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What Is The HVAC Duct Sizing Rule Of Thumb?

By Aztil –

  • An HVAC system is a complex piece of equipment. There are several parts that come into play to make the temperature in your home just right while also providing you with the best air quality possible.

    With that said, even if you purchase the best HVAC unit available it might get bottlenecked by some of the other factors like ductwork. This is where the HVAC duct sizing rule of thumb comes into play. 

    You probably might think that it’s redundant, after all, vents just expel air, however, your ducts play an important role in air distribution in your air conditioning system.

    Keep on reading to learn what makes ducts a crucial link in your AC chain and what is the HVAC duct sizing rule of thumb.

    Why Duct Size Matters

    AC ducts are passageways that help your HVAC system eliminate the stale, and distribute fresh cool air. 

    How it works is quite simple – the plenum which stores the air is attached to multiple air ducts (there are usually two of these plenums in your system). First, the HVAC fills the supply plenum with air that gets expelled through the ductwork. This new air pushes the existing air into a different set of ducts that are connected to a return plenum. 

    Consequently, that air is expelled outside or filtered back into the system. 

    For this process to work perfectly, the ducts need to be the right size. If your ducts are too small, the air that your HVAC pumps will increase in pressure which will back up into the system. Over time, this will reduce the efficiency of your AC and its lifespan due to unnecessary resistance to the blower fan. 

    Bigger ductwork on the other hand can handle a lot of airflow, but if they’re too big for your HVAC they won’t distribute the air evenly through your living space and your unit will end up working too hard. This might shorten its lifespan and increase your energy consumption. 

    HVAC duct sizing rule of thumb can help you determine the exact size of ducts you need for your AC for it to operate most efficiently. 

    How to Calculate the Correct Duct Size

    Calculating the exact size is an arduous task. One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home.

    According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.

    However, we advise against eyeballing it because even though an HVAC duct sizing rule of thumb is a great start, there are many factors at play here. Additionally, this rule of thumb can vary depending on the home and the HVAC system itself. We recommend contacting an HVAC professional to get the exact size of the ducts you need.

    If you prefer a more hands-on approach and want to do the process of sizing out your ducts yourself, there is another method that is a lot more precise. The problem is that it’s also very complex and requires some knowledge and serious brain power. 

    The formula will require multiple exact numbers, such as:

    • Square footage of your home
    • The size of your AC unit
    • Airflow velocity
    • Friction loss
    • Your HVAC’s unit static pressure

    As you can tell, there is a lot of technical talk (no one said it was going to be easy), so grab a pen and paper, your trusty old calculator and strap yourself in because it’s time to start crunching numbers:

    1. Check the plate on your furnace to see how many British thermal units (BTUs) it produces. This number needs to be divided by 10.000.
    2. Multiply that number by the airflow of your furnace which is measured in Cubic feet per minute (CFM). This number depends on the type of furnace: a natural draft furnace produces 100 CFM, an induced furnace produces 130 CFM, and a condensing furnace produces 150 CFM.
    3. Divide this number by 10 and you’ve calculated the airflow that your HVAC unit permits.
    4. Multiply the length of your ducts with the width to calculate the duct size. As an example, if the size of your vent is five by nine inches, the CFM is 45 per vent.
    5. Divide the CFM per duct by the complete amount of CFM of the furnace.
    6. Multiply the length of every room with the width to calculate the square footage of each room.
    7. Divide the number of ducts by each room’s square footage.

    Generally, a room of about 100 square feet or smaller requires one vent. For bigger rooms, you might need upwards of 3 vents.

    If this seems overly complex, you can search online for an online calculator made specifically for figuring out the correct duct size. Additionally, even programmed calculators can help you. There’s no shame in that, as many HVAC professionals use these tools along with other even more sophisticated gizmos to get even more reliable calculations.

    Should You Go at This Alone?

    You could, and we know that designing the HVAC system in your residence will make you feel amazing. However, even if you do these calculations yourself, we still advise you to consult an expert to at least confirm these calculations. 

    We’ve seen it a million times, one tiny mistake can end up costing you a lot of time and money. 

    The ducts are not to be taken lightly as they can drastically influence the comfort levels in your home, the lifespan of your HVAC unit, and most importantly, your energy expenditure. So in order to keep things running smoothly without spending a fortune on electric bills, make sure that you get this calculation appropriately done.

    Everything Your AC Needs

    Who would have thought that AC vents play such an important role in your HVAC system and in keeping your home cool? This is why you cannot afford any mistakes while sizing your ducts because let’s face it – do you want your unit to go bad or pay an astronomically high electricity bill?

    The calculation is the hard part, so if you want to play it safe, we recommend you contact an HVAC company. They can send a professional to your address who can calculate the right duct size in record time. 

    After all, HVAC professionals have done the same calculation countless times and for them, it’s really second nature at this point. 

    If you don’t want to rely on HVAC duct sizing rule of thumb, and you don’t want to play a mathematician, contact Aztil AC for any air conditioning trouble you might have. We’ve got a team of experts who can help you size your AC ducts properly and help you get optimal airflow in your home.

    Aztil AC is a company you can trust. We’re available 24/7 in Palm Beach, FL, and we can help provide you with everything your AC needs in record time, at an affordable price.

    Call 561-559-2205 , or send an email to info@aztilac.com

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Angel Orbea did an excellent job today. He came in and listened to my explanation of what happened, checked his notes and went to work. He showed me everything and explained everything. He did a great job!!
Response from the owner: Hi Maria,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!
Excellent Serviceman, clean, and Professional
Alex, tech …..wonderful!!!!
Response from the owner: Hi Rick, thank you for your kind words! We're so glad to hear that Alex did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Excellent work by Dana. He arrived on time, worked very hard and did a great job cleaning up after. He explained everything and was very friendly. I will definitely request Dana next time I need service. Outstanding work !
Response from the owner: Hello Mark. We greatly appreciate your positive review. I'm glad Benny was able to provide you with good service. We appreciate you as a customer and thank you for your business. Please contact us if you ever have any questions or need support with anything.
Love Atzil and Albert always does an amazing job keeping my system running smoothly.
Response from the owner: Hi Devon, thank you for your kind words! We're so glad to hear that Albert did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
The A/C in one of our homes stopped working. I forgot to contact air Boca , which is now Aztil Air Conditioning. Instead, I contacted a competitor (paridise). The competitor came out was very nice and friendly and when they diagnose the problem, they said it would be over $900 to fix it. This didn’t seem right so I remembered to call Air Boca. I spoke with (Sabrina) and she was helpful and she sent my favorite former technician Yandis to look at the unit. They were able to solve the problem for less than $300. I highly recommend Aztil.
Response from the owner: Hi Norm, thank you for your kind words! We're so glad to hear that Sabrina and Yandis did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Technician prompt, courteous and efficient
Response from the owner: Hi Leah, thank you for your kind words! We're so glad to hear that Aztil did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Paul, Dana, Jordan and Adrian were absolutely amazing. They were thorough, clean and explained everything. Highly recommended!!’
Response from the owner: Hi Robin, thank you for your kind words! We're so glad to hear that Paul and Dana did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
I have never written a review for a business before.Last night my A/C quit. I called AZTIL in the morning and Ramon was there by afternoon. I have been a longtime AZTIL customer and have been satisfied. However Ramon was so friendly, polite and knowledgeable I feel the need to recommend them. I happened to have severalfriends visiting when Ramon arrived. To a person they were so impressed by him. The comment was made that he made one want to use AZTIL. Thank you Aztil. You have a great asset.
Response from the owner: Hi Donna, thank you for your kind words! We're so glad to hear that Ramon did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana was great. Identified the issue. Was super courteous and a gentleman. Thanks for everything.
Response from the owner: Hi Howard, thank you for your kind words! We're so glad to hear that Dana did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Aztil is always reliable and got me back to normal .. Tino came and fixed the problem immediately and it was all covered on the service contract .. thanks Tino and Aztil for fine job
Response from the owner: Hi Joseph, thank you for your kind words! We're so glad to hear that Tino did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana was quick, clean, informative and had the right amount of concern for the problem we were having with the drain clogging of two units. Well done Dana !!
Response from the owner: Hi Doug, thank you for your kind words! We're so glad to hear that Dana did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Ramon did a wonderful and thorough job on my A/C maintenance. He was very informative and explained everything. I highly recommend Aztil and Ramon is excellent!!
Response from the owner: Hi Marsha, thank you for your kind words! We're so glad to hear that Ramon did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana was amazing A++
Response from the owner: Hi Juana, thank you for your kind words! We're so glad to hear that Dana did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
My tech was knowledgeable and very polite. Got my ac taken care of and I am very happy with him. Jackie at the office was also amazing .
Response from the owner: Hi Maggie, thank you for your kind words! We're so glad to hear that Aztil did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana did a good job installing our Apco-x filter.
Response from the owner: Hi Sandra, thank you for your kind words! We're so glad to hear that Dana did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
10 years with Aztil Tino the service representative did an amazing job on my 6 month maintenance he was very professional and trustworthy
Response from the owner: Hi James, thank you for your kind words! We're so glad to hear that Aztil and Tino did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Everyone was courteous and knew just what needed to be done.I would recommend all of them.
Response from the owner: Hi Michael, thank you for your kind words! We're so glad to hear that Aztil did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Yandis Moria from Aztil was amazing. He repaired our AC, arrived on time, extremely pleasant and explained everything to me. And he left my home all cleaned up! Highly recommend him.
Response from the owner: Hi E. R., thank you for your kind words! We're so glad to hear that Yandis did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Albert was great! He arrived early after confirming we were home, conducted the HVAC maintenance efficiently, and made recommendations on how to make our system last longer. Definitely sticking with Aztil AC.
Response from the owner: Hi Catherine, thank you for your kind words! We're so glad to hear that Albert did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
The technician Alex Molina was courteous and he did a good job.
Response from the owner: Hi Nancy, thank you for your kind words! We're so glad to hear that Alex did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Let me preface the review by saying I have been a more than satisfied customer since moving here in 2016. The service I have received from AZTIL is first rate. Daryl was a superb technician. The duct sealing was a value add. Daryl increased my unit’s efficiency 68%. I highly recommend Daryl and AZTIL
Response from the owner: Hi Clark, thank you for your kind words! We're so glad to hear that Aztil did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana and Jordan were great and increased our air conditioning efficiency by 80%. They did a duct sealing job. Very happy and pleased.
Response from the owner: Hi Catherine, thank you for your kind words! We're so glad to hear that Dana and Jordan did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana and Jordan did a phenomenal job!!! A+
Response from the owner: Hi Les, thank you for your kind words! We're so glad to hear that Dana and Jordan did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Yandis was very helpful with his recommendations and he was very thorough when adjusting the airflow on our new unit to make sure we were happy. This is the best company we have worked with since we moved to Florida. This business is timely, professional, and has great customer service. (Hard to find in Florida) Thank you!
Response from the owner: Hi Jennifer, thank you for your kind words! We're so glad to hear that Yandi did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Tino came to do the 6 month servicing. He was kind and professional. Easy to work with.
Response from the owner: Hi Michael, thank you for your kind words! We're so glad to hear that Ricardo did an excellent job and that you were pleased with the outcome. Thank you for being a valued customer.
Dana, awesome job, great pro and really nice fellow!
Response from the owner: Hi Christophe,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!
They were on time, knowledgeable and cleaned up after the work was completed
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 Aztil was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
Aztil AC’s Dana was on time and performed the work quickly and efficiently. Appreciate his professionalism.
Response from the owner: Hi Rob,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 did a great job with the servicing of my HVAC units. Very pleasant and knowledgeable. Thanks, Dana
Response from the owner: Hi David,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!
Response from the owner: Hi Timothy,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!
I had a fan blower motor replaced in my AC unit. It was making a constant high pitched whistle when operating. Rick Cochcran, the service tech, came out. Even though he couldn't hear the sound I was talking about, he persisted and solved the problem. Turns out the fan was turned to maximum spin. This caused the whistle. He dropped it down a notch, and voila! Problem solved. I was totally impressed with his persistence and professionalism. Thank you, Rick!
Response from the owner: Thank you for your positive review. I'm glad you were happy with Rick's work. We appreciate your business and look forward to serving you again.
Excellent work !!Thank you .
Response from the owner: Hi Audrey,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!
Great installation by Angel and team for a new air conditioner.Very professional, thoughtful and knowledgeable. Thank you.
Response from the owner: Hi Constance,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 Angel and his team was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
I read the reviews after my apptsI was shocked to see some pf the commentsAfter our womderful apptDana jordan cody created a working atonpshere as of they were working for a family memberThroughout the process they kwpt me informedAnd made sure i received thw best serviceThe person who cleaned rhe ducts i think his nameWas tino aka yankee fan and he was amazing start to finishPS i recommended their service ro my communiry od 4000 homes and theu requested i came to our next meeting to present my experience with this corp and teamI
Response from the owner: Hi Joel,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!
Dana and Jordan were very professional. They diagnosed the issue with my AC and had me up and running in record time. Thank You team AZTIL👍🏽
Response from the owner: Hi Marlene,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!
Lazaro did a great job!!! Highly recommend
Response from the owner: Hi Joseph,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 Lazaro was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
The best AC company hands down! Quick response and resolution every time!!!
Response from the owner: Hi Diane,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!
Ricardo Campos is friendly and professional. I highly recommend Aztil for A/C and duct cleaning
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!
Lee was super. He explained everything all his work performed perfectly. Very neat professional. A pleasure to deal with him.
Response from the owner: Hi Toby,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 Lee 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 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!!
Response from the owner: Hi Marta,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!
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!!!
Response from the owner: Hi Valerie,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 Iker & Inder was able to provide you with such great service. We appreciate your feedback and hope to see you again soon!
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!
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