Duct systems are designed to properly distribute air throughout a building. Ducts that are not well designed result in discomfort, high energy costs, bad air quality, and increased noise levels. Choosing the right-sized duct is critical to maintain proper air flow in the system. Below is a simple model for determining duct sizes.
The model applies to system up to 60 feet from unit to register. Because of the complexity of designing duct systems, we highly recommend using the ACCA 'Manual D' for sizing ductwork, or consulting a professional HVAC contractor. Refer to the links on the left for more information. Determine your duct size based on the required CFM (cubic feet per minute).
Duct Installation Guide to the basics of installing residential duct systems.
Example of a duct calculator tool (). Manual-D and Duct Calculators Frequently, it is assumed that the use of a “duct calculator” or “duct wheel” is the same thing as a Manual-D. This is not a quality alternative to implementing a full Manual-D calculation. How Strong is The Blower? College Physics Reasoning And Relationships Solutions Manual on this page. The basic concept of the process begins with the through the system.
Fuel Cell Fundamentals O Hayre Solution Manual. Perfect-Home-Hvac-Design can provide Manual J, Manual D, Manual S, and simplified Duct Design Services for minimal cost. All of the above steps are an integral production of your Wrightsoft based hvac system design prepared for you by Perfect-Home-Hvac-Design. Residential Hvac Duct Drawings Article by: Geoffrey R. Stoddard, Hvac Designer.
One must first know how strong the blower is. If it’s a really strong blower, your ductwork can be relatively small. If it’s not, the ducts need to be larger. It’s a bit more complicated than that. But, that’s the basic gist. You need to “match” the ductwork to the capability of the blower. There will be a future post on how to determine the blower power in your system.
When it’s available, I’ll place a link here. You don’t want to run out of blower power before you get to the end of the duct. If you run out of blower power, you won’t have enough air coming out of the supply register. The next main variable has to do with the “effective length” of the duct system. Or, another way to look at this is: Through how much ducting will my blower have to push the air? When calculating this length of ducting, one includes the physical length as measured by a tape measure, as well as the effect of the various fittings. Here’s a great example of two different fittings and their “equivalent lengths” that would be used in this calculation Friction Rate and Manual-D You don’t want to run out of blower power before you get to the end of the duct.
If you run out of blower power, you won’t have enough air coming out of the supply register. To avoid this low-airflow potential, one should use the “effective length” of the system to determine the “” that is to be used in sizing the duct system. Then, you simply match this friction rate to the desired cfm on a duct calculator to determine the appropriate duct size. Notice that you have to calculate the friction rate, and that it’s based on the effective length of the duct system. This is not just a number you pull out of your hat. And, it’s not the same for every system.
In fact, it’ll most likely be different for every system you work on. Rule of Thumb = Comfort Problems The old rule-of-thumb that has resulted in millions of homeowners being uncomfortable is: “I always use the same friction rate of 0.10 IWC.” Yes, this is the friction rate the is approximately in the middle of the friction rate spectrum. But, being in the middle does not make it any more appropriate than any other friction rate that is guessed. Manual-D Training To learn more in-depth information about Manual-D, including how to use Wrightsoft HVAC design software to implement Manual-D, sign up for our. How Are Your Ducts? Do you think that your ducts are not sized properly? Tell us about it in the comment section below.