VAV box reheat airflow setting

[DuctHunter]

What sequence of operation is best to use on the VAV box when you have a reheat coil.
One company I worked for had it customary to have the VAV box in maximum airflow setting when needing heating from the reheat coil. They would size the reheat coil based on the VAV box maximum airflow rate. Other engineers I talk to size the reheat coil for the VAV box minimum air flow setting. So on call for heat the VAV box goes to the minimum airflow setting and the slowly ramps up as more heat is needed.

The logic behind keeping the box airflow at the maximum is because when heating the air, if you cut the air to the minimum the velocity will be so low that the heated air will never drop down and will just stay near the ceiling.

I'd like to know what sequence others use and what airflow rate is the reheat coil sized for.

Thanks for your input.

 

[willard3]

Hot air is going to rise no matter how high the air velocity.

Given that the delta t of water/air is higher for heating than cooling, it stands to reason the air volume can be lower for the same amount of btu's.

 

[EnergyProfessional]

it all depends on the airflow you calculate you need for heating. Relying on coil sizing is dangerous, because you don't know AHU DAT, and water temp at a given time. Modern systems use hot water temp reset and also DAT reset.

I'd control it to modulate the valve to not exceed 90°F or whatever you need. that way you have less buoyancy.

It is counter-intuitive to most people, inc. mechanics that adjust the system, but making the air warmer, gives you less heating capacity in ceiling systems.
At some point you either have ridiculous air flow rates (which also create noise and drafts), or you make the smart choice and heat with radiators at the perimeter. I had to fix VAV systems where the space never reached above 65°F and at the ceiling you had over 85°F. The complete-mixing that ceiling-based VAV systems rely on only works in cooling.

 

[ME27272727]

Check out ASHRAE Standard 90.1 - 2013. They talk about Dual Max setpoints, and some other new guidelines for VAV reheat control. You may also want to look into supply air temperature reset while you're on the subject of VAV reheat and temperature control.

 

[cry22]

Take your primary airflow from 55 to 95 (well, 85 can be OK for not so cold climates), i.e. min VAV box setting PLUS perimeter heating load (wall, glass, infiltration loads served by the box)