# How do you measure CFM?



## Guest (Jul 7, 2004)

What do you want to mesure the CFM of ??
The Blower is rated at a Cfm at a specific Static Pressure . Ducts are Rated at a specific CFM for a Specific Static Pressure Also. Please Let us Knpw What Your Problem Is . 
There are instruments to measure the Velocity of the air comming out of the Registers. But CFM is a Volume Measurement. On Most Duct systems the Cfm of a 6" Round duct is approx. 100cfm. 8" Round 200Cfm Etc.The rule of thumb is 600 Cfm Required Per Ton of Cooling.


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## Guest (Oct 6, 2004)

*CFM question*

I have the same question for you guys. We have a window A/C unit from wich a rigid ducting was installed years ago. Two registers are located downstream, the second one about 5 meters away from the A/C unit. From that register a flexible 6" duct was attached to a point about 2 meters away whre it hits the last register. The problem is that in between the second and last register a beam cuts across the ducting which forced the installer to make an "S" bend in order to go below it and again up (whole installation is up against the ceiling). As you can see, the flow to the last register is minimal. Someone installed a "booster fan" aproximately a third of the way down from the second register and still the flow is negligible. I was thinking about buying a "real" booster fan from HOME DEPOT, it is rated a 80cfm. Someone was offering me a register wich has a fan attached directly inside. the actual "booster" and the one I just mentioned look like an old insdustrial fan inside of a square metal box, they do not look very efficient, no rating is available either. Will the HOME DEPOT unit solve my problem ? from your explanation I gather no more than 100CFM will be available from my ducting, is that right ? [email protected]


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## carmon (May 8, 2010)

Unregistered said:


> I have the same question for you guys. We have a window A/C unit from wich a rigid ducting was installed years ago. Two registers are located downstream, the second one about 5 meters away from the A/C unit. From that register a flexible 6" duct was attached to a point about 2 meters away whre it hits the last register. The problem is that in between the second and last register a beam cuts across the ducting which forced the installer to make an "S" bend in order to go below it and again up (whole installation is up against the ceiling). As you can see, the flow to the last register is minimal. Someone installed a "booster fan" aproximately a third of the way down from the second register and still the flow is negligible. I was thinking about buying a "real" booster fan from HOME DEPOT, it is rated a 80cfm. Someone was offering me a register wich has a fan attached directly inside. the actual "booster" and the one I just mentioned look like an old insdustrial fan inside of a square metal box, they do not look very efficient, no rating is available either. Will the HOME DEPOT unit solve my problem ? from your explanation I gather no more than 100CFM will be available from my ducting, is that right ? [email protected]


really ducting a window shaker,,,,,, not such a good idea


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## joepierson (Oct 26, 2011)

Unregistered said:


> I am trying to find out how to measure cfm, can anyone help?


measure the static pressure (before and after the blower), look up the CFM from manufacturer manual


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## freeagent (Nov 11, 2011)

Unregistered said:


> What do you want to mesure the CFM of ??
> The Blower is rated at a Cfm at a specific Static Pressure . Ducts are Rated at a specific CFM for a Specific Static Pressure Also. Please Let us Knpw What Your Problem Is .
> There are instruments to measure the Velocity of the air comming out of the Registers. But CFM is a Volume Measurement. On Most Duct systems the Cfm of a 6" Round duct is approx. 100cfm. 8" Round 200Cfm Etc.The rule of thumb is 600 Cfm Required Per Ton of Cooling.


600cfm/ton seems very high - I thought the rule of thumb was more like 400-450cfm/ton. A typical 3-ton unit would have a flow of 1200cfm. Minimum would be 350cfm/ton.


Regarding measuring flow. One not very accurate way, is to use an anemometer. If you only have one or two return grilles, that may be the easiest place to measure. You have to try and get an average face velocity an inch or so from the grille by moving the anemometer in a grid pattern. Some anemometers will do teh averaging for you. Once uou have the average velocity, then multiply that by the face area of the grille. fpm x A=CFM.

There are more accurate ways but they require more sophisticated equipment. Using the fan curve as already posted is another relatively easy way but getting the sp probes in just the right place may be a trick.


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