What Is The Free Area For A 4x10 Floor Register
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Calculating CFM from 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 only started using some other visitor. The concluding company sized the supply duct work a little dissimilar, and the pressure dosen't roughshod like I am used to, and anyway I bought a TIF 3220 to bank check the CFM of the system to satisfy myself. And besides to balance the airflow when we get done with the floor and install the vents.
For case I just had a 3 ton arrangement installed and I am bold that the organization air deportation is 1200 cfm for the system. The meter volition show velocity (ft/min) and CFM, but measurments accept to be inputed in the device to measure the CFM. My main question is how tin can I measure the CFM from the velocity readings?
The supply vents are four"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 y'all fourth dimension folks!!!
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CFM/duct cross-sectional area = velocity
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A Hart&Cooley #420 4X10 flooring register has a free area of .17 sq ft.
Velocity time AK/Free area in sq ft equals CFM.
EG: 600 FPM 10 .17 = 102 CFM
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FPM X Area in Sq.Ft. = CFM
It is hard to use velocimeter readings at grills to become readings accurate within normal tolerance of +/- 10%. The velocimeter itself volition exist accurate equally 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 gratis 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 volition publish the net expanse of the grill. Another problem is that many grills will not have the aforementioned velocity beyond the whole grill, 1 side will exist college 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 take tried to utilize 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.
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"Quote" And then a 4x10 grill with a gratis expanse 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 problem, how did y'all come upward with these numbers?
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I causeless a gratis area of 60%. (4x10)x.6 = 24; 24/144 = 0.167; 0.167x700 = 117.
Beware, you must know what grills are in use, and go the complimentary area from the maker, at that place is no good way to calculate costless area by measurement. I assumed a free area per centum just for the instance.
Edit: I pulled the 700 out of thin air, it seemed like an approximate velocity one might look to get a desirable amount of CFM from a 4x10 grill.
- Don't expect to find 1200 CFM at every grill. You lot'd be improve off finding a BPI or Comfort Institute certified HVAC tech (or someone familiar with those principles and knows how to utilise information technology) who tin mensurate actual airflow through your system. If you're dying of marvel, 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.
Originally Posted past kywallace69 
Hey folks,
I am new to this forum and I'll tell y'all about me and what I accept going on.
I build houses for a living and accept been using the same HVAC guy for years. He recently retired and I take just started using another visitor. The last company sized the supply duct work a fiddling 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 too to balance the airflow when we get done with the floor and install the vents.
For case I but had a 3 ton system installed and I am bold that the system air displacement is 1200 cfm for the arrangement. The meter will show velocity (ft/min) and CFM, just measurments have to be inputed in the device to measure the CFM. My main question is how tin 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 merely went in circles. Thanks for you time folks!!!Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.
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For the units total airflow; if yous accept a good vane anemometer the Render Air Grille can be used.
Many mfg'ers publish the Ak/open up air area of their RA Grilles on the Internet.
There are Videos showing how to become the delivered CFM airflow.
You lot could click my image & due east-post me.
Knowing at to the lowest degree what the ballpark airflow is critically of import to effiocient performance.
In the cold north we take former, mode 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 expert way of finding full arrangement CFM is to use a digital manometer and check the total external static pressure (ESP). Accept static pressure readings in supply plenum between furnace and a/c coil (if no curlicue then but in plenum shut to furnace) and in return air between furnace and filter and add the two numbers together. The render reading will exist negative but add together them together as if it were positive. It's just negative pressure, not a negative number, ie .26"wc supply, -.24"wc return is .five total ESP. Cheque the appropriate blower performance curve chart in the manual to come across how many cfms that fan is moving. Hopefully they left the manual backside for the new furnace
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I was told (and you know how that goes) that I can take CFM measurements at all the supply vents and ad them upwardly to the systems total CFM that is beingness displaced. Then to take a static pressure reading at the return duct right before entering the evap surface area and and then to take a CFM measurement at the return and so review it with the manufactures data.
Is it true that near systems run well-nigh .5 inches of water colume at the return?
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It would exist skillful to get an expert to test and residual all the duct work but in the area that I am in for some reason, no one seems to want to practise that or has the test equipment. Later the chore is complete, they pack up and there gone. I take spoke of this and they companies that I told me they don't have the equipment to do this. They have yet measured static pressure. But never use an anemometer or a balometer or such.
For some reason likewise, every company I accept used dosen't practise duct layouts, they come up in and measure the new home, practice a load calc and then start installing the duct work afterward. Most homes that I take built have round duct piece of work and every vent is a 6" round metallic duct running to every supply vent. For case the house they just got done doing is 1745 sq.ft and they installed a iii ton American Standard split unit. they started with 14" round pipe coming out of the supply on the furnace and then reducing downward to a 12" then near 10 feet later on a 10" and and so capping it. The duct is near 45' in length. and they take insulated with R-four bubble wrap insulation. The home has 12 - half dozen" vents in the ceiling through out and a 25"x25" return with 16" round metal pipe running to it. The system is doing well simply I just got curious nigh information technology. I have never seen anyone do a duct layout or blueprint. They just become with rule of thumb I guess. The inspector never asks for it for the let, simply the load calc.
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Okay.
Originally Posted by kywallace69 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 aforementioned HVAC guy for years. He recently retired and I take merely started using another company. The concluding company sized the supply duct work a lilliputian different, and the pressure dosen't fell like I am used to, and anyhow I bought a TIF 3220 to bank check the CFM of the system to satisfy myself. And likewise to balance the airflow when we get washed with the flooring and install the vents.
For instance I just had a 3 ton arrangement installed and I am assuming that the organisation air displacement is 1200 cfm for the system. The meter will show velocity (ft/min) and CFM, merely measurments have to exist inputed in the device to measure the CFM. My primary question is how tin can I mensurate the CFM from the velocity readings?
The supply vents are 4"x10" in the floor area.
I asked this from the HVAC company just they actually didn't reply my question. They just went in circles. Cheers for you fourth dimension folks!!!
The simplest and quickest way to run into if information technology'due south in the ballpark, is to read the MFG data plate; run across the Model #. This should tell you lot the size of the equipment (chapters) and blower CFM chapters. It should definitely exist more than 1200 that you lot need. If non, definitely it will not be enough due to the static pressure level of the duct work.
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I volition likewise say the all-time way is to utilize a menstruation hood but expensive. ESP would too be a expert fashion you lot 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 xvi" supply, 18" return and a couple more outlets. Only my
.MH
What Is The Free Area For A 4x10 Floor Register,
Source: https://hvac-talk.com/vbb/threads/810892-Calculating-CFM-from-Velocity
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