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# How to choose air duct dimensions – Profesional Guide

The required airflow determines the air duct dimensions of the HVAC ducts. Ducts come in a variety of sizes, shapes, and forms. A ductulator is used by engineers to calculate the size of a duct. The duct ratio is often restricted to no more than a 1 to 4 ratio, and the head loss is fixed at 0.1 in.wg every 100 ft. A square-shaped duct is typically more affordable than a rectangular-shaped duct.

Nowadays, air duct dimensions diameters may be quickly calculated by engineers using software built on ductulators. You can call professionals for HVAC Cleaning Buford if the ducts are already dirty. To determine the most economically advantageous size for their ducts, the majority of engineers still use manual methods.

## How to Choose the Proper Duct Size

You must manually calculate and compare the duct area of the sizes you’ve chosen to determine the ones that are most cost-effective, whether you’re using software or a physical ductulator.

### Calculating the Duct Area

You first need to know the duct length in order to calculate the duct area. So let’s use an illustration.

#### Method no. 1:

If the duct length is 12 inches (1 foot), then the duct area of a rectangular duct of 40 inches by four inches is 66 e.

Duct Area – (Duct Width x Duct Length x 2) + (Duct Height x Duct Length x 2)

Duct Area = (40′′ x 12′′ x 2) + (4′′ x 12′′ x 2)

Duct Area = 960 + 96

Duct Area: 1056 square inches (7.33 sqft)

If the duct length is 0.3 meters in SI units, the duct area of a 1000 x 100 mm rectangular duct is as follows:

Duct Area = (Duct Width x Duct Length x 2) +  (Duct Height x Duct Length x 2)

Duct Area = (x 0.3 x 2) +(0.1 x 0.3 x 2)

Duct Area Equals 0.6 + 0.06

Duct space = 0.66 m2

However,  we discovered that the following method for determining duct area is quicker:

#### Method no. 2:

(Duct Width + Duct Height) x 2 x (Duct Length) = Duct Area

(40′′ + 4′′) x (2′′ x 12′′) = Duct Area

Air duct dimensions: 1056 square inches (7.33 sqft)

In general, you want to use square-shaped ducts as often as you can because they have the lowest duct area and are the most cost-effective shape (and thus, material usage).

Metal is a commodity, and sheet metal is what ducts are constructed of. Thus, we are unable to request that manufacturers get their metals from a less expensive vendor. Therefore, the most efficient strategy to save costs is to employ square-shaped ducts, which have the least amount of surface area.

However, because of the constrained area above the ceiling, square is not the most desired shape for ducts, as you would know if you have ever worked on ducts. Because of this, most ducts have a rectangular shape.

You can install UV Light Buford in your ducts to remove bacteria that can cause allergies.

## How to measure rectangular ducts:

You can use a number of rectangular ducts of various lengths and widths to carry a single airflow as air duct dimensions.

For 3000 cfm (5100 m3/h) of airflow, for example, you may use 2018′′ (500x450mm), 2614′′ (650x350mm), or 3812′′ (950x300mm). Which size should you therefore use?

The duct area of the aforementioned three duct sizes can be calculated, and you’ll discover that 2018′′ (500x450mm) has the smallest duct area and 3812′′ (950x300mm) has the largest duct area. Therefore, if there is enough room, you should always utilize 2018′′ (500x450mm) for 3000 cfm (5100 m3/h).

However, as it is “flatter” and can fit in the space above the ceiling, you will probably utilize 2614′′ (650x350mm) for 3000 cfm (5100 m3/h). You rarely use something like 3812′′ (950x300mm) for 3000 cfm (5100 m3/h) unless you really have a small room.

## Remember to Take insulation In Consideration:

When planning duct routes, many individuals have a tendency to neglect insulations, which leads to conflicts between ducts and other services owing to a lack of available space.

Duct diameters listed on HVAC drawings are typically internal sizes. These duct sizes did not account for the insulation’s breadth (if any). Ventilation ducts are typically not insulated, so if you are working with them, you should be alright.

However, you need to know the insulation thickness and, consequently, the external dimension of the ducts if you are working with air conditioning ducts.

Some air conditioning ducts employ polyethylene (PE) insulation, whereas the majority use fiberglass insulation.

Depending on the project requirements, you need to add either 1′′ (25mm) or 2′′ (50mm) for fiberglass-insulated ducts. Insulation is typically 50mm thick for AHUs and 25mm thick for FCUs.

Depending on the requirements of the project, you usually need to add 3/8′′ (9mm) or more for PE insulated ducts. According to my knowledge, the majority of PE insulated ducts are 3/8′′ (9mm) thick, while some are 1/2′′ (12.5mm) thick.

Additionally, you should exercise caution when choosing internal duct sizes because AHUs ducts typically need to have 2 ft (1.2 m) of internal duct insulation at the supply air outlet for noise reduction. Usually 50mm thick, these internal duct insulations.

Therefore, if you have internal insulations, the effective duct size is smaller. Because of this, you must employ a larger duct size such that, after internal insulations, the net duct size is in compliance with friction loss of 0.1 in.wg per 100 ft (0.816 Pa per meter).

## Duct Size vs. Wall and Slab Opening

Some ducts must pass through concrete slabs and walls. In this situation, if the duct is rectangular, the aperture must be at least 4′′ (100mm) wider than the duct’s external dimension on each of the four sides.

For instance, a 4420′′ (1000x400mm) duct should have the right opening size (1100x500mm).

You can, however, add an extra 1′′ (25mm) to either side to have a little bit more room, but I wouldn’t advise going beyond that because you would have difficulties subsequently sealing off the opening.

Remember to allow enough space for wall and slab openings when working with insulated ducts, keeping in mind the thickness of the insulation and, consequently, the external dimension of the ducts.