# need some help

(Zoltan Pohl) #1

Hi guys

I need some help in my HVAC air flow calculation:** As stay in the course:
**

"Example in the course: For a 1,150-square-foot, 3-bedroom home with 8’ ceilings, the required CFM will be the greater of the number of bedrooms plus one (with a minimum of five) = 5 x 15 CFM = 75 CFM or 0.35 ACH x (1,150 cu ft x 8’) = 0.35 x 9,200 cu ft = 3,220 CFH/60 minutes = 53.7 CFM. This home requires 75 CFM. This calculation was made for min 5 persons."

Firstly in the course I did not understand well, why there are 2 different results: 53.7 and 75, and the 75 is the good one for that home. Could you clarify me? I am a bit confused.

Secondly: OK. Let say I want to optimize each office in an office building house.

Let say that I want to measure the supply air register efficiency, in one of the 300 sqft x 8’= 2400 ft3 volume office, with 6 occupants. How much air the supply register have to supply? How much CFM do I have to measure with my air flow hood in order to not pressurize or depressurize the office?

According ASHRAE there is 15 CFM per person or 0.35 ACH. OK.

According to the above calculation into the course I would need 15CFM x 6 persons= 90 CFM or 0.35ACH x 2400ft3=840 CFH/ 60=14 CFM air change rate for this office? But which one is good , 90 or 14? CFM14 represents the CFM /person with 0.35 ACH? Do I need to multiply 14 with the number of persons into the office to get the right CFM/ person/ office? In this case is it 14 x 6= 84?
Question:Why in the course calculation, above mentioned, 53.7 was not multiplied with the number of occupants, because was calculated for 0.35 ACH which means per person and not for 5 persons. Here is where I get confused.

I would appreciate your help to clarify where am I wrong.

Regards
Zoltan