blue and red swoop graphic

HVAC Duct Sizing Rule Of Thumb

By Aztil –

The HVAC unit installed in your home is responsible for its heating and cooling as the weather demands. The unit alone does not determine how efficiently it will work. There are many other factors that play a role in deciding so. One such decisive factor is the duct size. Are you surprised to hear that? But that’s the truth. You must be wondering how on earth can the duct size be important, after all, it just expels air? Moreover, what is the correct duct size and is there an HVAC duct sizing rule of thumb? Well, the purpose of this article is to answer these questions and help you understand the importance of ducts.

AC Ducts

Ducts are channels or passages that are used in heating, ventilation, and air conditioning (HVAC) to distribute fresh and get rid of stale air. The HVAC unit fills the supply plenum with either cooled or heated air, as desired. It is directed via the ductwork of the system. When fresh air enters a room, it pushes out the already present air from the room into yet another set of ducts. These ducts then deliver the stale air to the return plenum.

HVAC Duct Sizing Rule Of Thumb

A lot of times, your AC contractor would want to size your AC using the rule of thumb. Since you don’t know what it is, you must be flabbergasted. In the simplest of terms, the rule of thumb states – for every 500 square feet of air-conditioned floor area install an AC of one-tonne capacity.

Why Does Duct Size Matter?

One thing pretty clear is that you need to have appropriately sized ducts for the HVAC to work effectively. But why is that so? Here’s the answer to your question. Ducts that are small in size make your HVAC system work harder to keep your home at the desired temperature. If your duct size is big, the velocity of air gets compromised. It means you will not be able to feel the air flowing through the air vents.

How To Calculate The Duct Size

You have read above about the HVAC duct sizing rule of thumb. Now, let’s have a look at all the things you need to know in order to calculate the best duct size for your home. The formula is a little complicated and involves the following things –

1. Area of your home in square feet.
2. The size of your air conditioning unit.
3. Airflow velocity.
4. Friction loss.
5. The static pressure of the HVAC unit.

In order to calculate the duct size, proceed as follows –

1. Look on your furnace to see how many BTUs (British Thermal Unit) it produces. This information is usually given on the furnace plate. Divide that number by 10,000.

2. Multiply the answer by the Cubic Feet per Minute (CFM) airflow of your furnace. Commonly, 100 CFM is produced by a natural draft furnace, 130 CFM by an induced furnace and 150 CFM by a condensing furnace.

3. Divide the answer you get in step 2 by 10. What you get is the measure of airflow permitted by the HVAC unit in your home.

4. Now calculate the duct size by multiplying the length with width. For example, if the vent size is 4 by 8 inches, the total CFM is 32 (4×8) per vent.

5. Now divide the CFM per vent by the total amount of CFM for the furnace.

6. Calculate the square feet of each room by multiplying the room length with its width.

7. Finally, divide the number of vents by the total square feet of each room.

This calculation is confusing, and we know that. Your best option is to use an online calculator for figuring out the duct size for your home. Ideally, every room which is 100 square feet or less needs at least one vent and two or three vents for a room size bigger than that.

Using An Online Duct Size Calculator

Even professionals use online calculators or programmed calculators to figure out the duct size. If you also use one, there is no harm. They are simple to use and give reliable output. However, HVAC professionals have more sophisticated tools at their disposal, and hence, their calculations are more reliable and precise.

Our Recommendation

It is good that you want to design the HVAC duct system of your home. Still, we strictly advise that you consult a professional with expertise in this context to validate your calculations. It is because a mistake on your part can prove to be costly, both in terms of money and effort. The size of your ducts can alter the comfort level inside your home drastically. It also determines the amount that you spend to keep your home appropriately cooled or heated. Since the biggest contributor in your electricity bill is your HVAC unit, you want to get your calculations correct in the first instance itself.

Conclusion

Now that you know how vital ducts are in keeping your home comfortable and managing your electricity bills, you know why they need to be sized precisely. However, you also know that the calculation itself is complicated and based on a number of other factors. Making your job easy are the HVAC professionals. You need not worry about calculating the duct size. Just ask your HVAC company to send somebody who can do the needful and give you the precise size. These people are trained for the job and can deliver accurate results with far less trouble than yourself. In the state of Florida, Aztil AC is at your disposal to help you in calculating the duct size and other problems related to your HVAC. The AC is your home is its very heart, and we are ready to take care of your heart for you. Our team consists of qualified people who know their job well. You can place your trust in us without second thoughts. We guarantee your complete satisfaction.

Frequently Asked Questions

There are 4 things you need to know to calculate the HVAC duct size for your home:

  1. Square footage or surface area of the house.
  2. Square footage of every room in the house.
  3. CFM or cubic feet per minute.
  4. The friction loss rate of the ducts.

Unfortunately, the formula to calculate duct size is not an easy one. If you have the above information handy, we suggest using an online HVAC duct size calculator. Else, you should contact a professional HVAC technician to help you out.

For a room size of 12×12 feet, a supply duct of 4×8 inches will be adequate as it translates to about 144 CFM. But do bear in mind that this is just the “rule of thumb,” which tends to vary from home to home and HVAC system to HVAC system. Safety lies in letting the HVAC contractor do the sizing of the ductwork. This way, you will never go wrong.

1 ton of cooling is equal to 400 CFM approximately.

 

Hence, the airflow of a 3-ton heat pump is roughly 1200 CFM (400 x 3). Divide the CFM by 2 to get the filter grill size, which in this case will be:

122/2 = 600 square inches

 

Therefore, the total area of all the grills must be equal to a minimum of 600 square inches for adequate air supply to the system.

The size of the ducts in your home does not solely depend on the size of the residence. It also takes into account the size of each individual room. Therefore, to know the duct size, you will need to know the square footage of each room in addition to the square footage of the complete house.

When designing ducts, there is no standard rule of thumb that applies. Nevertheless, there are a few factors that you may consider. These are:

 

1. Try to maintain a 1:4 ratio of height to width.

2. If the ceiling in your home is low, the duct height should be less than 250 mm.

3. Assume velocity as 500 fpm when designing ducts for grills and diffusers.

4. When designing the main duct, consider the friction factor as .08 inch/100 feet.

Request Service

What Our Clients Think Of Us


Aztil Air Conditioning
Excellent
4.4
Based on 1051 reviews
review us on
Ricardo did an amazing job servicing our dryer and showing me how to keep it clean. He was efficient and courteous.
Response from the owner: Hi Richard, thank you so much for your kind words about Ricardo! We are so glad that he was able to provide you with excellent service and helpful tips. We appreciate your feedback.
Ricardo cleaned our air ducts today and did a great job! He was professional, kind, and extremely thorough. He went above and beyond any other tech who has done any work in our home by going the extra mile…in a nutshell he treated my home like it was his own, which I could tell from the minute he walked through the door. He is a true professional, and it was obvious that not only is he knowledgeable and skilled, but he truly takes pride in what he does, which is a rare thing to find. I highly recommend him, and I can only hope that he is the tech who shows up whenever I call Aztil for work to be done!!
I scheduled an appointment for yesterday for AC maintenance for a newly purchased property. As it turns out, a new AC was recommended because of mold found inside the old unit. Aztil was able to schedule my new AC installation right away for today. Iker & Inder were professional, efficient, friendly, & very knowledgeable. I know I will be signing up for Aztil’s preventative AC maintenance program. I highly recommend Aztil AC for any of your AC needs!!!
Tino fixed my problem of no heat on my new RHEEM air conditioner on a Sunday. He knew where to look for the problem, a bad connection. It's great to know that Aztil has such professional and experienced technicians.
Response from the owner: Hi Michael,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Outstanding service!!!
Response from the owner: Hi Rich,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana and Alex were the ultimate professionals. They repaired my air conditioner quickly and did a great job cleaning up.
Response from the owner: Hi Neila,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana Dussault is awesome.After 4 hellish days without A/C, he came to our rescue. My family and I thank you for your swiftness and professionalism.God bless you
Response from the owner: Hi Germaine,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was awesome! Clean, courteous, efficient and polite. Fixed the A/C and replaced a capacitor and got a quote for a new product Aeroseal that sounds like a good investment for the a/c ductsThanks Aztil for such an excellent experience today!
Response from the owner: Hi Pamela,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Roberto just came and did a very thorough maintenance job on our system. He was very professional and did not leave any stone unturned. We will be very happy to have him come next time!!
Response from the owner: Hi Andy,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Roberto was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Tino the service representative did an awesome job. He called to confirm ahead of time. Very courteous and professional. Outlined his scope of work before starting. Gave options to consider for future to improve performance. I would recommend him to anyone seeking service.
Response from the owner: Hi John,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Tino was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Paul was professional and knowledgeable with the repairs made on our air conditioning. He arrived on time and informed us of what needed to be done with an estimate of the costs.
Response from the owner: Hi Rhonda,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Paul was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
My new HVAC unit suffered a major mechanical failure. Technicians Dana and Rick are incredible and are true experts. Sabrina Corradetti, a manager, is one of the best customer care people I have ever had the pleasure of working with. Getting the required parts from the manufacturer was a time-consuming ordeal, but Ms Corradetti stayed on top of the issue, kept me informed, was ALWAYS courteous and very helpful. Aztil has some really good folks working for them.
Response from the owner: Hi John,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana and Rick was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ricardo did an amazing job on my contract maintenance tune up. Very thorough and explained to me what he was doing every step of the way. Thank you Ricardo
Response from the owner: Hi Howard,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana and Jordan went above and beyond on our AC service today. They were professional and thorough. They communicated very well and answered any of my questions.
Response from the owner: Hi Nicole,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana and Jordan was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
I continue to recommend this business to those in the area. When I have questions, if the technician is unable to answer I am immediately able to get an answer from those in the office.
Response from the owner: Hi Ashley,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana, outstanding technician
Response from the owner: Hi Giovanni,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ricardo was very thorough and knowledgeable. He explained everything he did . He was on time, polite and very nice. I would like him to come for all my routine maintenance of my AC.
Response from the owner: Hi Anne,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was the Aztil tech who came by to fix my AC issue on a Sunday. He was very professional and did explain everything to me very well.This is the second time I renew my 5 year contract with Aztil.
Response from the owner: Hi Ayman,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Paul was excellent, experienced. Recommend completely. Explained everything thoroughly.
Response from the owner: Hi Mary,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
WOW. David did a wonderful job. Nice when a company has an employee who cares enough to do a good job.
Response from the owner: Hi Debra,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that David was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Knowledgeable, Professional, efficient, personable, and timely.The experience From the office personal to the technician was excellent
Response from the owner: Hi Steve,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Last week we had an issue with our A/C...our request for service was handled promptly and professionally. The work took an extra day and this morning, DANA & ALEX showed up as promised. They completed the work, explaining what they did making sure we were completely satisfied. You have two outstanding employees representing your company.
Response from the owner: Hi Hanna,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana and Alex were able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ricardo is Amazing!!!!!! He did such an incredible job helping my ducts get clean!! We can now finally breathe healthier and live happier!!!! Overall, everyone that has come to my house has done more than a 5 star job!! I wish we had been dealing with this company from day one. I will NEVER use another company again!!!
Response from the owner: Hi Melissa,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was very professional. Explained every step of the way
Response from the owner: Hi Jennifer,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Paul g is awesome and so is the company
Dana and Cody from Aztil arrived when scheduled. He had the part needed and proceeded to replace the broken piece. Everything was fixed quickly, and our air conditioning is working perfectly, and he left his work area clean and neat. It’s the kind of experience one wants for a repair.
Response from the owner: Hi Patti,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana and Cody were able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was great. Quickly fixed the problem and advised me on how to avoid future issues.
Response from the owner: Hi James,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Wr have been under service with Aztil since 2013 and they have kept us going. Our technician, Ricardo is excellent, very knowledgeable. He takes his time for little things and keeps our AC going smoothly. Couldn't recommend more!. Thanks!
Response from the owner: Hi Czarree,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
I have been under contract with this AC company since 2013. They have kept me going since. Just got my usual 6 month tune-up, and my technician, Ricardo was excellent and very helpful, and answered all my questions. Highly recommend. 6 months later, Ricardo is back! Excellent as always!
Response from the owner: Hi Bret, We're glad to hear that the technicians were courteous of your belongings and that Patty was helpful in scheduling the service. We take pride in our customer service and we appreciate your feedback. Thank you for choosing us for your duct cleaning needs.
We had a great experience with Dana from Aztil. He was super thorough and very helpful. He communicated great and was right on time! Highly recommend !
Response from the owner: Hi lindsay,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ricardo went above and beyond to ensure my AC was functioning well. He took the time to review routine maintenance . He also cleaned the dryer duct and again with above and beyond. He was polite and efficient.
Response from the owner: Hi Barbara,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Jordan Henry was very nice and helpful to us at late night. We much appreciated his work on our HAVC. We support this company.
Response from the owner: Hi Claudia,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ramon is absolutely amazing! Very informative and an excellent technician! This is the second time we’ve had him and he’s great
Response from the owner: Hi Kathy,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ramon was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana & Cody gave excellent service to my freaking out self, answered all my many questions about my cracked water tray and more. Also explained about a condensate pump. Very pleased with both them and the way they work together as a team to ensure my problem was repaired.
Response from the owner: Hi Dottie,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana & Cody was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ramon from Aztil cleaned out the A/C drain and was very thorough, in addition to being polite!
Response from the owner: Hi Marna. Thanks for your positive feedback. Glad you were satisfied with the service from Lee. We value our employees and treat them well, that's why they stay with us. They also enjoy working with great clients such as yourself :) We appreciate your support and business.
We had our six month maintenance and had a great experience. The technician went above and beyond to ensure our unit was cleaned out.
Response from the owner: Hi Ashley,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Ramon took the time to be very thorough and informative. Answered any questions we had and explained recommendations well. Feel comfortable that we are in good hands with Aztil. Great service and fast response to urgent calls for service.
Response from the owner: Hi Steven,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ramon was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana from Aztil completed a major repair to my 3 year old Carrier A/C unit. Dana worked efficiently, carefully and neatly in my home. The replacement of the defective part was covered by the carrier warranty and all of the labor was covered by my Aztil maintenance contract. Dana was extremely fair and honest and did not do any upsell or extra work to run up a big bill. It was all covered by my warranty and maintenance contract. My home is cool and comfortably. I highly recommend Dana and will ask for him for all routine maintenance and repairs.
Response from the owner: Hi Michele,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
We had an A/C tune up today. I am happy to give 5 stars to Ricardo who was meticulous in doing the job right. He went above and beyond and was very courteous. 👍
Response from the owner: Hi Robert,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ricardo was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Great Job Dana !
Response from the owner: Hi Mario,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was professionally trained and efficient and quick to get my unit working
Response from the owner: Thanks Linda for your positive feedback. We appreciate your business.
Ramon Serpa was professional and made helpful recommendations to maximize our air conditioning efficiency. Thank you!
Response from the owner: Hi Claudia,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Ramon was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Dana was fantastic. Very thorough.
Response from the owner: Hi Bella,Thank you so much for your wonderful review! We're glad to hear that you had an excellent experience with our service. We pride ourselves on our knowledgeable staff, and it's great to know that Dana was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
js_loader

Join The Club

Request more information by filling out the form below to learn about our Elite Cool Membership Program.

Highlights of the Elite Cool Membership Program include:

  • Members will receive priority service: Get top priority when scheduling service calls.
  • No overtime charges: We’re here for you anytime, day or night, 365 days a year, without any overtime charges.
  • A 15% member discount: Receive a 15% preferred discount on repairs not covered by the manufacturer.
  • Two maintenance calls per year: Benefit from two maintenance calls per year, spaced about six months apart.
  • AND MORE!

Need more information? Click below to call now!

Call Now

Our Partners


  • Carrier logo
  • Honywell logo
  • Mitsubishi Elctric logo
  • York logo
  • American Standard logo
  • Amana logo
  • Trane logo
  • Rheem logo
  • Ruud logo
  • Daikin logo