Designing a duct system for your HVAC unit involves several steps, including sizing the unit, choosing the appropriate duct material and size, calculating airflow requirements, laying out the duct system, and building a material list. Most HVAC units are designed to have a 400 CFM of airflow for every 12000 BTU or 1 ton of cooling capacity. To determine the required CFM for each room, divide the BTU needed for each room by 30.
In this article, we discuss best practices for designing and installing proper ductwork in a home. The article covers the basics of HVAC ductwork sizing and layout, as well as the importance of duct components, materials, shapes, sizing, pressure losses, and fan. We also learn how to achieve efficient and effective ductwork design for heating and cooling your home, identify signs of bad ductwork, and understand factors that influence duct performance.
Duct systems for heating and cooling homes should be designed using principles of air flow, friction, and turbulence. Key components of duct system design include duct shape, which impacts air velocity and friction, and duct size, which determines the internal cross-sectional area.
For an efficient layout, use ducts for all air distribution, avoid building cavities like walls or raised floors, and install ductwork in the most direct and closest route from the air source to the living space. If possible, do not install ducts in unconditioned spaces.
In conclusion, proper ductwork design is essential for your home’s comfort level and energy efficiency. Factors such as frame construction, mechanicals, and free paths around the house can impact the placement of ducts and overall system performance.
📹 How to Design Duct Work for a 3 Ton Air Conditioning system
Ductwork design for a 3 Ton air conditioning unit. Have you ever wondered if your ducts are sized right? Follow along to get a …
How do you size ductwork for each room?
A 12′ x 12′ room requires a duct with a capacity of at least 1 CFM per square foot. The duct size should be based on the square footage of the home, the size of each room, and the required air velocity in CFM for proper heating or cooling. The friction loss rate is also important and influenced by duct length, the number of turns, and various HVAC components. An HVAC technician can determine the friction loss rate based on the number of filters, grills, registers, dampers, and other system parts.
They also use a more advanced calculation that factors in static pressure, measuring the entire length of the ductwork, multiplying the static pressure measurement by 100, and dividing it by the total length to determine the required airflow volume in CFM.
What is the best method of duct design?
The process of manual duct sizing, whether conducted manually or through the use of computerized methods, is of paramount importance in determining the airflow requirements of individual rooms or zones. Among the various techniques employed, the Equal Friction Method is arguably the most prevalent.
How do you calculate ductwork?
In order to ascertain the appropriate duct size for each room, it is necessary to calculate the cubic feet per minute (CFM) by dividing the total square footage of the residence by the number of heating, ventilation, and air conditioning (HVAC) units and the amount of heat generated per minute.
What is the lifespan of HVAC ductwork?
Ductwork typically lasts 10-15 years before needing repair or replacement. However, it often begins to deteriorate sooner than this due to its movement, which expands in winter for heating and contracts in summer for cooling. This movement, along with vibrations from fans pushing air through the house, can cause damage. Over time, seams will pull apart, creating leaks that make the HVAC system less efficient. The cool or warm air that is directed into the home leaks into the attic or crawlspace.
Additionally, the metal in the system can rust, mainly during summer when condensation collects on the metal, causing rust to corrode and create holes in the work, further reducing the efficiency of the HVAC system.
How do I know what size ducts to get for my house?
The cubic feet per minute (CFM) is a measure of the efficiency of an HVAC unit. It is calculated by multiplying the unit’s size in tons by 400 and dividing it by your home’s square footage. This CFM is then used to determine the appropriate HVAC duct size. Square footage is calculated by multiplying the length and width of every room, and if the room is oddly shaped, it is divided into smaller rectangles.
The friction loss rate, which is the pressure loss in a channel, is affected by various factors such as coil number, duct length, grilles, filters, turns, dampers, and registers. Accurate measurements are crucial for ductwork efficiency.
To determine if your ductwork is sized correctly, you should look for signs such as odd noises or high energy bills. If you notice constant HVAC noises, it is likely that your ductwork is undersized, as it cannot handle the load being pushed through it. A higher energy bill indicates that the HVAC system must work harder than it should, resulting in higher energy bills.
What is the formula for ducting calculation?
Duct size is a crucial aspect of HVAC system design and installation, often determined by the airflow rate and air velocity. The cross-sectional area of a duct (A) can be calculated using the equation A = Q / V, where Q is the airflow rate and V is the air velocity. Accurate duct sizing is essential for ensuring efficiency, performance, and longevity. This guide covers the importance of accurate duct sizing, the consequences of incorrect sizing, typical sizes, and detailed formulas for circular and rectangular ducts.
What is a common mistake in ductwork installation?
Improper HVAC mistakes can lead to issues such as incorrect ductwork sealing, incorrect thermostat placement, and improper vent placement. Cracks in ductwork can cause forced air to seep out, and if there are any, they should be sealed using a permanent sealer. Installing the thermostat incorrectly can affect the AC’s efficiency, causing it to run harder due to inaccurate temperature readings. Proper vent placement ensures adequate airflow from the HVAC, and it’s recommended to spread vents evenly throughout the home.
What is the most commonly used method for the design of duct?
The Equal Friction Method is a commonly used conventional design method that maintains the frictional pressure drop per unit length in both main and branch ducts. This method is generally better than the velocity method as most pressure drops are dissipated as friction in duct runs, rather than in balancing dampers. It is suitable for ducts that are not too long and can be used for both supply and return ducts.
What is the most common duct system layout for a house?
Whole-house ventilation systems can be arranged in two ways: the ‘branch and duct’ layout and the ‘home run’ layout. The ‘branch and branch’ layout has a large main supply and exhaust duct connected to the ventilation unit, while smaller ducts (branches and runouts) feed fresh supply air to the living space or pull moist air from the living space into the exhaust trunk. This layout is common for whole-home ventilation systems and space heating/cooling systems.
On the other hand, the ‘home run’ layout has small ducts running to the supply or exhaust grilles from a divided mixing box attached to the ventilation unit. This layout provides more precision and efficiency in the final distribution location, as there are no trunk ducts, branch ducts, or run outs, maximizing air flow.
What is the rule of thumb for duct design?
The HVAC duct sizing rule of thumb is a widely used method for determining the correct size of AC ducts in a home. It suggests installing an air conditioner of one-tonne capacity for every 500 square feet of floor area. However, this rule is not a foolproof method, as it can vary depending on the home and the HVAC system. It is recommended to consult an HVAC professional for the exact size of the ducts needed.
If a more hands-on approach is preferred, a more precise method is available, but it is complex and requires knowledge and brainpower. It is essential to consult an HVAC professional for the best results.
What are the general rules for designing ductwork?
Ductwork design principles focus on location in conditioned zones, ensuring a clear air path, perfect balance, approved materials, tight sealing, and testing. These principles aim to correct thermal losses, pressure imbalances, and other energy-wasting errors. Ducts should be installed in conditioned spaces to reduce heat gain and loss, as unconditioned areas like attics and crawl spaces may be exposed to extreme temperatures and draw air with dust, mold spores, and allergens into the HVAC system.
Ideally, ducts should be installed in each room with a supply vent, or a central return in a hallway or common area if this is not feasible. It is crucial to ensure an unobstructed air path from the supply vents in all rooms to the central return. Modern ductwork design principles aim to improve home efficiency and comfort.
📹 Residential Ductwork : HVAC Duct Design Basics
Learn all the basics of HVAC ductwork sizing and ductwork design. Along with the basics of HVAC ductwork, this HVAC training …
I am planning to install a new American Standard 120,000btu (5 ton) Up flow furnace (S9V2D120U5PSBB) and the HVAC designer said I need a return box placed under the furnace (code). I ask Him what would be the height of the box and he said it would be 6″ X 28″.75 X 24.5″. The question I have, with the 6″height, that will cut down on the height of my plenum (17″ – 6″ = 11″). Is there a minimum height that a plenum has to be? Another question, with the main supply (14″ X22″) and elbow opening into the filter housing (20″ X22″) and other side of filter housing going into the return box opening (6″ X 20″) are you choking the air flow going into supply box? Is the 11″ plenum allowed? Great article.
Are these CFM figures you’re using based on 400 FPM. I’m trying to get a good grasp on this ductwork sizing stuff, but seems like there’s some info left out or just an assumed FPM number most guys are using when teaching. When someone says, “this size duct can handle this much CFM” I’m thinking, “Yes, at a set velocity of xFPM.” But if your FPM changes does the CFM not change….??
This is how I always done it. 2x 4 supply airs= 8 8+2= 16×8 duct. That would be my first section of duct coming off my upper Plenum. Let’s say coming off the front. That would be the closest heat runs next to the very furnace pretty much. Let’s say I got a few more further ahead so now I wanna break it down. I’m only gonna have 3 coming off that duct. 2×3=6 +2= 12×8 duct. (Always x2 +2 ) keep breaking it down further in your path. Try to make those run straight into whatever duct. Make it pretty! Less elbows, the better! Now your return air duct . 3x RA+ 3 . Break it down if you have to. Single supply air ran in 6 inch round pipe and double supply air ran in 7 to 8 inch. always run metal all the way up to the return air inlet grill or supply air dampered register. Always put a damper in your duct right before your first supply airs. Around damper in every supply air 1 foot or so off of the duct. Try not to have them at the floor boot as they loosen and may vibrate. That would be annoying For a supply air duct, never ever tap off the endcap. This will jack your flow all messed up. That air has to ricochet off that end cap then it will go to the supplies. Return airs can tap off the endcap. That one’s just pulling air back to the furnace. If you have to split your supplier, air duct or return air duct midway then you need a pair of pants. That’s a different story. That I could keep going, but this tinner mommy is tired😂🤣😂 journey, mama From Minnesota. That’s the easy way.
Nice job however you should have finished with a floor layout showing the trunk & flexible duct layout. The reason I mentioned that is that it is usually better if you connect the flexible duct toward the end of the trunk such that it works more like a plenum. For instance, if you do your 1st 6″ flex duct say at 90 degrees off the trunk maybe 3′ from the start & the 2nd one say toward the end, 10′ in this example, the 2nd one will have much more air flow than the 1st one.
So? Probably a totally stupid question !!! I’m designing a duct layout for a 2 ton system for my small house. The AHU will be centered with the house, I’ll need (approx- 500 cfm’s on one side of the house,, and 300 for the other half,,, Would I be correct in sizing the trunk accordingly for each side? The trunk for for 300 cfm’s come out to roughly 8 x 8 ( with a .07 friction rate,,, calculated, not assumed !) Only Asking because I’ve searched like crazy, and this is the only article I’ve seen where you’ve split the load,, ( make’s sense to me btw,,) Just want to make sure I size accordingly,, I’ve searched at nauseum, and every example shows one main trunk and sizing down as needed, so, I’m a little confused with that part, Otherwise,, its approx,, a 8 x 20 trunk straight across,for a 2 ton/800 cfm’s unit,, if I’m using the calculator correctly 🙂 🤔 thx!!!
Nothing about this guide is of any value other than in the beginning when he said to do load calculations. EVERY “rule of thumb” mentioned is of no value. Also, if you don’t know how much air is supposed to go to each room, which you don’t without load calculations, how can you determine the correct size duct/s to route to that specific room? HINT: You can’t.
I am a new residential installer in the Midwest looking to move into design and layout. There are many things missing in your article that I have been taught were needed. Reduction of duct size along the run for pressurization. Friction difference between flex and hard pipe. You didn’t even cover return systems. What about differences between brands? I’m just curious.
Hi Adam I have a questions total 1700 sq house the size first floor is 25ft Width by 30 ft length and the first floor ceiling height is 9 ft tall and the furnace is to the other end basement not in middle the house,and Upstairs 4 room all the size is equal 12 ft by 13 ft and ceiling upstairs from the floor is 8 ft tall .what is the size duck work ? Here I used Rectangular 8 inch by 16 inch and 16 ft long but it couldn’t reach upstairs floor
Here’s a good question: can the A-coil be adjusted (it’s horizontal position) by removing the trunk (short straight section of square ducting) connecting the A-coil cabinet & the plenum? Have an A-coil that is not sitting like it should. Condensate water 💦 doesn’t drain out of the regular drain line, but it drips out of the cabinet. Some I have collecting in a bucket, but some is going into the drain pan. Thank you the heating and cooling system work just fine, but there is a condensate drain problem, b/c the A-coil is not sitting as it should (level) for proper drainage
nice salesman training vid, as to how to supper simplify for the customer. i am guessing you are no more than 3 years in and have a lot of potential in sales. If your trying to show the masses how to calc and size ducts you have miss some of the most important things. you used a bad rule of thumb for older homes and homes built in the last 30 years 600 SQ per ton is not for every house. then you skip load OK not recommended by any Pro, but then you don’t explain total effective feet of the duct as you have it layed out, you dont tell the most important part of what friction rate is left after pressure drop of filter coil, to even select a airflow for any of what you had in the vidio. take some more air flow clases and if i am wrong and you know more than this vid then stop posting half ass vids to confuse DYI people.