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# CFM calculation

## CFM calculation

#1
10-06-09, 08:41 AM
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Join Date: Jul 2009
Location: malaysia
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CFM calculation

Does anyone can help me the below:

1) How to calculate the CFM from the floor plan? Any standard
values or figures per square feet
2) What are the CFM different between Hotel Coffee House
& Office Area?
3) How to find out the actual CFM from the A.H.U ?

Thank you.

#2
10-06-09, 08:24 PM
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Join Date: Dec 2005
Location: VA
Posts: 5,491
#1 and 2 This should be with the heat load cal. Nothing is standard.

#3Give M# of AHU that will tell us what size it is.

#3
10-07-09, 01:24 AM
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Join Date: Jul 2009
Location: malaysia
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#Could i know the airflow CFM & cooling capacity with a area 6,066 square ft (Hotel
Coffee Hse) which is using either A.H.U (chilled wtr system) Or DX cooling Coil system?

# How many air flow CFM / ton (Min & Max)?

Thank you.

#4
10-07-09, 08:40 AM
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400 cfm a ton. You need to do a heat load to know how many tons of cooling you need.

#5
10-22-09, 04:13 PM
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Location: Akron Ohio
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400 CFM per ton is a common estimate but is that always correct? I did an actual CFM calculation for my Manual-J (which stated I needed about 22k cooling) and the figures showed a total CFM of about 900. Possibly I made a mistake in the calculations or that I used the wrong variable in the equation but the formula was pretty simple and the variables were taken from a manual-d book (although the info was posted on a forum, I didn't actually read it from the book myself)

#6
10-23-09, 04:37 AM
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Location: VA
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2 tons of cooling at 900 cfm you are in range.

Originally Posted by shane21
400 CFM per ton is a common estimate but is that always correct? I did an actual CFM calculation for my Manual-J (which stated I needed about 22k cooling) and the figures showed a total CFM of about 900. Possibly I made a mistake in the calculations or that I used the wrong variable in the equation but the formula was pretty simple and the variables were taken from a manual-d book (although the info was posted on a forum, I didn't actually read it from the book myself)

#7
10-23-09, 07:34 AM
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Join Date: Oct 2009
Location: Akron Ohio
Posts: 194
well at 800 CFM I can use a standard 16x25 filter. But with 913 CFM it kinda forces me into the 20x25 filter range which increases cost on filter rack, return boot and it will cause me to have to fabricate a transition piece from the filter rack to the return opening on the furnace as it isn't quite large enough to accommodate the 20x25 filter rack.

I guess my question was if I calculated my CFM at 913 and it's just below 22K cooling wouldn't a better estimate be 500 CFM per ton? On a 1 ton or even a 2 ton unit the difference isn't too bad but if figuring duct work for a 4 ton unit the difference in duct needed for 1600 CFM vs 2000 CFM is pretty substantial.

I am no pro when it comes to HVAC just done a lot of reading on it the last 4 weeks and have seen the 400 CFM per ton both ridiculed for being too small and endorsed by people in the industry for 20+ years as being reliable. I don't know the answer I just know it was too small an estimate for my system when I did the actual calculations.

#8
10-23-09, 09:29 AM
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Return and supply duct should always be sized for the max cfm of air. Y not use a filter grille. Return duct should be around15x14 and supply around 13x13

#9
10-23-09, 10:10 AM
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Location: Akron Ohio
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do you mean use a filter grille at every cold air return? I'm not sure how a filter grille would address my specific situation.

#10
10-23-09, 07:37 PM
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If you don't have a common return a filter grille will not work. Why not use the filter rack that is in the AHU?

#11
10-24-09, 09:04 AM
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Location: Akron Ohio
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Do you mean installing a filter inside the AHU through the front access panel? There isn't an actual rack there but I suppose making one would be pretty easy. Can I do that?

Sorry I am an admitted novice when it comes to forced air units. The only real furnace install/service work I have ever done was on hydronic heating systems.

#12
10-24-09, 03:15 PM
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I have never seen a unit that does not have a rack. Are you sure it does not have one?

#13
10-24-09, 06:33 PM
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I checked again now that I'm home and it does not have a filter rack inside the AHU sealed cavity. This furnace doesn't even have a return hole cut in the AHU sealed cavity. It is made to be a left/right/bottom fed return so the actual hole has to be cut in the field. I'm guessing this is why no filter rack?

#14
11-02-09, 04:41 AM
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Filter

Some manufacturers actually sell a separate filter rack assembly to be purchased at time of installation , most do not do this .
I would trace the complete return duct from air handler to filter grill , if you have room to room , them it should be at air handler.
If you have one return i would put in a filter grill making sure all return duct in sealed. If you have room to room , I would do a lil work and measure a diagonal distance close to your return to a common filter size ; say your return duct is 18 up x 20 across you could cut in your angle in as on the 18 side 20 or 25 inches .
Then go to any duct or hvac shop tell them what you did and they should know to bend you up a filter rack and cover .You will have to screw in the bakk side with 1/4 inch screws so c mace sure you have access or it will move.

Just a suggestion and MAKE SURE YOUR COIL IS CLEAN ON RETURN SIDE