CFM/duct cross-sectional area = velocity
Hey folks,
I am new to this forum and I'll tell you about me and what I have going on.
I build houses for a living and have been using the same HVAC guy for years. He recently retired and I have just started using another company. The last company sized the supply duct work a little different, and the pressure dosen't fell like I am used to, and anyway I bought a TIF 3220 to check the CFM of the system to satisfy myself. And also to balance the airflow when we get done with the flooring and install the vents.
For instance I just had a 3 ton system installed and I am assuming that the system air displacement is 1200 cfm for the system. The meter will show velocity (ft/min) and CFM, but measurments have to be inputed in the device to measure the CFM. My main question is how can I measure the CFM from the velocity readings?
The supply vents are 4"x10" in the floor area.
I asked this from the HVAC company but they really didn't answer my question. They just went in circles. Thanks for you time folks!!!
CFM/duct cross-sectional area = velocity
A Hart&Cooley #420 4X10 floor register has a free area of .17 sq ft.
Velocity time AK/Free area in sq ft equals CFM.
EG: 600 FPM X .17 = 102 CFM
FPM X Area in Sq.Ft. = CFM
It is difficult to use velocimeter readings at grills to get readings accurate within normal tolerance of +/- 10%. The velocimeter itself will be accurate as described by TIF, for one location of measurement. One problem is what is the actual area of the grill? A 4x10 grill doesn't have a net area of 40 square inches, it may only have a free area of 24 square inches, depends on the grill. So a 4x10 grill with a free area of 24 sq.in. divided by 144 sq.in. per sq.ft. = 0.167 sq.ft. A velocity of 700 FPM would be 117 CFM. The grill manufacturer will publish the net area of the grill. Another problem is that many grills will not have the same velocity across the whole grill, one side will be higher than the other. So you must average the velocity readings if the velocity is not the same. For a 4x10 grill, maybe middle left, middle, and middle right averaged will work.
I have tried to use velocimeter grill readings, and gave up, too many variables and too much guesswork compared to other methods. Now I use an air hood (balometer) at the grills or a hot wire velocimeter (anemometer) inserted into the duct.
"Quote" So a 4x10 grill with a free area of 24 sq.in. divided by 144 sq.in. per sq.ft. = 0.167 sq.ft. A velocity of 700 FPM would be 117 CFM.
If not too much trouble, how did you come up with these numbers?
I assumed a free area of 60%. (4x10)x.6 = 24; 24/144 = 0.167; 0.167x700 = 117.
Beware, you must know what grills are in use, and get the free area from the maker, there is no good way to calculate free area by measurement. I assumed a free area percentage just for the example.
Edit: I pulled the 700 out of thin air, it seemed like an approximate velocity one might expect to get a desirable amount of CFM from a 4x10 grill.
Don't expect to find 1200 CFM at every grill. You'd be better off finding a BPI or Comfort Institute certified HVAC tech (or someone familiar with those principles and knows how to apply it) who can measure actual airflow through your system. If you're dying of curiosity, find a TAB outfit (testing and balancing) to test the output of every grill in your house using a balometer. Not a velocimeter, a balometer. There are balometers made that do well measuring residential grill outputs.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.
For the units total airflow; if you have a good vane anemometer the Return Air Grille can be used.
Many mfg'ers publish the Ak/open air area of their RA Grilles on the Internet.
There are Videos showing how to get the delivered CFM airflow.
You could click my image & e-mail me.
Knowing at least what the ballpark airflow is critically important to effiocient performance.
In the cold north we have old, way oversized Oil furnaces, that require around 1200-cfm per 100,000-BTUH output; some of those Oil furnaces won't even deliver 600-cfm.
They are way off for both heating or cooling...
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Another good way of finding total system CFM is to use a digital manometer and check the total external static pressure (ESP). Take static pressure readings in supply plenum between furnace and a/c coil (if no coil then just in plenum close to furnace) and in return air between furnace and filter and add the two numbers together. The return reading will be negative but add them together as if it were positive. It's just negative pressure, not a negative number, ie .26"wc supply, -.24"wc return is .5 total ESP. Check the appropriate blower performance curve chart in the manual to see how many cfms that fan is moving. Hopefully they left the manual behind for the new furnace
I was told (and you know how that goes) that I can take CFM measurements at all the supply vents and ad them up to the systems total CFM that is being displaced. Then to take a static pressure reading at the return duct right before entering the evap area and then to take a CFM measurement at the return and then review it with the manufactures data.
Is it true that most systems run about .5 inches of water colume at the return?
It would be good to get an expert to test and balance all the duct work but in the area that I am in for some reason, no one seems to want to do that or has the test equipment. After the job is complete, they pack up and there gone. I have spoke of this and they companies that I told me they don't have the equipment to do this. They have however measured static pressure. But never use an anemometer or a balometer or such.
For some reason also, every company I have used dosen't do duct layouts, they come in and measure the new home, do a load calc and then start installing the duct work after. Most homes that I have built have round duct work and every vent is a 6" round metal duct running to every supply vent. For instance the house they just got done doing is 1745 sq.ft and they installed a 3 ton American Standard split unit. they started with 14" round pipe coming out of the supply on the furnace and then reducing down to a 12" then about ten feet later a 10" and then capping it. The duct is about 45' in length. and they have insulated with R-4 bubble wrap insulation. The home has 12 - 6" vents in the ceiling through out and a 25"x25" return with 16" round metal pipe running to it. The system is doing well but I just got curious about it. I have never seen anyone do a duct layout or design. They just go with rule of thumb I guess. The inspector never asks for it for the permit, just the load calc.
The simplest and quickest way to see if it's in the ballpark, is to read the MFG data plate; see the Model #. This should tell you the size of the equipment (capacity) and blower CFM capacity. It should definitely be more than 1200 that you need. If not, definitely it will not be enough due to the static pressure of the duct work.
I will also say the best way is to use a flow hood but expensive. ESP would also be a good way you just would not know what is at each register.
I also don't think your moving 1200 CFM with the 14" supply. Would have liked to see a 16" supply, 18" return and a couple more outlets. Just my .
MH