive to handle the heating load, the supply air temp setpoint is reset (lowered).
( 2024-06-11 21:39:21 -0500 )edit

@rraustad Anyway, the simulation result is the most important, and AirTerminal:SingleDuct:VAV:HeatAndCool:Reheat seems to work closer to what I expect. Thanks again for your advice.

( 2024-06-11 22:05:17 -0500 )edit

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I did review the code for each of these terminal units but it is difficult to say if there is an actual difference. The original intent was to allow the HeatAndCool model to modulate air flow AND coil capacity at the same time but after this review I don't see that operation. It may be that the original intent has change over time, not necessarily in the model inputs but in the model code, or that these terminal units were not really that different from the start. The one thing to remember about these terminal units is they are used with different systems, one that will usually hold a constant inlet temperature (VAV systems) and one that will vary the inlet temperature based on all zone loads (CBVAV). This will make the analysis below more difficult.

The best way to see the performance is to compare the results by creating the plot shown above for each terminal type. Use a full annual simulation to report terminal unit outlet temp and air flow rate. Then scatter plot these data on a single plot versus Zone Predicted Sensible Load to Setpoint Heat Transfer Rate. The result will be a mirror image of the figure above on the x-axis unless the Zone Predicted Sensible Load to Setpoint Heat Transfer Rate magnitude is multiplied by -1. Quite frankly I would not be able to tell the difference in performance without this type of plot. Also, this is the type of analysis I use during bug investigations.

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The original intent was to allow the HeatAndCool model to modulate air flow AND coil capacity at the same time

Interesting. but in actual automatic control, modulating airflow and coil capacity at the same time sounds difficult. I think modulating coil capacity usually takes priority over modulating airflow.

( 2024-06-11 12:16:03 -0500 )edit