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Duct Sizing, Is there a Rule of Thumb?

Mike_Leger's picture

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I'm an owner builder, designing my heating system. Is there a rule of thumb for duct size vs. cfm? cfm needed per sq ft? I will have a 1000 cfm fau and need to calculate sizes for distribution and design of the plenum. I live on the coast in central ca. so no ac is necssary.

thanks,
mike

(post #164826, reply #1 of 7)

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go to the library and get a book on Heating and Air Conditioning..duct sizing is dependent on several variables... and improper duct sizing is the most common reason for Bad hot air / Air conditioning systems..

coastal areas often need A/C more than others becase of the humidiity.. A/C systems not only cool... they dehumidify....

(post #164826, reply #2 of 7)

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While you are at the library, a good brief discussion of the equal-friction and static drop methods of duct sizing (along with numerical step by step calculation examples and tables) can be found on pgs 2-94 to 2-103 of"Standard HDBK of engineering calculations", T. Hicks, 1972, McGraw Hill.

(post #164826, reply #3 of 7)

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Good rule of thumb is don't use rules of thumb for this stuff. Really.

Bob

(post #164826, reply #4 of 7)

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Art,
Thanks for the input, sounds like it will be helpful. I am not afraid of numbers.... I'll check it out.

Thanks,
Mike Leger

(post #164826, reply #5 of 7)

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You can get a cardboard slide rule to calculate the air flow per foot of run for different sizes of duct, rectangular or round, plus the effective extra feet of resistance caused by various transitions. This gets complicated and boring real quick, and then you have to balance the returns. If it's done wrong, and it looks like it usually is, air will be forced out of the house in some places, sucked in at others ... closing doors will change the relative temps of rooms ... etc. The house won't be comfortable or efficient -- that is, it'll be like mine.

That said, I'll admit I'm trying on my own to work out some improvements to the ridiculous setup we have, leaving access to the key parts and putting in a damper or two to permit tweaking later. The desirable air flow per room will depend on how much heat is created, gained or lost there -- sun exposure, number of machines, lights and people in it, etc. Then the total airflow MUST be enough to keep the temperature rise across the furnace heat exchanger from cracking and trying to poison you. And the furnace room must not be depressurized so as to backdraft flue exhaust. Rules of thumb don't sound so appealing, do they?

I understand there are a number of software design packages. It might be worth it to pay a good HVAC company to design the system for you. An advantage the HVAC people also have is the fancy tools to measure the actual air flow, pressure, temperature, etc.

(post #164826, reply #6 of 7)

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Been there, done that with and without the use of an engineer.

For me, you get close, but that is about it. There are minimum rates of air flow that are comfortable and quiet. After that it gets to be a mess. Technically, there should be scoops in the main duct that drop into the air flow and divert some of the air to each register instead of a dampter that increas static pressure with all sorts of nasty things happening to all the lines.

I have always oversized the ducts to reduce the friction and noise, and accepted a less than perfect result.

We tried to engineer my office building and used a good union sheet metal shop, but the ducts get awfully big, and the mechanical spaces very crowded.

Good luck,

Dennis

(post #164826, reply #7 of 7)

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I'm an owner builder, designing my heating system. Is there a rule of thumb for duct size vs. cfm? cfm needed per sq ft? I will have a 1000 cfm fau and need to calculate sizes for distribution and design of the plenum. I live on the coast in central ca. so no ac is necssary.

thanks,
mike