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Duct Work Size by CFM?

Posted by Ritt123 (My Page) on
Thu, May 3, 12 at 18:22

What size duct should I use for a Goodman Air Handler ARUF 30316 (2.5 Ton I Think)
Blower CFM (High/Med/Low) = (1240/1230/980)
6 registers
1) 5ft from AC
2) 10ft from AC
3) 20ft from AC
4) 20ft from AC
5) 35ft from AC
6) 25ft from AC
Same size on all Ducts ok with me. The all go in to one Big Room except one.

I don't know what I'm doing but I don't want to pay someone to do the duct work, I will get an AC person to hook up the unit when I can afford it. I am just trying to get the duct work in, before I put the blow in insulation. Thanks for any help you can give.


Follow-Up Postings:

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RE: Duct Work Size by CFM?

Ritt -

I understand your desire to save money - this could end up costing you more money in the long run if not done properly. Your self proclamation of not knowing what you are doing is scary to say the least.

You would be better off to have the help of someone with HVAC knowledge, and you help him / her with simple sweat equity, along with an agreed price for their help.

Simply put - you can pay now, or pay later.

Good luck!


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RE: Duct Work Size by CFM?

Even if the ductwork is rigid and 6 inches in diameter per register, that would be 600CFM total.

You still may be oversized. You may experience problems such as the coil freezing up unless you install an evaporator freeze thermostat. You may also experience noisy registers if the system cannot handle the CFM.

BTW: I agree with the above post by bpChill

SR


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RE: Duct Work Size by CFM?

you bought your equipment online didn't you? Seek professional help now before you hurt yourself.


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RE: Duct Work Size by CFM?

No, I did not purchase this air handler on line. I only would like some information I don’t need advice thanks. I would think that anyone with the correct information could run a little flex duct to a register and over to where the AC will be. I know that you need to keep the lines straight as possible and to avoid restrictions. I’m not an AC person but I can do a simple task thanks. I don’t need for people to thank that I’m an idiot. When I said I don’t know what I’m doing what I meant that I don’t know how to do the math. If the Blower CFM on High is 1240 should the total duct work with 6 runs need to add up to 1240? If this is the case and an 8” Flexible Duct CFM is rated at 160 CFM I could use 8 of them. But since I only need 6 I could Run two 10” Flexible Ducts rated at 300 CFM and four 8” and the total CFM would come out to 1240. I believe that it would be better to have more duct CFM capacity then to little CFM capacity for the AC Unit. Thanks for you feed back but I am looking for answers.


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Thanks

Well let me try this again. Did not look before I leaped last time.
No, I did not purchase this air handler on line. I only would like some information I do not need advice thanks. I would think that anyone with the correct information could run a little flex duct to a register and over to where the AC will be. I know that you need to keep the lines straight as possible and to avoid restrictions. I am not an AC person but I can do a simple task thanks. I do not need for people to thank that I�m an idiot. When I said I do not know what I am doing what I meant that I do not know how to do the math. If the Blower CFM on High is 1240 should the total duct work with 6 runs need to add up to 1240? If this is the case and an 8 inch Flexible Duct CFM is rated at 160 CFM I could use 8 of them. But since I only need 6 I could Run two 10 inch Flexible Ducts rated at 300 CFM and four 8 inch and the total CFM would come out to 1240. I believe that it would be better to have more duct CFM capacity then to little CFM capacity for the AC Unit. Thanks for you feed back but I am looking for answers.


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RE: Duct Work Size by CFM?

Buy a ductulator.


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RE: Duct Work Size by CFM?

I am not a HVAC guy but I did run ducts prior to drywall in a basement finishing project. I was just tying into another system but I wanted it to handle the entire 4 tons if I zoned the system. That would only be for heat and it is an Infinity system so it would cut back the flow if the ductwork was inadequate - this makes a huge difference compared to what you are doing.

I did a lot of research and in the end, I overdid the ductwork to be safe. Yes you can go too big but it is harder and the distances were short and the loads are low. The ducts are not oversized for the possible max airflow but they are oversized for the load.

By memory, I had 2 8 inch runs that were 10 feet and 3 9 inch runs that were 20 feet (roughly).

I haven't zoned it yet and it works fine. So it really is just stealing a ton or so off the main system.

The one thing about flow is that a rating does not guarantee the flow. Obviously a 25 ft run of 8 inch flex duct is not going to flow 160 CFMs as easily as a 5 ft run will. The longer runs need to be sized bigger to compensate - I believe that is what a ductulator does for you.


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RE: Duct Work Size by CFM?

Assuming you get all the ducts sized correctly, how do you know a 2.5 ton AC is the correct size to cool the space?


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RE: Duct Work Size by CFM?

"I don't know what I'm doing but I don't want to pay someone to do the duct work"

Then you better learn how to design duct work.


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