HVAC air flow calculation

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."
Question:Why in the course calculation, above mentioned, 53.7 CFM was not multiplied with the number of occupants, because was calculated for 0.35 ACH which means per person and not for 6 persons. Here is where I lost myself.

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 the air flow in an office building house (starting with office x).

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 are15 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?

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


Zoltan, what you are describing is way beyond the scope of a home inspection.
If you want to go down that road I wish you all the best.

What he is describing is not what he is trying to describe.

Ask a good question and we’ll give you a good answer.


OK. I know that this kind of calculation is beyond the scope for a general inspection but it is very important in IAQ inspection, because depressurizing or over pressurizing an office could create problems for the occupants comfort. If the supply and return are not balanced the air quality can be affected.

My question simplified: In an 300 sqft office with 6 occupants how much CFM the supply register need to deliver for maximum occupants comfort?

Many thanks


The calculations you initially talked about appears to me to be about ventilation.

If, your talking about CFM for a load calculation, it has nothing to do with the “comfort of the occupant”. It h as to do with the balance point of the building envelope based upon where you are (and you didn’t fill out your profile).

You don’t measure ventilation cfm with an Alnore Hood in the room.

Hi David,

Ventilation, particulates and moisture are the 3 main components of IAQ.

You mention: “If, your talking about CFM for a load calculation, it has nothing to do with the comfort of the occupant”. I disagree. For instance if I measure a bathroom fan proper CFM and is not min 50 CFM (as ASHRAE standard) the moisture will not be efectively removed and problems with IAQ occur.
The balance of a building, properly designed HVAC and airflow (measured in CFM) has a lot to do with the occupants comfort and automatically affect IAQ according to ASHRAE 62.1 and 62.2.
You can measure, for instance, airflow in the the supply and return in an office (if both are in the same area in that office) in order to see if that particular office is pressurized or depressurized and if the volume of air is enough for the number of occupants for that specific area. The rule of thumb is 1cft3 in =1cft3 out. If this is not achieved would be an IAQ problem.

One of the main equipment to measure CFM and pressure is flow hood.

Many thanks


Again, CFM/person does not insure comfort. BTU does.

BTU and CFM have a relationship based upon system design. But you can’t use an Alnor hood to determine BTU.

I still don’t understand what your asking about. Comfort, Ventilation, Moisture, or whatever. Your mixing everything up and they do not go together.

What is this about?

Don’t you think the size and room load has something to do with the ventilation rate?

I think his question has to do with table 403.3 of the IMC.


I would think so too, but he keeps talking about a bunch of other crap that does not relate. Ventilation has nothing to do with comfort. It is about ventilation. You can live with bad comfort. You can’t live without ventilation.

This must be one of those NACHI course posts (in the wrong place).

Hi guys

Jeff! You get to the point.

David: Maybe I was not enough clear due to my bad english:)

I am working here in Romania as IAQ consultant especially for clients with asthma and other respiratory allergies but not only. For this reason, for me, the ventilation, pollutants and moisture play a very important role as a whole. In IAQ they are interrelated. I inspect generally moisture intrusion, pollutants (specific measurements PM 2.5, PM 10, pollen, dust mites, molds, VOC) and ventilation (airflow, air changes rates, contaminants).

Ok. Of course CFM is not BTU. CFM is about airflow rate and this is what I was talking about. Ventilation rate is directly related to the airflow.
If in an office room the supply register supply 100 CFM conditioned cool air in summer and the return suck 120 CFM it means that office is depressurized. Do you agree? If the office is depressurized means that the outside warm air moisture will be sucked into the office through the building envelope . Do you agree? What the occupant will feel? The occupant will feel a bit draft due to the pressure difference (supply-return) on one side and on the other side that office will be warmer than others (I assume the thermostat is not in this office).
Warm humid air can bring in pollutants and moisture. High level of moisture can cause condensation on cold surfaces and can form molds etc. Draft, pollutants and moisture get into the office and affect the occupants comfort (sneezing, coughing, etc). This is a long story…

What I wanted to mention is that: moisture, draft, indoor pollutants are all directly linked to ventilation. Air quality is directly related to ventilation. Air quality directly affect occupant comfort and health.

I really appreciate any comments even if you do not agree. We are here to clarify things. Isn’t it?



OK, you are now making some sense.

But you are measuring with the wrong device. You need to measure pressure, not CFM. CFM is a huge number and if you can measure 20 cfm in a room/bld you have a huge problem. You also need to measure pressure in piscals not psi.

Air duct balancing does not always effect IAQ. You can have more SA/RA ratio that does nothing bad because it is a closed system within the building as a whole. It’s when the indoor pressure with reference to the exterior atmosphere gets out of balance. This can be from simple air duct leakage.

All of this has nothing to do with comfort, and everything to do with health.

You talked about vent fans.
You can’t just suck out bad air. For every 50cfm you blow out, you must suck in 50cfm from outside the building. This generally comes from a basement/crawlspace where all those bad things your trying to control come from.

You must replace vented air through the HVAC/filter system to control the contents of the OA being replaced in the bld. This can be verified more accurately with a micromanometer than an Alnor hood. One problem being that if you have equal supply/return duct leakage producing a 0 net pressure differential, you could have a huge indoor/outdoor air transfer going on an not pick it up. This test is not qualitative. It simply indicates the pressure differential and helps in locating the source.

Measuring cfm produces the same problem. You may measure volume, but you don’t know where it is coming from. And if it is not coming from a polluted source, it is not your IAQ problem.

Hi David,

Thank you for your kind explanation.

  1. I usually measure the airflow (CFM) with flow pan as in this video:
    https://www.youtube.com/watch?v=9vwmP1T2ReI. I have a Fluke digital manometer, something similar with the Retrotec in this video and the opening of this box is set into the instrument settings. The opening into the box is set in such a way the pressure to be between 2-8Pa. This is the way to measure the CFM I was talking about at a fan or at a supply or return register.

  2. What do you mean by: “CFM is a huge number and if you can measure 20 cfm in a room/bld you have a huge problem.” According to ASHRAE, a person need 15CFM or 0.35 ACH for a heathy environment. Maybe we are talking about different things. I am talking about balancing the CFM at the supply register (positive pressure) and at return register (negative pressure), assuming that each office has their own return register. Make sense?



These room vents, what do they connect to?
The HVAC unit, or direct vents to the exterior?

Why are we focusing just on a room? Why not the building total?

If you have a room with a high ventilation requirement, we generally suck out the bad air, vent it outdoors and bring in replacement air through the HVAC economizer which provides the ventilation requirement for the total building occupancy, not just one room in the building.

When you are measuring a volume differential in CFM you are measuring a volume of air. If you measure Pascals, you are measuring the driving force that is causing the volume difference.

You can not assume that all of the air you are measuring is all inclusive in your attempt to quantify a result. Some of the air may not be going into the return. It may go under the door, along plumbing/electrical penetrations and never be affected by the exterior elements.

We are talking about ventilation. We can not throw HVAC SA/RA into that mix.
If you have HVAC SA/RA vents thrown into the mix, you need to isolate them from each other to test them. Do you have vents just for “ventilation” supply and return air?

If you are doing air barrier testing, you can not assume that CFM in/out measurements have anything to do with the OA leakage rate. Your testing must be done “with reference to the exterior”, not volume differentials of a closed system. You need to only measure CFM IN for compliance. You need to do air mixture calculations for anything else.

If you have air quality issues from OA entering the building, you need to look at the pressure causes not the volume.

ie. I had a sick building investigation where the engineers were blaming dog urine because they couldn’t come up with anything else.

I was doing the thermal imaging. I identified a significant negative building pressure with reference to the exterior (thermal air patterns). When I investigated the cause of the pressure (not measuring the volume of air infiltration), I found that the building had a supply vent fan and an exhaust vent fan. The supply fan broke last year. No one fixed it, thinking that like your bathroom fan you only need to suck out the air, not put in back in. The building was so large that forced ventilation was required to keep the building pressure neutral.

Humid air off the adjacent river was being sucked through the building envelope which met the A/C and turned to water and IAQ issues. And they were looking for dog urine… You need to know what your looking for to test correctly.

I could not have solved the issue by testing SA/RA volume.