Expert: Dan Griffin - 6/4/2009

Question
I have a rooftop a/c unit for a 3000 s.f. medical office and I need to calculate the "mechanical ventilation by supply air and return or exhaust air", what does that mean? how do I do this?

Answer
Carlos, you don't explain why you are doing these calculations.  Here are a few issues.

A typical building is designed to run at a slight positive pressure.  This is so the conditioned air is forced out the doors when the door is opened rather than drawing uncontrolled quantities of outside air.  Any given air handler is designed to draw some total amount of CFM to the return.  This volume is controlled by duct size, blower size, and blower RPM.  Each supply grille is designed to deliver a specific CFM, again controlled by duct size and damper controls.

Exhaust fans draw air out of the building at a specific CFM based on size and intake grille.

Current mechanical codes require fresh air based on occupancy load of the building.  The return air is required to draw in this fresh air to avoid "sick building syndrome".

Grille CFM can be measured with flow hoods.
CFM can be calculated by  duct size, fan size, and fan RPM with adjustments made for duct material, offsets, and length of runs.

These calculations can be performed by a mechanical engineer or a competent HVAC man.  For the information to be usable with a code authority or health department the calculations will have to be done by a certified individual.