What size supply duct for 3 ton unit

30 Oct 2023 ... Are you considering upgrading to a variable speed heat pump, but you're unsure if your existing duct system can handle it?

What size supply duct for 3 ton unit. A furnace return system must collect all the air being put out by that system. If the furnace is putting out 1,000 cubic feet per minute of air flow, the return system must collect 1,000 cubic feet and take it back to the unit. An insufficient return system or one too large will reduce the efficiency of the heating and increase energy bills.

The airflow capacity of an HVAC unit is measured in CFM per square foot, and this number is important when it comes to determining if a unit is big enough for a building as well as sizing ducts. To calculate it, you need to know the output of the unit in tons, where 1 ton = 12,000 British thermal units (BTUs) of heating or cooling capacity.

The return grill size would be 24x30. The run would be approximately 10-15 feet. Thanks! In many cases 18" will be large enough. But I can't see how its being installed in your home to know for sure. However, a 24x30 return grille …For a three-ton unit, you will require an 18″ round air duct size for your ductwork. If you intend to use a rectangular air duct, you need a ductwork size of …It's a two ton unit and I can't figure out how to size the plenum. The Air handler is central in the attic, only 6 runs so I'm going to do a radial duct system. Example. System Info-. 2 ton system with supply air opening of 16.375 x 11.3 Inches. 6 flex duct runs total; 5 8" runs 1 4" run. Longest run is 15 feet. 1.19 Jan 2021 ... Understanding the procedure of reducing the size of an air duct based on sound principles, not rules of thumb. Proper Duct sizing, no rules ...If you dropped the CFM to 350-cfm per ton of cooling the 18 inch metal RA duct would deliver at 991-fpm, which is really too high for RA flow, friction rate is also high for RA @0.09". For RA @2000-CFM I'd want a 22" metal duct @758-fpm & 0.04" friction Rate/100'.A plenum is a distribution box attached to the supply outlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. It is also used to balnce the air and attenuate the sound caused by …A good HVAC duct sizing rule of thumb is to measure rooms in your home, the necessary airflow rates, and friction loss rate. Using these numbers you can calculate the appropriate HVAC duct size. For example, a one-to AC system typically uses 10-inch ducts, while a 4-ton AC system requires 18-inch ducts. There are several variables to consider ...Average New AC Unit Costs. The average cost of installing a new central air conditioner in 2024, including equipment and labor costs, is $5,750 for a 3-ton unit in a 2,000-square-foot home. On average, a new AC unit cost will …

Figure 8. Step one. Screw the sides and ends together with the 1/4x5/8 screws provided. Assemble a long (46.18") duct support between the two sides (Return section): insert the first duct support 18.25" from either end by inserting the tabs on each end of the duct support into the appropriate triangular shaped slots.We would like to show you a description here but the site won't allow us.So if you go 25% bigger, I'd expect to notice much improvement in capacity & efficiency you'd certainly need more duct and register capacity. I'd bet staying 2 ton and having the duct system checked and improved would be all you need. Reply. 08-02-2013, 08:27 PM #7. kangaroogod.the duct system to be adequate for a .1 friction rate. • The chart should not be used to design complete duct systems. • To insure proper distribution of air, volume dampers must be used for all branch runs. • The chart is based on metal duct. Flex duct has a higher pressure drop and as such may require the size shown on this guide beFor example, my manual j calc suggest a 1.5 ton air conditioner. At 400 cfm per ton, my supply trunk must be large enough to deliver (400x1.5) 600cfm. Then the total cfm of the branches should be equal to the cfm of the trunk, so six 6" branches would be 600 cfm.All tests were run utilizing a 40-ton rooftop fan assembly at 20,000 cfm of airflow at 2" T.S.P. The 40-ton fan assembly consisted of two parallel, forward-curved, 20" diameter fan wheels. The different combinations and the resulting sound reduction in the critical first three octave bands are shown in Figure 9, p. 19.Everything I've read on HVAC forums says I need 1200 CFM for a 3 ton unit and that equates to 18" flex duct. My contractor is insisting that 16" flex is adequate and that I …

BlueStar Ductable AC Air-Cooled Ducted Split Air Conditioners 3, 4.3, 5.5, 8.5, 8.75, 11, 16.5, 22 Tons (TR). Blue star ducted split air conditioners are ideal when the floor space is limited but the Air Conditioning unit can be installed or mounted above the false ceiling.2. Ductwork can easily be installed using the shortest amount of duct. 3. Location allows easy installation of drain pipe. 4. Space is left around the unit as required in Fig. 1.1 for future service and maintenance. 5. Air inlet and outlet registers of the unit should never be blocked, so airflow can reach every corner of the room. WARNING Fig ...However, upon request, many grille suppliers do provide custom-made sizes at a higher price. Return air grilles are standardized based on 2″ per size increase. The smallest return air grille is usually starts at 4 inches by 4 inches. So, the next corresponding return air grille size includes 4×6, 6×6, 6×4, 8×6, 4×8 and so on.Room air conditioner size for a 3-bedroom house Room Air Conditioner Size for 4-Bedroom Houses. Below are room air conditioner sizing charts for 4-bedroom houses (2000 sqft and 2400 sqft). The air conditioner sizes (BTU) are rounded to the actual common air conditioner sizes rather than just the result of the calculated cooling capacity.

Jared allen restaurant colorado.

I have a 1973 mobile home. it is a double wide it is 1400 sq ft. and has a 4 ton coleman a/c unit 12" return. and 12" supply that y's to 2 10" that connect at the end of the trunk lines that is about 40' long one on each side of the mobile home the trunk lines are 10"x5" aluminum. 5 4x8 floor registers out of each trunk line. the problem I am having is only about 1 1/2 ton of air flow is ...Ducted Air Conditioner. Explore the most comprehensive range of Ducted air conditioning systems at Daikin. They are perfect for homes, open-plan offices, shops, schools, restaurants and any other big-size spaces that require balanced air distribution around the room. The most practical choice for heating or cooling large air conditioning ...It looks like duct tape, but it's not duct tape, and it's sometimes found on airplanes, where passengers have been known to snap photos of it just prior to f... It looks like duct ...Our previous 5 ton unit only had a 16x16 (leading to a 12 inch round) and a 12x12 (leading to a 8 inch round). We had many issues with our previous unit which i believed to be linked to our low return air duct sizes. In the 15 years we owned that unit we had to replace the condensor 3 times (about every 5.

Mobile home package units operate at a higher static pressure than a standard split system. The duct in the mobile home is usually 12 * 4. Running a 16 inch flex to that is not going to help. Want to help the effiencency install a larger return. Get rid of some of the static pressure on that side.Central AC Btu Calculator. Locate your climate zone on the map above and provide your square footage and we'll determine how many BTUs your heating system should have. Climate Zone: See the climate zone map above to find your home's climate zone. Home Size (sq ft): Insulation Condition: Sun Exposure: Recommend AC Unit Size: 2 - 2.5 Ton.The right size of return duct for a 3.5 ton AC is 22 X 22 inches. In the case of a 4 ton AC, it’s 24 X 24 inches. And for a 5 ton AC, the correct size is 27 X 27 inches. Calculate the return duct size. And multiply AC tonnage by 3.14 for round and 144 square inches. This is for rectangular ducts. Now, we have all the answers and calculations ...I have a 5 Ton Goodman package unit with gas heat. The HVAC guy upgraded this from a 3 Ton because he said the 3 Ton wasn't big enough to cool/heat the area it was setup with. (He sized the original 3 Ton unit.) The unit goes to three rooms primarily: 12x15 w/9ft ceiling, 1 vent (14 inch opening) 16x20 w/vaulting ceiling from 8ft to 16ft, 3 ...A 2-ton air conditioning unit is capable of cooling between 800 and 2,000 square feet of interior space. A building’s energy efficiency, including air leaks, insulation and shading...Min. Breaker Size: 22.8 Amps: Tonnage: 3 Ton: Fuel Type: Gas: Height: 34 3/4" Width: 47" Depth: 51" Warranty: 20 Years Heat Exchanger and 10 Years Parts with Online Registration: ... Horizontal Duct Cover for Goodman Packaged Units View Item Add To Cart. Qty. Horizontal Duct Cover for Goodman Packaged Units. $34.99.Since the size of your ductwork can increase or lower this measurement, you’ll have to find the necessary CFM for each room before you can get the right duct size for each space. Cubic Feet Per Minute = (HVAC Unit Tons x 400) / total square footage of home. Calculate for each individual room. To calculate CFM, you need to know the size …LanzaJet, the renewable jet fuel startup spun out from the longtime renewable and synthetic fuel manufacturer LanzaTech, has inked a supply agreement with British Airways to supply...

Step 3. Calculate the return duct size based on the air conditioner's size and the type of duct you are using. Use the following formulas: For rectangular duct: Size of air conditioner (example: 4 tons) x 144 square inches = return air duct in square inches. For round duct: Diameter of duct x 3.14 = return air duct in square inches.

If you dropped the CFM to 350-cfm per ton of cooling the 18 inch metal RA duct would deliver at 991-fpm, which is really too high for RA flow, friction rate is also high for RA @0.09". For RA @2000-CFM I'd want a 22" metal duct @758-fpm & 0.04" friction Rate/100'.The original Small Duct High Velocity central heating and cooling system, SpacePak is the perfect choice for retrofits and new construction. Easily installed in any home. A seamless design blends in with your home's decor. Superior comfort with unique draft-free delivery. With whisper-quiet operation and no bulky wall units, you'll never even ...Frequently Asked Questions. What return duct size do I need for a 2 Ton unit? For a 2 ton unit you can use either a 19″ flex duct or one of the following rectangular ducts: 14″x13″, 15″x12″, 17″x11″, 18″x10″, …In a rule of thumb sense the return duct should be the same size as all the supply ducts combined. But a rule of thumb is just an initial guide, not anything to bet your comfort on. You have to consider the length of duct and the type of duct. HVAC engineers have developed manuals to cover all the ins and outs of how to size duct work.A duct smoke detector should shut off a typical blower and fresh air and turn on the exhaust. Duct detectors aren't a replacement for room sensors. If the duct system is designed to carry more than 2000 CFM (5 tons nominal) of air, you need one in the return if IMC is being followed and the supply if NFPA is being followed.Room Size. However, most HVAC contractors use a rule of thumb to determine the number of air vents needed. For example, for a standard residential HVAC unit, you'll generally need one supply vent and one return vent for every 100 to 150 square feet of living space. This means that a 10-foot by 12-foot bedroom (120 square feet) would need one ...This unit is one of the lightest, most compact packaged units ever designed. It's light weight (230 lb [104 kg]) for the 024 size) makes the unit easier to handle. The low height keeps ductwork connections to a minimum and makes units less visible. This unit utilizes a structural beam design to form the four sides of the cabinet.A common 5 ton system would be 2000 CFM (400 CFM per ton) so a 24x30 register or grill (-.02" sp) with (10) feet of 24x12 duct (-.092"/100') with a duct transition to the RA opening of the unit (if duct is needed) would be a fairly low loss system. Reply. 05-29-2013, 06:08 PM #3. HVAC_HUSKER.

How to send money to yourself venmo.

Homestead exemption denton county.

The cfm should be matched to the condensing unit. 1200 cfm max for your 3 ton unit. Even at 1600 cfm your temp rise should still be up around 17 deg, so either the system isn't performing well or you're taking inaccurate measurements. 1350 is fine for a heat pump, in heating mode. 1600 a bit too much. Low-noise fans and vibration-reducing technology reduce outdoor sound levels to keep your neighborhood quiet, while precision controls lets you adjust temperature settings conveniently and accurately. Condenser Model Number: PCE4A3621. Questions? Speak with an AC Expert! Call Us at 1-877-895-1155. I am currently finishing my basement and trying to size my supply and return ducts. My current 4 ton systems barely can handle the current load(1900 sq ft house) and my wife has ruled out a zoned system due to cost. Current Supply System. 4 Ton: Three Branches off the Plenum: 12" x 10" : Upstairs (700 sq ft) 14" x 8" : LV and Family Rm (700 Sq ft)Buy O General Duct Air Conditioner 3 TON ARGA36FMTA-U online at the best price in UAE from Sharaf DG. Shop from a large selection of Air Conditioners, Cooling & Heating, Home Appliances, Window Air ConditionersApr 17, 2020 · Forum Rules. I currently have a 3 ton with 16 inch metal return duct work. I was told this by a contractor: 3 ton heat pumps with the new r410a refrigerant recommended size is 18" on the return duct. Otherwise you should expect issues with your system typically in winter time. I wanted to verify with you guys since the cost of upsizing the ... Jul 27, 2008. #2. You are really pushing it on the trunk size. I would say it will work, but you will be increasing the "static pressure", and you may end up with some noisy grille outlets. Your furnace is pushing 1200 cfm which is good for 3 tons of AC. Your 2 ton AC system would only require 800 cfm. The rule of thumb for AC is 400 cfm per ton.Run a large size plenum under home from Pkg unit. Run new branches to existing registers using six round 26ga pipe or insulated flex. Just slow, not stupid. ... on the master bedroom side. Shorten the helper up. So that it taps into the supply duct between the first supply for the master, and the last supply for the dining room. When posting in ...For floor registers, they follow the same rules as return grilles, where you state the dimension parallel to the louvers first. This means that floor registers will often have a smaller number first, like 4×10 or 4×12. Ceiling and some sidewall registers will usually just be described as long side first, such as a 10×6 or 12×8, but that can ... ….

The size of the HVAC unit you'll need for the whole house can be found using the unit's average output and the square footage of your home. As a rule of thumb, you'll need 400 CFM of airflow per ton of air conditioning; it takes about 1 CFM of air to heat or cool 1 to 1.25 square feet of floor area. Last, you'll need to understand duct friction.How large should return air duct be? A typical supply vent is 4 by 10 to 12 inches and a typical return vent is 16 by 20 inches or larger. ... your return air duct and grill size will be 24 by 24 inches.) Always place return ducts as far away from the supply air vents as possible. ... You have a 3 ton unit which is 1200 cfm’s. As r mentioned ...Take this number and multiply by 1,000. Then divide the answer by 12,000. The answer you get is the tonnage or size of your system. Let's say there's a 24 in the model number. Then the tonnage is (24,000/12,000) or 2 tons. If you're putting in a central air conditioning unit you'll be incurring quite an extensive cost.Make sure the pieces are pushed completely together using the metal pieces. Use the crimpers if you have to cut the pipe. Attach trunk line to wood using 1.25″ screws ( use two for both sides) but DO NOT over tighten. Attach the bottom of the trunk at an angle hammering into place.Take this number and multiply by 1,000. Then divide the answer by 12,000. The answer you get is the tonnage or size of your system. Let’s say there’s a 24 in the model number. Then the tonnage is (24,000/12,000) or 2 tons. If you’re putting in a central air conditioning unit you’ll be incurring quite an extensive cost.Apr 17, 2020 · Forum Rules. I currently have a 3 ton with 16 inch metal return duct work. I was told this by a contractor: 3 ton heat pumps with the new r410a refrigerant recommended size is 18" on the return duct. Otherwise you should expect issues with your system typically in winter time. I wanted to verify with you guys since the cost of upsizing the ... A supply duct can be too big if it exceeds the capacity of the HVAC system or if it results in inefficient airflow distribution. Proper duct sizing is essential to avoid these issues. ... What size duct does a 5-ton unit need to operate properly? A 5-ton HVAC unit typically requires ductwork sized to handle around 2000-2400 CFM. The exact duct ... What size supply duct for 3 ton unit, 24 Apr 2018 ... I replaced the 2 ton carrier heat pump package unit with a 2 ton goodman an had a few questions on duct sizing ... Most will run at the same size ..., The following sizes are what are commonly required for line sets. The first number is for the outside diameter (OD) of the copper liquid line, and the second number is for the OD of the suction line. Note that this is not the inside diameter of the lines. 1/4” x 3/8”. 1/4” x 1/2”. 1/4” x 5/8”. 3/8” x 5/8”., A plenum is a distribution box attached to the supply outlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. ... Download McQuay Duct Sizer for HVAC Duct Sizing. 10/30/2016 01:14:00 AM. Download AutoCAD Fonts Library - Free SHX Fonts ... Download Electrical Cable Size ..., Qty. $133.00. 8 Kilowatt 23,800 BTU Goodman Heater Coil View Item. Add To Cart. Qty. $127.00. Designed with an enhanced comfort in mind, the ARUF37C14 air handler lives up to Goodman's expectations to be efficient and inexpensive, ready to heat and cool your home when paired with a condenser whenever you need it., Average New AC Unit Costs. The average cost of installing a new central air conditioner in 2024, including equipment and labor costs, is $5,750 for a 3-ton unit in a 2,000-square-foot home. On average, a new AC unit cost will …, Location. North Richland Hills, Texas. Posts. 15,732. Post Likes. 3/4 should be allowable with every manufacturers 3 ton R-410A system, and is the recommended size for some. It won't void any warranties, and the 200-400 btuh of capacity difference will never make up for the cost of the refrigerant lines., A TXV is used for precise metering on the 6 ton to 12.5 ton and high SEER 3 ton to 5 ton products and a fixed orifice is used to keep the cost of the product down on the 3 ton to 5 ton product. Two independent refrigerant circuits and compressors are used on the 7.5 ton to 12.5 ton units for economical and precise control., A duct smoke detector should shut off a typical blower and fresh air and turn on the exhaust. Duct detectors aren't a replacement for room sensors. If the duct system is designed to carry more than 2000 CFM (5 tons nominal) of air, you need one in the return if IMC is being followed and the supply if NFPA is being followed., Multiply it by 144 to get the grille size in square inches and choose your preferred grille size based on that. For instance, 400 cfm ÷ 350 x 144 = 164.57 sq.in. So, the possible grille sizes are 20×8 (160 sq.in), 16×10 (160 sq.in) or 14×12 (168 sq.in) depending on your duct size. The above way is based on the grille sizing formula which:, However the main supply is only 18"x12" and runs 50 feet, it branches off to four 8x8 supplies on the 2nd floor, five 8x8 supplies on the first. By my calculations an 18"x12" main supply will only provide 430CFM or enough for a 1-ton system. So it looks like I would need 1400-1600CFM or something closer to 26"x26" or 28"x28"., The static pressure caused a windstorm in each room. I had two more supply ducts added, and the rest of them increased to a larger size (probably 9" or 10" flex) to lower the static pressure. The 3 ton system has finally quit (compressor locked rotor amps), so I'm looking at replacing it with a 2 ton system (per Manual J calcs)., Ducted Air Conditioner. Explore the most comprehensive range of Ducted air conditioning systems at Daikin. They are perfect for homes, open-plan offices, shops, schools, restaurants and any other big-size spaces that require balanced air distribution around the room. The most practical choice for heating or cooling large air conditioning ..., Low-noise fans and vibration-reducing technology reduce outdoor sound levels to keep your neighborhood quiet, while precision controls lets you adjust temperature settings conveniently and accurately. Condenser Model Number: PCE4A3621. Questions? Speak with an AC Expert! Call Us at 1-877-895-1155. , The maximum rises in temperature for PVC and steel pipes are 10.3 and 12.7 °C respectively. HVAC Duct Size Rules of Thumb - Cooling. Cooling mode: Air flow required: 400 CFM of air flow per Ton of A/C (1 Ton equals 12,000 BTU) ... So a 1 ton A/C unit moves say 400 CFM. ... The actual supply register location, size, design, and the boot ..., It's a two ton unit and I can't figure out how to size the plenum. The Air handler is central in the attic, only 6 runs so I'm going to do a radial duct system. Example. System Info-. 2 ton system with supply air opening of 16.375 x 11.3 Inches. 6 flex duct runs total; 5 8" runs 1 4" run. Longest run is 15 feet. 1., 10-year Parts, Labor, and Compressor. The Central Ducted Hyper Heat System comes with a Standard 10-year parts and labor warranty plus an additional 10-year compressor warranty., Wednesday, July 1, 2015. Click here to download the .pdf version of the duct size chart. Click here to read the blog post about duct sizing. This entry was posted in. HVAC - Tech Charts., The smallest return air grille is usually starts at 4 inches by 4 inches. So, the next corresponding return air grille size includes 4×6, 6×6, 6×4, 8×6, 4×8 and so on. The largest return air grille is typically stops at 48 inches by 24 inches. Here are some examples of a standard return air grille: 4×4. 6×6., Forum Rules. I currently have a 3 ton with 16 inch metal return duct work. I was told this by a contractor: 3 ton heat pumps with the new r410a refrigerant recommended size is 18" on the return duct. Otherwise you should expect issues with your system typically in winter time. I wanted to verify with you guys since the cost of upsizing the ..., Lowering the fan speed will reduce the noise level, but the temperature of the air coming out of the coil will drop as the fan speed is reduced. You don't want it to get too low. Just as an example, in my system the air going in is at 75 F and coming out of the closest duct is 59 F. - Jim Stewart., Flat Elbows & Air Duct Reducers (19) Category Flat+Elbows+%26+Air+Duct+Reducers Flat Elbows & Air Duct Reducers 5' Length (14) Category 5%27+Length 5' Length End Caps (10) Category End+Caps End Caps, Post Likes. The (minimum) rule of thumb is 5 outlets per ton. A long supply run will have an effect on the airflow it can deliver. The last Unico (or it may have been a SpacePac) I bid needed to have the "plenum" enlarged from the original 7" to 9" and then I need to re-route some of the supplies off of the 9"., This is a 2.5" solid White oak unfinished surface mount supply outlet with screen. It comes in a box quantity of 1. ... It can deliver up to 9 tons of air conditioning in the same space as a 3-ton traditional unit. Technical Specifications: Duct Size: 2.5: Matching Duct Type: Sound Attenuator: Material: Wood: Duct Connection Type: Clamp: Supply ..., System Size (Tons): 3. System Size (Tons): 4. System Size (Tons): 5. System Size (Tons): 2. MRCOOL. MRCOOL ROYALTON 4 5 1 3 Residential Commercial/Residential. 26 products in Central Air Conditioners. Sort By ... Commercial/Residential 3.5-Ton 17600-BTU 14-Seer Central Air Conditioner. Find My Store., Feb 28, 2010 · What I gather, is that duct work is designed around the a/c unit cfms. My house needs a 1.5 ton air conditioner (18k heat gain) and at 400 cfm/ton is 600cfms. My current sized trunk would have the 600 cfms in mind and then reduce from there correctly. My load cal program also gives the cfm/room and suggested duct sizes. , Return Duct Size for 3 Ton A/C. I am getting my A/C and furnace replaced, and the old grille and duct is way undersized (20"X20" and 14" flex). Everything I've read on HVAC forums says I need 1200 CFM for a 3 ton unit and that equates to 18" flex duct. My contractor is insisting that 16" flex is adequate and that I should trust his 50 years of ... , Buy Goodman GPC1460M41 5 Ton, 14 SEER Self-Contained Packaged Air Conditioner, Multi-Position. ... Supply Air Opening Size. side 16w x 18h bottom 22w x 11d Inches. Return Air Opening Size. ... 50" x 53" x 3" Condensing Unit Pad $239.99. Add to Cart. Thermostats. Cielo Smart Thermostat Eco ..., The quote was for a new furnace and 3 ton heat pump (American standard 14 seer) $ with original ductwork or $ with all new. ... my sister's trailer had ducts in ceiling, but no access. just curious as to how they would access the ducts. ... if you are doing duct and all i would go to a package unit. Reply . 09-30-2016, 09:24 AM #14. DOGBOY ..., The following sizes are what are commonly required for line sets. The first number is for the outside diameter (OD) of the copper liquid line, and the second number is for the OD of the suction line. Note that this is not the inside diameter of the lines. 1/4" x 3/8". 1/4" x 1/2". 1/4" x 5/8". 3/8" x 5/8"., This unit is one of the lightest, most compact packaged units ever designed. It's light weight (230 lb [104 kg]) for the 024 size) makes the unit easier to handle. The low height keeps ductwork connections to a minimum and makes units less visible. This unit utilizes a structural beam design to form the four sides of the cabinet., With only 423 sq in of return your air velocity through the grille would be just over 400 ft/min, a good number as far as noise is concerned. Air velocity of more than 500 ft/min will be too noisy for a residential system. As important as air velocity is for noise, also consider the pressure drop across the grille and filter., You have a 3 ton unit which is 1200 cfm's. As r mentioned 1 ton = 400 cfm's. If that 14" round return is all you have it is undersized for your unit. A 14" round pipe is equivalent to 750 cfm's. If it's flex it's 700. Your return trunk should at least match your supply or even 10-20 percent larger., Step 3. Calculate the return duct size based on the air conditioner's size and the type of duct you are using. Use the following formulas: For rectangular duct: Size of air conditioner (example: 4 tons) x 144 square inches = return air duct in square inches. For round duct: Diameter of duct x 3.14 = return air duct in square inches.