Duct Size Calculations

santarevJune 9, 2008

I am having a new HVAC put in, and doing some homework to spotcheck my contractor. I was looking to calculate the size of a return, but this looks a bit more complicated then anticipated. I am trying to figure out a discrepancy between different contractors installing a residential HVAC system. 2 state I need a new return, one does not.

The one that does not has very good references, but I am concerned that he is in the minority on the thoughts of the size of the return. He is also the one that seem all around to know what he is doing (except this one point)

I have a old furnace/AC unit, going to a Trane2.5 ton AC unit (15 SEER) and 95% furnace (85K BTU) - itÂs a 1900 sq ft house in PA.

The current return is 8X18 giving me either 725CFM (0.08) or 820 CFM at .1SP. The 2.5 ton requires 1000 CFM - meaning we should increase to 8"X22" @0.8. My contractor says I only need to be at 80% of the 1000CFM for the system to work correctly (where I am with the current return) and is fighting me on increasing the return size, stating its not necessary.

I would think if the 2.5 ton needs 1000CFM, it needs 1000CFM. Right now with my 8"X18 I am at around 800CFM (80%). Will this work? Is my contractor just not wanting to do the work or am I reading into things?

I don't know what to do, or how to convince him either way. He wrote in the contract "all ductworkd requried, etc", but when I realized that he wasn't planning on increasing the return for the reasons he said above, I am now questioning.

This isn't my profession, so I will have a difficult time to present a case and reason for my inquiry

Thank you


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Actually anyone exceeding .05" ESP on residential will be unhappy. Besides air flow requirements for the AC 350-400 CFM across the coil, that much static and add a filter and the blower will surely drag which results in excessive current draws, excessive motor loads and decreased life and efficiency.

What you donÂt say when you mention return is, are we talking only the drop to the furnace or the duct that returns 100% of the air from one direction from the first return register? That matters?

For instance if it is a 18x8 drop that is 5Â long tied into a 18x8 going both ways with returns of appropriate openings on both sides, while it should be bigger it could work.

    Bookmark   June 9, 2008 at 4:48PM
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z:700 is correct, keep the (ESP) External Static pressure at or below .5"; I am sure that is what he meant. I like to keep the Return air ducting .05 Friction Rate per 100 foot of duct run.

However, "when sizing duct systems," I always like to go "450-cfm per ton of cooling through the A-Coil." Also, I would size for 450 because for Heat pumps 450-cfm per ton is required.

For 1125-cfm, 201-sq.in. duct area (16" Rd duct) Velocity would be 806-fpm, faster than I want, FR per 100 feet 0.07".

I thought I answered this question before.

For a 2.5-ton, 450-cfm per ton is 1125-cfm. I would size the return for that all the way, including an oversized filter rack.

Air Filter Rack Sizing
Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton. For a 2.5-Ton system, that would mean 25X20 open area filter rack or furnace return area.

All filter mfg'ers should print the free air area of the clean filter on the edge of the filter, along with the pressure drop data. Divide the rated CFM the duct is carrying by the free area sq.ft. of the filter for airflow velocity in FPM.

"Try to reduce the air velocity through a clean filter to 350-FPM and to reduce the resultant pressure drops as the filter loads."

Use the topic links at the top of the linked page for insight toward more efficient duct system airflow, lower static pressures & higher cfm. - udarrell

Here is a link that might be useful: Correct Sizing of Residential Ductwork Systems

    Bookmark   June 9, 2008 at 10:19PM
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Dear All

The 8X18 is the size of the return to the furnace. It does drop from the ceiling - about 6 ft of it from the horizontal returns (each from a different side of the house). (to answer LZ700's question)

My concern is if this is big enough of a return for the stated resons.... I thought A) we needed to be within +/- 10% of the delivery of air for optimal work. Undersizing can cause too many problems ( noise, fan power, unit effiency, etc).

This contractor may not be wanting to go the extra mile. I just need to know as a consumer how to show him what is best for the system. my calculations show 1000CFM travels thru an 8X18 duct at 1000FPM and 800 CFM travels through 8X18 at 800FPM. Both above Manual D recs of 700 FPM max return velocity. Obviously high velocity causes increased friction (restriction to flow) and standard filters work best belwo 400FPM.

I am trying to stick to my guns for a 8X22 return..... Unless someone thinks otherwise. Let me know...appreciate the help


    Bookmark   June 10, 2008 at 9:22AM
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I would like to know the the calculation metheod for ducting work. I have client wants to install 25 ton 3 package units in a room size 20x30m for the temperature should be below 27 deg. Please tell me the duct size to be used and the details

    Bookmark   May 18, 2011 at 12:01PM
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