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

I'm searching for further clarification on the differences between VAV:Reheat and VAV:HeatAndCool:Reheat. The distinctions between the two for me became muddled after reading through this question.

In the input output reference under VAV:HeatAndCool:Reheat the airflow rate of the VAV:Reheat object is said to be...

"...reduced to the minimum value (max air flow rate x zone minimum air flow fraction) when zone heating is required."

But there are parameters in the VAV:Reheat object that in my eyes don't agree with that statement such as Maximum Flow Fraction During Reheat. This field is a fraction that...

"...is multiplied by the Maximum Air Flow Rate to determine the maximum volume flow rate (m3/s) allowed during reheat operation..."

This, to me, indicates that the air flow is indeed modulated during heating for VAV:Reheat.

If what is said about VAV:Reheat under VAV:HeatAndCool:Reheat is true shouldn't the air flow for VAV:Reheatduring heating operation simply be whatever is defined in the object by the Zone Minimum Air Flow Method?

Documentation verbiage aside, here's how I would expect the two objects to behave. VAV:Reheat modulates the reheat valve to 100% first, then increases air flow whereas VAV:HeatAndCool:Reheat increases the air flow to maximum first, then modulates the reheat valve to 100%. Is this correct? Am I just doing a poor job of reading the documentation?

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Actually, a very good interpretation of the documentation. The VAV:Reheat object was one of the first VAV box models in EnergyPlus. As you describe, it meets a heating load, by modulating the heating coil capacity at minimum flow, then opening the VAV damper if load exceeds delivered capacity. However, it also meets cooling loads by also modulating the damper.

Now enter a new model, the VAV:HeatAndCool:Reheat model, this was created when the VAVChangeoverBypass model was added to EnergyPlus. This model needed the VAV damper to modulate in heating and cooling, not holding the minimum damper position in heating mode until the load exceeded the delivered capacity. So this VAV box model modulates air flow in both heating and cooling to meet the load.

The fact that you see these subtle difference means you understand what E+ offers and what you want for a VAV box model. Choose according to the system selected.

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Perfect answer! Thanks for clearing that up.

( 2016-04-22 17:24:45 -0600 )edit