A variety of factors affect ductwork estimating. These include sizing, friction loss rate, material, and labor productivity.
Use a mechanical estimating software that performs accurate on-screen takeoffs directly from CAD drawings. This allows for faster estimating with fewer errors.
Specs are set up using SMACNA labor rates and standard allowances, modified for each project. This method also allows for adjusting the labor productivity based on historical feedback.
Size
Properly sized ductwork enhances airflow, improves comfort and efficiency, reduces energy costs and extends the life of the HVAC system. But calculating duct size is a complicated process that considers many factors. The first step is determining the air flow requirements of the space to be conditioned. This is done by calculating the heating or cooling load, taking into account the room size and number of occupants. Then, the optimum duct size is determined by considering the system output, duct resistance (elbows, turns), static pressure and other parameters.
The ductwork sizing is calculated by multiplying the required airflow by the system output in tons per square foot. This reveals the number of cubic feet per minute, or CFM, needed for each space. Then, a contractor adds up the square footage of each space. It is important to do this on a room-by-room basis since the temperature may vary in different areas of the house or office.
The friction loss rate is also an important factor. It helps determine the static pressure throughout the length of the ducts, which has a significant impact on air flow. This is accomplished by using a formula that includes the total duct length, and the number of coils, filters, grills, registers, dampers and turns in the ductwork system. This is a complex measurement that should be handled by an experienced contractor.
Friction Loss Rate
In laminar flow, friction losses are proportional to fluid velocity (v) over a fixed length of pipe. This is known as the Darcy energy loss equation. The governing parameters are the relative roughness of the pipe and Reynolds number. In turbulent flow, the form friction factor is much more sensitive to the Reynolds number than to roughness.
In the duct world, we use an equation that accounts for all of these things to give us the major head loss term in a pressure drop. You can find this in Manual D or Wrightsoft software and it is the same thing as the friction rate chart.
This is what we take away from the total external static pressure, or TESP, which is what is left after subtracting pressure drops for things that aren’t ducts or fittings like the evaporator coil, filters, air cleaners, humidifiers, balancing dampers, registers and grilles. We want this available static pressure to be as high as possible so we have enough push to move air through the whole duct system and all of the fittings.
We then divide this by the total effective length, or TEL. We would like to have a low TEL as well so we have a big available static pressure for the long runs of ducts in the house. Of course, things may change on the jobsite that affect TEL and ASP. For example, a support beam might be in the way of one of your ducts so it has to be moved or rerouted. This will change your FR and could impact duct size requirements.
Material
Ducts enclose and carry air from one room to another for heating, cooling and ventilation. A duct’s size is determined by the amount of air it must move, or its CFM (cubic feet per minute). Accurately-sized ducts ensure that each room gets the right amount of air and that the system works efficiently. Air leaks or incorrectly sized ducts can result in higher utility bills, damage to rooms and the property and health problems for building occupants.
Ducting is made from a variety of materials, from fiberglass to steel. Sheet metal is the most popular choice because it’s sturdy, easy to work with and has a long lifespan. It is also flexible, so it can be curved to fit around obstacles. Aluminum and galvanized mild steel are commonly used for HVAC sheet metal ducts.
Other types of ducts include fiberglass reinforced plastic and fabric ducts. Fiberglass ducts are constructed of fiberglass strands bound with resin, and foil is often added for insulation purposes. These ducts are less expensive than sheet metal, but their textured surface is more difficult to clean and can trap dust and dirt over time.
The estimating process for ductwork is complex and requires the use of advanced software tools to produce accurate real-world pricing and takeoff estimates. FastDUCT uses award-winning estimating technology to automate and streamline the ductwork acquisition process. Users can estimate rectangular and round duct in any material, with or without insulation, perform job-based item adjustments, apply job factors and view the results all within seconds.
Labor
Ducts are made of metal and may be located in hard to access places like attics or crawl spaces. They also need to be bent around obstacles, fit into joists and curves and be constructed in an airtight way. These factors add up to a significant price tag for the labor that goes into the project.
Sheet metal estimators use historical field productivity feedback to determine a reasonable expectation for how much work can be done in a day or a week on a given job type. The estimating software will allow you to adjust these numbers up or down depending on the difficulty of the current project relative to your historical projects.
Another method that can be used is the “per piece” estimating method. This will provide you with a more accurate estimation of labor as it takes into consideration the typical mixture of straight duct, gauge sizes and fitting types that are used for a given ventilation system.
Using a specialized ductwork estimating software can help you get an accurate estimate of the ductwork on any project. It will give you calculations for square feet, pounds, material and shop hours allowing you to quickly and accurately allow for bought in components, manufacturing materials, labor productivity, site time and other estimating factors. This helps you save money and gives you the confidence that your bid is competitive.