VAV system 1

[ZDR1985]

Hi I am just new in HVAC. I have a DX system split type comprising of Air Handling Unit and Air cooled condensing unit. I will be using VAV boxes for zoning. Question is how do I maintain the fresh air and pressure requirement for my indoor conditions?

 

[cdxx139]

Have you used ASHARE 62 to calculate the fresh air in each space? What pressure requirements do you have?

knowledge is power

 

[willard3]

You should seek help from someone in your office who has done this.

 

[trashcanman]

Outside air should be provided by an outside air fan. The O/A requirements do not change, day or night, summer or winter. The O/A should be ducted to the mixed air plenum for the air handling unit. Constant O/A is dictated by the IMC.

 

[Ziggypump]

Per ASHRAE 62.1-2007, the outside air requirements can change based on the time of year, s well as the supply and return diffuser locations. Well at least in the North here, it's common. The zone air distribution effectiveness can change, causing the amount of outside air required to change.

With a rooftop unit the air is probably supplied and returned in the ceiling. Cold air supplied from the ceiling and returned at the ceiling has an effectiveness of 1.0. Warm air supplied at more than 15 degrees F warmer than return will have an effectiveness of 0.8. This means that more outside air is required in the winter than in the summer (stratification).

I designed a VAV AHU system for a building, where the winter outside air requirement equaled the total supply air (VAV reheat volume), and the summer outside air requirement was less than the winter outside air requirement. Both conditions were calculated in the 62MZCalc spreadsheet and documented to prove the results.

In figuring out your ventilation calculations, find and read ASHRAE 62.1-2004. Then read it again. Then read it as you are trying to calculate. Most state codes, if not all by now, utilize the ASHRAE method, or allow an engineered calculation as an alternative to the state requirements.

 

[cry22]

Trashcan:
OA does not need to be supplied by a supply fan. a duct with airflow measuring station (for VAV system) is adequate and meets code. for CV, air balance is sufficient.

Ziggy:
Winter has more OA than summer? supply and return registers changing location?
OA min as a percentage of total is larger in winter for VAV systems, not the actual amount. One would think that if anything, air quality being better in winter than it is in summer, if you were to use CO2 sensors, you actually would need less winter OA than summer

ZDR
If you split system is small (say 5000 CFM or less), you should not use VAV system at all. A return fan is necessary (or relief dampers directly from space) for VAV systems to maintain space pressurization, along with OA air flow measuring station to comply with code.