Key Factors for Calculating HVAC Duct Sizing
- Home Size. The size of your home will directly impact the size of your ductwork. A larger home will require more...
- Number of Rooms. The number of rooms in your home will also play a role in determining the size of your ductwork. More...
- Blower Size. The size of your HVAC system’s blower depends on the size of your home. A...
How to properly size ductwork?
How To Properly Size Duct Work In Residential Homes? In order to calculate CFM, you must know how big your heating or cooling unit is. The average output of an HVAC unit is 400, so multiply this by 400. You can then divide the square footage of your home by the number of rooms. By doing this, you will be able to multiply the CFM of all your rooms.
How do you calculate duct work size?
- The Pick HVAC Ductwork Calculator
- Ductwork Sizing
- Duct Size Chart
- Ductwork Sizing Calculator – Step by Step Step 1: CFM – System Size & Blower Size Sizing an HVAC System Step 2: Maximum Friction Rate Friction Rate and Ductwork Length ...
- Summary of Ductwork Sizing Factors Our Recommendation
How do you calculate the CFM of a duct?
CFM calculation requires the size of your HVAC unit in tons to be multiplied by 400 (the average output of an HVAC unit). The total must then be divided by the square footage of your house. The air flow from your system also depends on the rate of friction loss of ducts. 2000 cfm (0.94 m3/s) of air flows through a 10 x 12 inches rectangular duct.
How to properly size duct work in residential homes?
- Situate the air handler close to the center of the layout to minimize the length of ductwork spans.
- Make the layout as symmetrical as possible with branch ducts spaced evenly along the trunk duct and each branch of similar length.
- Use a minimum number of fittings, diffusers, elbows, and bends to reduce the system’s total effective length.
How do you calculate ducting?
To calculate the required equipment size, divide the HVAC load for the entire building by 12,000. One ton equals 12,000 BTUs, so if a house or office needs 24,000 BTUs, it will take a 2-ton HVAC unit. If you get an uneven number, such as 2.33 for a 28,000 BTU load capacity, round up to a 2.5-ton unit.
How do you calculate ductwork from CFM?
Cubic Feet Per Minute Size Calculations 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.
How do you calculate square footage of ducts?
1:322:44How to calculate square foot of a duct - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo the square foot of duct will be equal to 2 multiplied by area of side a plus 4 multiplied by areaMoreSo the square foot of duct will be equal to 2 multiplied by area of side a plus 4 multiplied by area of side B.
How do you calculate HVAC ducts?
Measure the diameter of the round duct across its widest point.Divide the diameter by 2 to get the radius of the duct.Multiply the radius by itself to get the square of the radius. ... Multiply the square of the radius by 3.14 to get the area of the duct in square inches.More items...
What size duct is needed for 1000 CFM?
16 inchesCFM Sizing Chart For Flex Round Ducts (50-1,700 CFM)Duct Size (Inches)Flex Duct Airflow (CFM)12 inches480 CFM14 inches700 CFM16 inches1,000 CFM18 inches1,300 CFM7 more rows
What size duct do I need for 2000 CFM?
2000 cfm (0.94 m3/s) of air flows through a 10 x 12 inches rectangular duct.
What size ductwork do I need for a 5 ton unit?
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.
How many CFM do I need per square foot?
One cfmOne cfm is needed per square foot (1 cfm/sq ft) of floor area. This is the average air quantity required for a room or an entire building. This number is based upon an averaged heat load calculation for comfort cooling.
What size HVAC Do I need calculator?
Using Square Footage to Determine What Size HVAC System is NecessaryHouse Square FootageBTUs Needed100 – 1505,000150 – 2506,000250 – 3007,000300 – 3508,00010 more rows•Jan 4, 2022
How do you calculate ductwork per pound?
4:066:02How to measure HVAC Duct Weight ? | SMACNA Standards - YouTubeYouTubeStart of suggested clipEnd of suggested clipAnd l will be the length of your duct piece so from that you will get the area. And from that areaMoreAnd l will be the length of your duct piece so from that you will get the area. And from that area you can multiply with the nominal weight that is unit weight kilogram per meter square.
How to find the size of a duct?
With a ductulator, all you have to do is mark the friction loss at 0.1 in.wg per 100 ft (0.816 Pa per meter) and then rotate your airflow until it aligns with the marked friction loss (blue color section). Then, you see the bottom part of the ductulator to find the rectangular duct size (green color section).
How are HVAC ducts sized?
HVAC ducts are sized based on the required airflow. Ducts have different forms and shapes with different dimensions. So, how do you size a duct? And, how do you know which duct size is the best one?
How to Find the Best Duct Size?
Whether you are using a physical ductulator or software, you’ll need to manually calculate and compare the duct area of the sizes you’ve selected to find the most cost-optimized sizes.
How thick is fiberglass insulation?
Most air conditioning ducts use fiberglass insulations while some of them use polyethylene (PE) insulations. For fiberglass insulated ducts, you need to add either 1″ (25mm) or 2″ (50mm) depending on the project specification. Usually, AHUs have 50mm thick insulation while FCUs have 25mm thick insulation.
How wide should a duct be?
Generally, you want to keep the duct ratio not more than 1 to 4. Duct ratio is the ratio between the width and the height of the duct. If the height of a duct is 12″ (300mm) then its width should not exceed 48″ (1200mm).
Why is a duct ratio of 1:4 not efficient?
Ducts with a duct ratio of more than 1:4 are not efficient due to high friction loss. However, you may use a greater duct ratio as long as your fan has sufficient static head.
Why use square shaped ducts?
Generally, you want to use square-shaped ducts as much as possible because they are the most cost-optimized shape with the least amount of duct area (and thus, material usage).
How to find the pressure drop of a duct?
Then we locate where the velocity line is of 5m/s and we draw a line across until we hit that. Then to find the pressure drop we draw a vertical line down from this intersection. In this instance we see it comes out at 0.65 pa per meter. So add this figure into the chart. As we’re using the equal pressure drop method we can use this pressure drop for all the duct lengths so fill those in too. Then we scroll up again and align our intersection with the upward diagonal lines to see this requires a duct with a diameter of 0.45m so we add that into the table also.
How to find total duct losses?
Now find the total duct losses for each duct and branch, that’s very easy to do simply multiply the duct length by the pressure drop per meter, in our example we found it to be 0.65pa/m. Do that for all the ducts and branches on the table.
What are the lines on a duct chart?
The layout of the chart does vary a little depending on the manufacturer but in this example the vertical lines are for pressure drop per meter of duct. The horizontal lines are for volume flow rate. The downward diagonal lines are for velocity, the upward diagonal lines are for duct diameter.
What is the perimeter of a round duct?
A round duct with a cross sectional area of 0.6m2 has a perimeter of 2.75m. A rectangular duct with an equal cross sectional area has a perimeter of 3.87m. The rectangular duct therefore requires more metal for its construction, this adds more weight and costs to the design.
How to calculate dynamic loss of a straight path through a tee?
Now we calculate the dynamic loss for the straight path through the tee, using the formula co multiplied by rho multiplied by v squared divided by 2. If we drop our values in we get the answer of 0.934 pascals so add that to the table.
Which type of duct is the most energy efficient?
Round duct is by far the most energy efficient type and that’s what we’ll use in our worked example later on. If we compare round duct to rectangular duct we see that: Round duct and rectangular duct comparison. A round duct with a cross sectional area of 0.6m2 has a perimeter of 2.75m.
How to find velocity ratio?
If we look at the air travelling straight though first, we find the velocity ratio first using the formula velocity out divided by velocity in. In this example the air out is 3.3m/s and the air in is 4m/s which gives us 0.83
What factors affect the size of ducts?
Duct sizing depends on multiple factors like size of your home, air flow velocity, friction loss and static pressure of the HVAC system.
Why is duct size important?
The wrong sized ducts will cause similar problems like those caused by the wrong sized unit, eventually putting too much pressure on your unit. Duct sizing depends on multiple factors like size of your home, air flow velocity, ...
What is the CFM of a room?
Cubic Feet Per Minute determines the velocity of air required to heat or cool each room of your house. The air velocity or air flow is directly proportional to the size of the duct work. Hence it is imperative to find the CFM of each room before deciding the size of ducts to be installed.
How to calculate CFM?
CFM calculation requires the size of your HVAC unit in tons to be multiplied by 400 (the average output of an HVAC unit). The total must then be divided by the square footage of your house.
Is ductwork sizing proportional to HVAC?
Ductwork Sizing Computation For Your HVAC: All You Need to Know. The power of an HVAC system may be directly proportional to its size but that does not mean that you will benefit from buying an over-sized HVAC system for your home. HVAC systems need to be adequately sized, depending on the size and area of your house.
What is duct size calculator?
A duct size calculator, commonly known as a ductulator, depends on factors like the size of the space you’re heating or cooling, air flow velocity, friction loss, and available static pressure of the HVAC system. Save time on the job and do less manual math by using our free, online ServiceTitan Ductulator to easily calculate the right size duct for your projects.
What is duct CFM chart?
Refer to the duct CFM chart in the HVAC manufacturer’s specs to determine the external static pressure of the blower for that specific HVAC model. It’s typically displayed as a CFM chart for HVAC that breaks down different blower settings and total CFMs required for the house or building.
How to determine friction rate?
It’s calculated by using the available static pressure (ASP) divided by total effective length (TEL) and multiplied by 100 to show how much pressure drop the system can accommodate per 100 feet of effective length. You want a higher friction rate, because it means you can use smaller, more restrictive ductwork than on an HVAC project designed with a lower friction rate, which requires larger ducts. With a low friction rate, one faulty component can severely hamper air flow because there’s less room for error.
What is the average airflow output in cooling mode?
NOTE: The average airflow output in cooling mode is between 350 and 400 CFM. Heating season airflow requires approximately 65 percent of the airflow needed for cooling. Therefore, to ensure there’s adequate airflow for both cooling and heating, use the high-end threshold of 400 CFM when referencing a duct sizing chart CFM resource.
What happens if you use the wrong size duct?
Using the wrong size duct for the space can prematurely wear out HVAC components and will likely increase customers’ energy expenses. Incorrect duct size can also cause inadequate airflow to certain areas and produce unwelcome noise. None of those scenarios result in happy customers after they’ve paid big bucks for a new, more efficient HVAC system or upgraded ductwork.
How many BTUs does a 2 ton furnace need?
As an example, say Room A needs 2,000 BTUs of heat gain based on HVAC room-by-room load calculations, and the home overall needs 24,000 BTUs, which requires a 2-ton furnace with a velocity of 800 CFM.
How is air velocity measured?
Air velocity, or airflow, gets measured in cubic feet per minute (CFM) and is directly proportional to the size of ductwork. You must find the duct CFM of each room to figure out the size of air ducts to install. It’s important to do room-by-room calculations, otherwise temperatures will likely measure uneven throughout the house or office.
1. Calculate the CFM for Each Room
Most HVAC units are designed to have a 400 CFM of airflow for every 12000 BTU or 1 ton of cooling capacity and 12000 BTU divided by 400 CFM is 30.
2. Design the Supply and Return Grille Location
The location of the supply and return grille determines how you will run the supply and return duct. So, draw the supply and return grille on your layout drawing.
3. Design the Supply Air Duct
Locate your HVAC unit and start by drawing the main duct first. Then, branch out the duct and extend it until it is close to the respective supply air diffuser.
4. Design the Return Air Duct
Follow the same procedure as the supply air duct to design the return air duct. You need to install a return air plenum box behind your HVAC unit to accommodate return duct connections.
5. Design the Fresh Air Supply Duct
It is better to include a fresh air supply duct to introduce outdoor air for ventilation. Typically, the fresh air is about 10% of the total airflow of the HVAC unit.
6. Check the External Static Pressure
Although you are most likely fine with the external static pressure, it is good to double check just in case you stretched the duct too long.
Finalization
By now, your layout drawing should be full of details. If I put together my ductwork design, it looks like the below layout drawing:
