VAV box - Max-min settings

[remp]

Corect me if Im wrong? (by the way im in Germany)

Typical VAV system.

Max. box position - full cooling
Min. box position - minimum cooling

The VAV modulated between both max & min position. Min position ensures min air change rates and grille air diffusion index is achieved

When heating is required at min position the box reheat coil modulated from 0-100% open to achieve room temperature.This is all stock standard.

Q. The designer now wants if the room is still not satisified when in heating mode, to open up the box again from min up towards max. ( the grilles are over sized and velocity at min. is too low and the hot air is stagnant at the ceiling) Looks like this is her get out solution to grille problem, but its a waste of energy as far as I can see having the main AHU speed drive ramping up when in heating

Is this crazy , especially on internal zone boxes, in the middle of Spring. I could understand perimeter zone in really cold environments with poor perimeter glazing & insulation, so your supply air temp is not too hot (above 30deg) ramp up then. What do you think???

 

[imok2]

Why not set min position on affected area box to a higher min position say inStead of 50% SET FOR 60% this shouldn't make much difference in supply fan ramp up besides a little more air my be what you need

 

[DrRTU]

With a DDC system, you should be able to set separate cooling and heating minimums. Upping the heat min. should not be a big fan penalty. I select minimum cooling valve position based upon occupancy based upon the applicable vent standard then select minimum heating valve based occupancy, minimum reheat coil flow and proper grill selection. Discharge air with full electric heat should be 90 to 100 F. You are correct in your assessment of the problem. Without correct selection of the air distribution you will have numerous problems. This sounds like the designer was overly concerned about noise or selected grilles to match the ceiling.

 

[mauricestoker]

For heating, on VAV with reheat, the minimum heat setting is going to be limited by energy code, 90.1-2007. The 0.4 CFM/SF, minimum OAR, or 30% of peak design flow would set your highest allowed minimum. Going from deadband to heating, using reheat prior to increasing air volume is easier on controls and energy than going directly to opening the damper for volume flow. It avoinds ramping up the supply unless the reheat (I wouldn't go over 95 *F for ASHRAE 55). I think a lot depends on population loading and if time-of-day scheduling would be applicable. If unoccupied for long durations, then the min can be set even lower by having no people OAR.